From 70946a44deec299ef54c0ec933e8d82ddd4bcc6a Mon Sep 17 00:00:00 2001 From: Yao Dongdong Date: Mon, 7 Mar 2016 16:02:14 +0800 Subject: documentation: Make sample code and documentation consistent In the chapter 'analogy with reader-writer locking', the sample code uses spinlock_t in reader-writer case. Just correct it so that we can read the document easily. Signed-off-by: Yao Dongdong Signed-off-by: Paul E. McKenney --- Documentation/RCU/whatisRCU.txt | 22 +++++++++++++++------- 1 file changed, 15 insertions(+), 7 deletions(-) (limited to 'Documentation') diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index dc49c6712b17..111770ffa10e 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -681,22 +681,30 @@ Although RCU can be used in many different ways, a very common use of RCU is analogous to reader-writer locking. The following unified diff shows how closely related RCU and reader-writer locking can be. + @@ -5,5 +5,5 @@ struct el { + int data; + /* Other data fields */ + }; + -rwlock_t listmutex; + +spinlock_t listmutex; + struct el head; + @@ -13,15 +14,15 @@ struct list_head *lp; struct el *p; - - read_lock(); + - read_lock(&listmutex); - list_for_each_entry(p, head, lp) { + rcu_read_lock(); + list_for_each_entry_rcu(p, head, lp) { if (p->key == key) { *result = p->data; - - read_unlock(); + - read_unlock(&listmutex); + rcu_read_unlock(); return 1; } } - - read_unlock(); + - read_unlock(&listmutex); + rcu_read_unlock(); return 0; } @@ -732,7 +740,7 @@ Or, for those who prefer a side-by-side listing: 5 int data; 5 int data; 6 /* Other data fields */ 6 /* Other data fields */ 7 }; 7 }; - 8 spinlock_t listmutex; 8 spinlock_t listmutex; + 8 rwlock_t listmutex; 8 spinlock_t listmutex; 9 struct el head; 9 struct el head; 1 int search(long key, int *result) 1 int search(long key, int *result) @@ -740,15 +748,15 @@ Or, for those who prefer a side-by-side listing: 3 struct list_head *lp; 3 struct list_head *lp; 4 struct el *p; 4 struct el *p; 5 5 - 6 read_lock(); 6 rcu_read_lock(); + 6 read_lock(&listmutex); 6 rcu_read_lock(); 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) { 8 if (p->key == key) { 8 if (p->key == key) { 9 *result = p->data; 9 *result = p->data; -10 read_unlock(); 10 rcu_read_unlock(); +10 read_unlock(&listmutex); 10 rcu_read_unlock(); 11 return 1; 11 return 1; 12 } 12 } 13 } 13 } -14 read_unlock(); 14 rcu_read_unlock(); +14 read_unlock(&listmutex); 14 rcu_read_unlock(); 15 return 0; 15 return 0; 16 } 16 } -- cgit 1.4.1 From 41abcf321d447b9987f6b7d1a9bb65831e786daf Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 11 Dec 2015 16:18:22 -0800 Subject: documentation: Add real-time requirements from CPU-bound workloads This commit records RCU's responsibility to avoid degrading latencies of CPUs running tight loops within properly configured workloads, both in kernel and in userspace. Signed-off-by: Paul E. McKenney --- Documentation/RCU/Design/Requirements/Requirements.html | 10 +++++++++- Documentation/RCU/Design/Requirements/Requirements.htmlx | 8 ++++++++ 2 files changed, 17 insertions(+), 1 deletion(-) (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index a725f9900ec8..3004baa71bcc 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1,5 +1,5 @@ - + @@ -2170,6 +2170,14 @@ up to and including systems with 4096 CPUs. This real-time requirement motivated the grace-period kthread, which also simplified handling of a number of race conditions. +

+RCU must avoid degrading real-time response for CPU-bound threads, whether +executing in usermode (which is one use case for +CONFIG_NO_HZ_FULL=y) or in the kernel. +That said, CPU-bound loops in the kernel must execute +cond_resched_rcu_qs() at least once per few tens of milliseconds +in order to avoid receiving an IPI from RCU. +

Finally, RCU's status as a synchronization primitive means that any RCU failure can result in arbitrary memory corruption that can be diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index 3a97ba490c42..61caffc86823 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -2337,6 +2337,14 @@ up to and including systems with 4096 CPUs. This real-time requirement motivated the grace-period kthread, which also simplified handling of a number of race conditions. +

+RCU must avoid degrading real-time response for CPU-bound threads, whether +executing in usermode (which is one use case for +CONFIG_NO_HZ_FULL=y) or in the kernel. +That said, CPU-bound loops in the kernel must execute +cond_resched_rcu_qs() at least once per few tens of milliseconds +in order to avoid receiving an IPI from RCU. +

Finally, RCU's status as a synchronization primitive means that any RCU failure can result in arbitrary memory corruption that can be -- cgit 1.4.1 From f43b62542eb61a52d97d6b82a786a912fa5e6c51 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 7 Jan 2016 09:12:43 -0800 Subject: documentation: Add synchronize_rcu_mult() to the requirements Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 92 ++++++++++++++++++++++ .../RCU/Design/Requirements/Requirements.htmlx | 82 +++++++++++++++++++ 2 files changed, 174 insertions(+) (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 3004baa71bcc..59acd82e67d4 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2231,6 +2231,8 @@ described in a separate section.

  • Sched Flavor
  • Sleepable RCU
  • Tasks RCU +
  • + Waiting for Multiple Grace Periods

    Bottom-Half Flavor

    @@ -2480,6 +2482,81 @@ The tasks-RCU API is quite compact, consisting only of synchronize_rcu_tasks(), and rcu_barrier_tasks(). +

    +Waiting for Multiple Grace Periods

    + +

    +Perhaps you have an RCU protected data structure that is accessed from +RCU read-side critical sections, from softirq handlers, and from +hardware interrupt handlers. +That is three flavors of RCU, the normal flavor, the bottom-half flavor, +and the sched flavor. +How to wait for a compound grace period? + +

    +The best approach is usually to “just say no!” and +insert rcu_read_lock() and rcu_read_unlock() +around each RCU read-side critical section, regardless of what +environment it happens to be in. +But suppose that some of the RCU read-side critical sections are +on extremely hot code paths, and that use of CONFIG_PREEMPT=n +is not a viable option, so that rcu_read_lock() and +rcu_read_unlock() are not free. +What then? + +

    +You could wait on all three grace periods in succession, as follows: + +

    +
    + 1 synchronize_rcu();
+ 2 synchronize_rcu_bh();
+ 3 synchronize_sched();
+
    +
    + +

    +This works, but triples the update-side latency penalty. +In cases where this is not acceptable, synchronize_rcu_mult() +may be used to wait on all three flavors of grace period concurrently: + +

    +
    + 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
+
    +
    + +

    +But what if it is necessary to also wait on SRCU? +This can be done as follows: + +

    +
    + 1 static void call_my_srcu(struct rcu_head *head,
+ 2        void (*func)(struct rcu_head *head))
+ 3 {
+ 4   call_srcu(&my_srcu, head, func);
+ 5 }
+ 6
+ 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
+
    +
    + +

    +If you needed to wait on multiple different flavors of SRCU +(but why???), you would need to create a wrapper function resembling +call_my_srcu() for each SRCU flavor. + +

    Quick Quiz 15: +But what if I need to wait for multiple RCU flavors, but I also need +the grace periods to be expedited? +
    Answer + +

    +Again, it is usually better to adjust the RCU read-side critical sections +to use a single flavor of RCU, but when this is not feasible, you can use +synchronize_rcu_mult(). +

    Possible Future Changes

    @@ -2901,5 +2978,20 @@ during scheduler initialization.

    Back to Quick Quiz 14. + +

    Quick Quiz 15: +But what if I need to wait for multiple RCU flavors, but I also need +the grace periods to be expedited? + + +

    Answer: +If you are using expedited grace periods, there should be less penalty +for waiting on them in succession. +But if that is nevertheless a problem, you can use workqueues or multiple +kthreads to wait on the various expedited grace periods concurrently. + + +

    Back to Quick Quiz 15. + diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index 61caffc86823..6ff4966672e2 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -2398,6 +2398,8 @@ described in a separate section.

  • Sched Flavor
  • Sleepable RCU
  • Tasks RCU +
  • + Waiting for Multiple Grace Periods

    Bottom-Half Flavor

    @@ -2647,6 +2649,86 @@ The tasks-RCU API is quite compact, consisting only of synchronize_rcu_tasks(), and rcu_barrier_tasks(). +

    +Waiting for Multiple Grace Periods

    + +

    +Perhaps you have an RCU protected data structure that is accessed from +RCU read-side critical sections, from softirq handlers, and from +hardware interrupt handlers. +That is three flavors of RCU, the normal flavor, the bottom-half flavor, +and the sched flavor. +How to wait for a compound grace period? + +

    +The best approach is usually to “just say no!” and +insert rcu_read_lock() and rcu_read_unlock() +around each RCU read-side critical section, regardless of what +environment it happens to be in. +But suppose that some of the RCU read-side critical sections are +on extremely hot code paths, and that use of CONFIG_PREEMPT=n +is not a viable option, so that rcu_read_lock() and +rcu_read_unlock() are not free. +What then? + +

    +You could wait on all three grace periods in succession, as follows: + +

    +
    + 1 synchronize_rcu();
+ 2 synchronize_rcu_bh();
+ 3 synchronize_sched();
+
    +
    + +

    +This works, but triples the update-side latency penalty. +In cases where this is not acceptable, synchronize_rcu_mult() +may be used to wait on all three flavors of grace period concurrently: + +

    +
    + 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
+
    +
    + +

    +But what if it is necessary to also wait on SRCU? +This can be done as follows: + +

    +
    + 1 static void call_my_srcu(struct rcu_head *head,
+ 2        void (*func)(struct rcu_head *head))
+ 3 {
+ 4   call_srcu(&my_srcu, head, func);
+ 5 }
+ 6
+ 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
+
    +
    + +

    +If you needed to wait on multiple different flavors of SRCU +(but why???), you would need to create a wrapper function resembling +call_my_srcu() for each SRCU flavor. + +

    @@QQ@@ +But what if I need to wait for multiple RCU flavors, but I also need +the grace periods to be expedited? +

    @@QQA@@ +If you are using expedited grace periods, there should be less penalty +for waiting on them in succession. +But if that is nevertheless a problem, you can use workqueues or multiple +kthreads to wait on the various expedited grace periods concurrently. +

    @@QQE@@ + +

    +Again, it is usually better to adjust the RCU read-side critical sections +to use a single flavor of RCU, but when this is not feasible, you can use +synchronize_rcu_mult(). +

    Possible Future Changes

    -- cgit 1.4.1 From d8936c0b7e29510ce8f5c85ff5fcc592a938e860 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 15 Feb 2016 17:29:47 -0800 Subject: documentation: Explain why rcu_read_lock() needs no barrier() This commit adds a Quick Quiz whose answer explains why the compiler code reordering enabled by CONFIG_PREEMPT=n's empty rcu_read_lock() and rcu_read_unlock() functions does not hinder RCU's ability to figure out which RCU read-side critical sections have completed and not. Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 130 ++++++++++++++------- .../RCU/Design/Requirements/Requirements.htmlx | 28 +++++ 2 files changed, 115 insertions(+), 43 deletions(-) (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 59acd82e67d4..2a56031bfdd4 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -583,6 +583,17 @@ The first and second guarantees require unbelievably strict ordering! Are all these memory barriers really required?
    Answer +

    Quick Quiz 7: +You claim that rcu_read_lock() and rcu_read_unlock() +generate absolutely no code in some kernel builds. +This means that the compiler might arbitrarily rearrange consecutive +RCU read-side critical sections. +Given such rearrangement, if a given RCU read-side critical section +is done, how can you be sure that all prior RCU read-side critical +sections are done? +Won't the compiler rearrangements make that impossible to determine? +
    Answer +

    Note that these memory-barrier requirements do not replace the fundamental RCU requirement that a grace period wait for all pre-existing readers. @@ -626,9 +637,9 @@ inconvenience can be avoided through use of the call_rcu() and kfree_rcu() API members described later in this document. -

    Quick Quiz 7: +

    Quick Quiz 8: But how does the upgrade-to-write operation exclude other readers? -
    Answer +
    Answer

    This guarantee allows lookup code to be shared between read-side @@ -714,9 +725,9 @@ to do significant reordering. This is by design: Any significant ordering constraints would slow down these fast-path APIs. -

    Quick Quiz 8: +

    Quick Quiz 9: Can't the compiler also reorder this code? -
    Answer +
    Answer

    Readers Do Not Exclude Updaters

    @@ -769,10 +780,10 @@ new readers can start immediately after synchronize_rcu() starts, and synchronize_rcu() is under no obligation to wait for these new readers. -

    Quick Quiz 9: +

    Quick Quiz 10: Suppose that synchronize_rcu() did wait until all readers had completed. Would the updater be able to rely on this? -
    Answer +
    Answer

    Grace Periods Don't Partition Read-Side Critical Sections

    @@ -969,11 +980,11 @@ grace period. As a result, an RCU read-side critical section cannot partition a pair of RCU grace periods. -

    Quick Quiz 10: +

    Quick Quiz 11: How long a sequence of grace periods, each separated by an RCU read-side critical section, would be required to partition the RCU read-side critical sections at the beginning and end of the chain? -
    Answer +
    Answer

    Disabling Preemption Does Not Block Grace Periods

    @@ -1127,9 +1138,9 @@ synchronization primitives be legal within RCU read-side critical sections, including spinlocks, sequence locks, atomic operations, reference counters, and memory barriers. -

    Quick Quiz 11: +

    Quick Quiz 12: What about sleeping locks? -
    Answer +
    Answer

    It often comes as a surprise that many algorithms do not require a @@ -1354,12 +1365,12 @@ write an RCU callback function that takes too long. Long-running operations should be relegated to separate threads or (in the Linux kernel) workqueues. -

    Quick Quiz 12: +

    Quick Quiz 13: Why does line 19 use rcu_access_pointer()? After all, call_rcu() on line 25 stores into the structure, which would interact badly with concurrent insertions. Doesn't this mean that rcu_dereference() is required? -
    Answer +
    Answer

    However, all that remove_gp_cb() is doing is @@ -1406,14 +1417,14 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx, so the very few places that needed something like synchronize_rcu() simply open-coded it. -

    Quick Quiz 13: +

    Quick Quiz 14: Earlier it was claimed that call_rcu() and kfree_rcu() allowed updaters to avoid being blocked by readers. But how can that be correct, given that the invocation of the callback and the freeing of the memory (respectively) must still wait for a grace period to elapse? -
    Answer +
    Answer

    But what if the updater must wait for the completion of code to be @@ -1838,11 +1849,11 @@ kthreads to be spawned. Therefore, invoking synchronize_rcu() during scheduler initialization can result in deadlock. -

    Quick Quiz 14: +

    Quick Quiz 15: So what happens with synchronize_rcu() during scheduler initialization for CONFIG_PREEMPT=n kernels? -
    Answer +
    Answer

    I learned of these boot-time requirements as a result of a series of @@ -2547,10 +2558,10 @@ If you needed to wait on multiple different flavors of SRCU (but why???), you would need to create a wrapper function resembling call_my_srcu() for each SRCU flavor. -

    Quick Quiz 15: +

    Quick Quiz 16: But what if I need to wait for multiple RCU flavors, but I also need the grace periods to be expedited? -
    Answer +
    Answer

    Again, it is usually better to adjust the RCU read-side critical sections @@ -2827,18 +2838,51 @@ adhered to the as-if rule than it is to actually adhere to it!

    Quick Quiz 7: -But how does the upgrade-to-write operation exclude other readers? +You claim that rcu_read_lock() and rcu_read_unlock() +generate absolutely no code in some kernel builds. +This means that the compiler might arbitrarily rearrange consecutive +RCU read-side critical sections. +Given such rearrangement, if a given RCU read-side critical section +is done, how can you be sure that all prior RCU read-side critical +sections are done? +Won't the compiler rearrangements make that impossible to determine?

    Answer: -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. +In cases where rcu_read_lock() and rcu_read_unlock() +generate absolutely no code, RCU infers quiescent states only at +special locations, for example, within the scheduler. +Because calls to schedule() had better prevent calling-code +accesses to shared variables from being rearranged across the call to +schedule(), if RCU detects the end of a given RCU read-side +critical section, it will necessarily detect the end of all prior +RCU read-side critical sections, no matter how aggressively the +compiler scrambles the code. + +

    +Again, this all assumes that the compiler cannot scramble code across +calls to the scheduler, out of interrupt handlers, into the idle loop, +into user-mode code, and so on. +But if your kernel build allows that sort of scrambling, you have broken +far more than just RCU!

    Back to Quick Quiz 7.

    Quick Quiz 8: +But how does the upgrade-to-write operation exclude other readers? + + +

    Answer: +It doesn't, just like normal RCU updates, which also do not exclude +RCU readers. + + +

    Back to Quick Quiz 8. + + +

    Quick Quiz 9: Can't the compiler also reorder this code? @@ -2848,10 +2892,10 @@ No, the volatile casts in READ_ONCE() and this particular case. -

    Back to Quick Quiz 8. +

    Back to Quick Quiz 9. - -

    Quick Quiz 9: + +

    Quick Quiz 10: Suppose that synchronize_rcu() did wait until all readers had completed. Would the updater be able to rely on this? @@ -2866,10 +2910,10 @@ Therefore, the code following in any case. -

    Back to Quick Quiz 9. +

    Back to Quick Quiz 10. - -

    Quick Quiz 10: + +

    Quick Quiz 11: How long a sequence of grace periods, each separated by an RCU read-side critical section, would be required to partition the RCU read-side critical sections at the beginning and end of the chain? @@ -2883,10 +2927,10 @@ Therefore, even in practice, RCU users must abide by the theoretical rather than the practical answer. -

    Back to Quick Quiz 10. +

    Back to Quick Quiz 11. - -

    Quick Quiz 11: + +

    Quick Quiz 12: What about sleeping locks? @@ -2914,10 +2958,10 @@ the mutex was not immediately available. Either way, mutex_trylock() returns immediately without sleeping. -

    Back to Quick Quiz 11. +

    Back to Quick Quiz 12. - -

    Quick Quiz 12: + +

    Quick Quiz 13: Why does line 19 use rcu_access_pointer()? After all, call_rcu() on line 25 stores into the structure, which would interact badly with concurrent insertions. @@ -2933,10 +2977,10 @@ is released on line 25, which in turn means that rcu_access_pointer() suffices. -

    Back to Quick Quiz 12. +

    Back to Quick Quiz 13. - -

    Quick Quiz 13: + +

    Quick Quiz 14: Earlier it was claimed that call_rcu() and kfree_rcu() allowed updaters to avoid being blocked by readers. @@ -2957,10 +3001,10 @@ next update as soon as it has invoked call_rcu() or grace period. -

    Back to Quick Quiz 13. +

    Back to Quick Quiz 14. - -

    Quick Quiz 14: + +

    Quick Quiz 15: So what happens with synchronize_rcu() during scheduler initialization for CONFIG_PREEMPT=n kernels? @@ -2976,10 +3020,10 @@ so it is still necessary to avoid invoking synchronize_rcu() during scheduler initialization. -

    Back to Quick Quiz 14. +

    Back to Quick Quiz 15. - -

    Quick Quiz 15: + +

    Quick Quiz 16: But what if I need to wait for multiple RCU flavors, but I also need the grace periods to be expedited? @@ -2991,7 +3035,7 @@ But if that is nevertheless a problem, you can use workqueues or multiple kthreads to wait on the various expedited grace periods concurrently. -

    Back to Quick Quiz 15. +

    Back to Quick Quiz 16. diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index 6ff4966672e2..98da30ca84c4 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -682,6 +682,34 @@ That said, it is much easier to fool yourself into believing that you have adhered to the as-if rule than it is to actually adhere to it!

    @@QQE@@ +

    @@QQ@@ +You claim that rcu_read_lock() and rcu_read_unlock() +generate absolutely no code in some kernel builds. +This means that the compiler might arbitrarily rearrange consecutive +RCU read-side critical sections. +Given such rearrangement, if a given RCU read-side critical section +is done, how can you be sure that all prior RCU read-side critical +sections are done? +Won't the compiler rearrangements make that impossible to determine? +

    @@QQA@@ +In cases where rcu_read_lock() and rcu_read_unlock() +generate absolutely no code, RCU infers quiescent states only at +special locations, for example, within the scheduler. +Because calls to schedule() had better prevent calling-code +accesses to shared variables from being rearranged across the call to +schedule(), if RCU detects the end of a given RCU read-side +critical section, it will necessarily detect the end of all prior +RCU read-side critical sections, no matter how aggressively the +compiler scrambles the code. + +

    +Again, this all assumes that the compiler cannot scramble code across +calls to the scheduler, out of interrupt handlers, into the idle loop, +into user-mode code, and so on. +But if your kernel build allows that sort of scrambling, you have broken +far more than just RCU! +

    @@QQE@@ +

    Note that these memory-barrier requirements do not replace the fundamental RCU requirement that a grace period wait for all pre-existing readers. -- cgit 1.4.1 From 514f1eb5f44520d5255b927ad5aabc00db5bc73d Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 15 Feb 2016 16:52:35 -0800 Subject: documentation: Document illegality of call_rcu() from offline CPUs There is already a blanket statement about no member of RCU's API being legal from an offline CPU, but add an explicit note where it states that it is illegal to invoke call_rcu() from an NMI handler. Signed-off-by: Paul E. McKenney --- Documentation/RCU/Design/Requirements/Requirements.html | 3 ++- Documentation/RCU/Design/Requirements/Requirements.htmlx | 3 ++- 2 files changed, 4 insertions(+), 2 deletions(-) (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 2a56031bfdd4..01e12b86e81f 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1354,7 +1354,8 @@ situations where neither synchronize_rcu() nor synchronize_rcu_expedited() would be legal, including within preempt-disable code, local_bh_disable() code, interrupt-disable code, and interrupt handlers. -However, even call_rcu() is illegal within NMI handlers. +However, even call_rcu() is illegal within NMI handlers +and from offline CPUs. The callback function (remove_gp_cb() in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index 98da30ca84c4..3355f1f9384c 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -1513,7 +1513,8 @@ situations where neither synchronize_rcu() nor synchronize_rcu_expedited() would be legal, including within preempt-disable code, local_bh_disable() code, interrupt-disable code, and interrupt handlers. -However, even call_rcu() is illegal within NMI handlers. +However, even call_rcu() is illegal within NMI handlers +and from offline CPUs. The callback function (remove_gp_cb() in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, -- cgit 1.4.1 From 11a65df5732167519937eabf16a870f5f8bde5ee Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Mar 2016 11:03:36 -0700 Subject: documentation: Remove unnecessary images from requirements This commit removes a cutesy cartoon and also a diagram that can just as easily be represented by text. Reported-by: Linus Torvalds Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/2013-08-is-it-dead.png | Bin 100825 -> 0 bytes .../RCU/Design/Requirements/RCUApplicability.svg | 237 --------------------- .../RCU/Design/Requirements/Requirements.html | 28 ++- .../RCU/Design/Requirements/Requirements.htmlx | 28 ++- 4 files changed, 40 insertions(+), 253 deletions(-) delete mode 100644 Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png delete mode 100644 Documentation/RCU/Design/Requirements/RCUApplicability.svg (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png b/Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png deleted file mode 100644 index 7496a55e4e7b..000000000000 Binary files a/Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png and /dev/null differ diff --git a/Documentation/RCU/Design/Requirements/RCUApplicability.svg b/Documentation/RCU/Design/Requirements/RCUApplicability.svg deleted file mode 100644 index ebcbeee391ed..000000000000 --- a/Documentation/RCU/Design/Requirements/RCUApplicability.svg +++ /dev/null @@ -1,237 +0,0 @@ - - - - - - - - - - - - image/svg+xml - - - - - - - - - - - - - - - - - - Read-Mostly, Stale & - - Inconsistent Data OK - - (RCU Works Great!!!) - - (RCU Works Well) - - Read-Mostly, Need Consistent Data - - Read-Write, Need Consistent Data - - Update-Mostly, Need Consistent Data - - (RCU Might Be OK...) - - (1) Provide Existence Guarantees For Update-Friendly Mechanisms - - (2) Provide Wait-Free Read-Side Primitives for Real-Time Use) - - (RCU is Very Unlikely to be the Right Tool For The Job, But it Can: - - diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 01e12b86e81f..c67a96a2a389 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1120,12 +1120,27 @@ These classes is covered in the following sections.

    Specialization

    -RCU is and always has been intended primarily for read-mostly situations, as -illustrated by the following figure. -This means that RCU's read-side primitives are optimized, often at the +RCU is and always has been intended primarily for read-mostly situations, +which means that RCU's read-side primitives are optimized, often at the expense of its update-side primitives. +Experience thus far is captured by the following list of situations: -

    RCUApplicability.svg

    +
      +
    1. Read-mostly data, where stale and inconsistent data is not + a problem: RCU works great! +
    2. Read-mostly data, where data must be consistent: + RCU works well. +
    3. Read-write data, where data must be consistent: + RCU might work OK. + Or not. +
    4. Write-mostly data, where data must be consistent: + RCU is very unlikely to be the right tool for the job, + with the following exceptions, where RCU can provide: +
        +
      1. Existence guarantees for update-friendly mechanisms. +
      2. Wait-free read-side primitives for real-time use. +
      +

    This focus on read-mostly situations means that RCU must interoperate @@ -1171,10 +1186,7 @@ some period of time, so the exact wait period is a judgment call. One of our pair of veternarians might wait 30 seconds before pronouncing the cat dead, while the other might insist on waiting a full minute. The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat, as -fancifully illustrated below: - -

    2013-08-is-it-dead.png

    +the final 30 seconds of the minute following the last heartbeat.

    Interestingly enough, this same situation applies to hardware. diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index 3355f1f9384c..d6a84f3e0451 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -1257,12 +1257,27 @@ These classes is covered in the following sections.

    Specialization

    -RCU is and always has been intended primarily for read-mostly situations, as -illustrated by the following figure. -This means that RCU's read-side primitives are optimized, often at the +RCU is and always has been intended primarily for read-mostly situations, +which means that RCU's read-side primitives are optimized, often at the expense of its update-side primitives. +Experience thus far is captured by the following list of situations: -

    RCUApplicability.svg

    +
      +
    1. Read-mostly data, where stale and inconsistent data is not + a problem: RCU works great! +
    2. Read-mostly data, where data must be consistent: + RCU works well. +
    3. Read-write data, where data must be consistent: + RCU might work OK. + Or not. +
    4. Write-mostly data, where data must be consistent: + RCU is very unlikely to be the right tool for the job, + with the following exceptions, where RCU can provide: +
        +
      1. Existence guarantees for update-friendly mechanisms. +
      2. Wait-free read-side primitives for real-time use. +
      +

    This focus on read-mostly situations means that RCU must interoperate @@ -1330,10 +1345,7 @@ some period of time, so the exact wait period is a judgment call. One of our pair of veternarians might wait 30 seconds before pronouncing the cat dead, while the other might insist on waiting a full minute. The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat, as -fancifully illustrated below: - -

    2013-08-is-it-dead.png

    +the final 30 seconds of the minute following the last heartbeat.

    Interestingly enough, this same situation applies to hardware. -- cgit 1.4.1 From 6146f8df48cb52c46c256424bd03b567b889b7bb Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Mar 2016 13:25:20 -0700 Subject: documentation: Get rid of duplicate .htmlx file This commit uses colors to obscure the quick-quiz answers, thus getting rid of the .htmlx file. Use your mouse to select the answer in order to see the text. Alternatively, use your favorite scripting language to remove all occurences of "" from the file. Reported-by: Linus Torvalds Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 889 +++--- .../RCU/Design/Requirements/Requirements.htmlx | 2872 -------------------- Documentation/RCU/Design/htmlqqz.sh | 108 - 3 files changed, 428 insertions(+), 3441 deletions(-) delete mode 100644 Documentation/RCU/Design/Requirements/Requirements.htmlx delete mode 100755 Documentation/RCU/Design/htmlqqz.sh (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index c67a96a2a389..acdad96f78e9 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1,5 +1,3 @@ - - @@ -65,8 +63,8 @@ All that aside, here are the categories of currently known RCU requirements:

    This is followed by a summary, -which is in turn followed by the inevitable -answers to the quick quizzes. +however, the answers to each quick quiz immediately follows the quiz. +Select the big white space with your mouse to see the answer.

    Fundamental Requirements

    @@ -153,13 +151,27 @@ Therefore, the outcome: cannot happen. -

    Quick Quiz 1: -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -synchronize_rcu()!!! -Just who are you trying to fool??? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Wait a minute! + You said that updaters can make useful forward progress concurrently + with readers, but pre-existing readers will block + synchronize_rcu()!!! + Just who are you trying to fool??? +
    Answer:
    + First, if updaters do not wish to be blocked by readers, they can use + call_rcu() or kfree_rcu(), which will + be discussed later. + Second, even when using synchronize_rcu(), the other + update-side code does run concurrently with readers, whether + pre-existing or not. +
     

    This scenario resembles one of the first uses of RCU in @@ -210,9 +222,20 @@ to guarantee that do_something() never runs concurrently with recovery(), but with little or no synchronization overhead in do_something_dlm(). -

    Quick Quiz 2: -Why is the synchronize_rcu() on line 28 needed? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Why is the synchronize_rcu() on line 28 needed? +
    Answer:
    + Without that extra grace period, memory reordering could result in + do_something_dlm() executing do_something() + concurrently with the last bits of recovery(). +
     

    In order to avoid fatal problems such as deadlocks, @@ -332,12 +355,27 @@ It also prevents any number of “interesting” compiler optimizations, for example, the use of gp as a scratch location immediately preceding the assignment. -

    Quick Quiz 3: -But rcu_assign_pointer() does nothing to prevent the -two assignments to p->a and p->b -from being reordered. -Can't that also cause problems? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + But rcu_assign_pointer() does nothing to prevent the + two assignments to p->a and p->b + from being reordered. + Can't that also cause problems? +
    Answer:
    + No, it cannot. + The readers cannot see either of these two fields until + the assignment to gp, by which time both fields are + fully initialized. + So reordering the assignments + to p->a and p->b cannot possibly + cause any problems. +
     

    It is tempting to assume that the reader need not do anything special @@ -494,11 +532,42 @@ The rcu_access_pointer() on line 6 is similar to code protected by the corresponding update-side lock. -

    Quick Quiz 4: -Without the rcu_dereference() or the -rcu_access_pointer(), what destructive optimizations -might the compiler make use of? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Without the rcu_dereference() or the + rcu_access_pointer(), what destructive optimizations + might the compiler make use of? +
    Answer:
    + Let's start with what happens to do_something_gp() + if it fails to use rcu_dereference(). + It could reuse a value formerly fetched from this same pointer. + It could also fetch the pointer from gp in a byte-at-a-time + manner, resulting in load tearing, in turn resulting a bytewise + mash-up of two distince pointer values. + It might even use value-speculation optimizations, where it makes + a wrong guess, but by the time it gets around to checking the + value, an update has changed the pointer to match the wrong guess. + Too bad about any dereferences that returned pre-initialization garbage + in the meantime! + + +

    + For remove_gp_synchronous(), as long as all modifications + to gp are carried out while holding gp_lock, + the above optimizations are harmless. + However, + with CONFIG_SPARSE_RCU_POINTER=y, + sparse will complain if you + define gp with __rcu and then + access it without using + either rcu_access_pointer() or rcu_dereference(). +

     

    In short, RCU's publish-subscribe guarantee is provided by the combination @@ -571,28 +640,156 @@ systems with more than one CPU: synchronize_rcu() migrates in the meantime. -

    Quick Quiz 5: -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of synchronize_rcu()? -
    Answer - -

    Quick Quiz 6: -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers really required? -
    Answer - -

    Quick Quiz 7: -You claim that rcu_read_lock() and rcu_read_unlock() -generate absolutely no code in some kernel builds. -This means that the compiler might arbitrarily rearrange consecutive -RCU read-side critical sections. -Given such rearrangement, if a given RCU read-side critical section -is done, how can you be sure that all prior RCU read-side critical -sections are done? -Won't the compiler rearrangements make that impossible to determine? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Given that multiple CPUs can start RCU read-side critical sections + at any time without any ordering whatsoever, how can RCU possibly + tell whether or not a given RCU read-side critical section starts + before a given instance of synchronize_rcu()? +
    Answer:
    + If RCU cannot tell whether or not a given + RCU read-side critical section starts before a + given instance of synchronize_rcu(), + then it must assume that the RCU read-side critical section + started first. + In other words, a given instance of synchronize_rcu() + can avoid waiting on a given RCU read-side critical section only + if it can prove that synchronize_rcu() started first. +
     
    + + + + + + + + +
     
    Quick Quiz:
    + The first and second guarantees require unbelievably strict ordering! + Are all these memory barriers really required? +
    Answer:
    + Yes, they really are required. + To see why the first guarantee is required, consider the following + sequence of events: + + +
      +
    1. + CPU 1: rcu_read_lock() + +
    2. + CPU 1: q = rcu_dereference(gp); + /* Very likely to return p. */ + +
    3. + CPU 0: list_del_rcu(p); + +
    4. + CPU 0: synchronize_rcu() starts. + +
    5. + CPU 1: do_something_with(q->a); + /* No smp_mb(), so might happen after kfree(). */ + +
    6. + CPU 1: rcu_read_unlock() + +
    7. + CPU 0: synchronize_rcu() returns. + +
    8. + CPU 0: kfree(p); + +
    + +

    + Therefore, there absolutely must be a full memory barrier between the + end of the RCU read-side critical section and the end of the + grace period. + + +

    + The sequence of events demonstrating the necessity of the second rule + is roughly similar: + + +

      +
    1. CPU 0: list_del_rcu(p); + +
    2. CPU 0: synchronize_rcu() starts. + +
    3. CPU 1: rcu_read_lock() + +
    4. CPU 1: q = rcu_dereference(gp); + /* Might return p if no memory barrier. */ + +
    5. CPU 0: synchronize_rcu() returns. + +
    6. CPU 0: kfree(p); + +
    7. + CPU 1: do_something_with(q->a); /* Boom!!! */ + +
    8. CPU 1: rcu_read_unlock() + +
    + +

    + And similarly, without a memory barrier between the beginning of the + grace period and the beginning of the RCU read-side critical section, + CPU 1 might end up accessing the freelist. + + +

    + The “as if” rule of course applies, so that any + implementation that acts as if the appropriate memory barriers + were in place is a correct implementation. + That said, it is much easier to fool yourself into believing + that you have adhered to the as-if rule than it is to actually + adhere to it! +

     
    + + + + + + + + +
     
    Quick Quiz:
    + You claim that rcu_read_lock() and rcu_read_unlock() + generate absolutely no code in some kernel builds. + This means that the compiler might arbitrarily rearrange consecutive + RCU read-side critical sections. + Given such rearrangement, if a given RCU read-side critical section + is done, how can you be sure that all prior RCU read-side critical + sections are done? + Won't the compiler rearrangements make that impossible to determine? +
    Answer:
    + In cases where rcu_read_lock() and rcu_read_unlock() + generate absolutely no code, RCU infers quiescent states only at + special locations, for example, within the scheduler. + Because calls to schedule() had better prevent calling-code + accesses to shared variables from being rearranged across the call to + schedule(), if RCU detects the end of a given RCU read-side + critical section, it will necessarily detect the end of all prior + RCU read-side critical sections, no matter how aggressively the + compiler scrambles the code. + + +

    + Again, this all assumes that the compiler cannot scramble code across + calls to the scheduler, out of interrupt handlers, into the idle loop, + into user-mode code, and so on. + But if your kernel build allows that sort of scrambling, you have broken + far more than just RCU! +

     

    Note that these memory-barrier requirements do not replace the fundamental @@ -637,9 +834,19 @@ inconvenience can be avoided through use of the call_rcu() and kfree_rcu() API members described later in this document. -

    Quick Quiz 8: -But how does the upgrade-to-write operation exclude other readers? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + But how does the upgrade-to-write operation exclude other readers? +
    Answer:
    + It doesn't, just like normal RCU updates, which also do not exclude + RCU readers. +
     

    This guarantee allows lookup code to be shared between read-side @@ -725,9 +932,20 @@ to do significant reordering. This is by design: Any significant ordering constraints would slow down these fast-path APIs. -

    Quick Quiz 9: -Can't the compiler also reorder this code? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Can't the compiler also reorder this code? +
    Answer:
    + No, the volatile casts in READ_ONCE() and + WRITE_ONCE() prevent the compiler from reordering in + this particular case. +
     

    Readers Do Not Exclude Updaters

    @@ -780,10 +998,25 @@ new readers can start immediately after synchronize_rcu() starts, and synchronize_rcu() is under no obligation to wait for these new readers. -

    Quick Quiz 10: -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Suppose that synchronize_rcu() did wait until all readers had completed. + Would the updater be able to rely on this? +
    Answer:
    + No. + Even if synchronize_rcu() were to wait until + all readers had completed, a new reader might start immediately after + synchronize_rcu() completed. + Therefore, the code following + synchronize_rcu() cannot rely on there being no readers + in any case. +
     

    Grace Periods Don't Partition Read-Side Critical Sections

    @@ -980,11 +1213,24 @@ grace period. As a result, an RCU read-side critical section cannot partition a pair of RCU grace periods. -

    Quick Quiz 11: -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + How long a sequence of grace periods, each separated by an RCU + read-side critical section, would be required to partition the RCU + read-side critical sections at the beginning and end of the chain? +
    Answer:
    + In theory, an infinite number. + In practice, an unknown number that is sensitive to both implementation + details and timing considerations. + Therefore, even in practice, RCU users must abide by the + theoretical rather than the practical answer. +
     

    Disabling Preemption Does Not Block Grace Periods

    @@ -1153,9 +1399,43 @@ synchronization primitives be legal within RCU read-side critical sections, including spinlocks, sequence locks, atomic operations, reference counters, and memory barriers. -

    Quick Quiz 12: -What about sleeping locks? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + What about sleeping locks? +
    Answer:
    + These are forbidden within Linux-kernel RCU read-side critical + sections because it is not legal to place a quiescent state + (in this case, voluntary context switch) within an RCU read-side + critical section. + However, sleeping locks may be used within userspace RCU read-side + critical sections, and also within Linux-kernel sleepable RCU + (SRCU) + read-side critical sections. + In addition, the -rt patchset turns spinlocks into a + sleeping locks so that the corresponding critical sections + can be preempted, which also means that these sleeplockified + spinlocks (but not other sleeping locks!) may be acquire within + -rt-Linux-kernel RCU read-side critical sections. + + +

    + Note that it is legal for a normal RCU read-side + critical section to conditionally acquire a sleeping locks + (as in mutex_trylock()), but only as long as it does + not loop indefinitely attempting to conditionally acquire that + sleeping locks. + The key point is that things like mutex_trylock() + either return with the mutex held, or return an error indication if + the mutex was not immediately available. + Either way, mutex_trylock() returns immediately without + sleeping. +

     

    It often comes as a surprise that many algorithms do not require a @@ -1378,12 +1658,27 @@ write an RCU callback function that takes too long. Long-running operations should be relegated to separate threads or (in the Linux kernel) workqueues. -

    Quick Quiz 13: -Why does line 19 use rcu_access_pointer()? -After all, call_rcu() on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that rcu_dereference() is required? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Why does line 19 use rcu_access_pointer()? + After all, call_rcu() on line 25 stores into the + structure, which would interact badly with concurrent insertions. + Doesn't this mean that rcu_dereference() is required? +
    Answer:
    + Presumably the ->gp_lock acquired on line 18 excludes + any changes, including any insertions that rcu_dereference() + would protect against. + Therefore, any insertions will be delayed until after + ->gp_lock + is released on line 25, which in turn means that + rcu_access_pointer() suffices. +
     

    However, all that remove_gp_cb() is doing is @@ -1430,14 +1725,31 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx, so the very few places that needed something like synchronize_rcu() simply open-coded it. -

    Quick Quiz 14: -Earlier it was claimed that call_rcu() and -kfree_rcu() allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + Earlier it was claimed that call_rcu() and + kfree_rcu() allowed updaters to avoid being blocked + by readers. + But how can that be correct, given that the invocation of the callback + and the freeing of the memory (respectively) must still wait for + a grace period to elapse? +
    Answer:
    + We could define things this way, but keep in mind that this sort of + definition would say that updates in garbage-collected languages + cannot complete until the next time the garbage collector runs, + which does not seem at all reasonable. + The key point is that in most cases, an updater using either + call_rcu() or kfree_rcu() can proceed to the + next update as soon as it has invoked call_rcu() or + kfree_rcu(), without having to wait for a subsequent + grace period. +
     

    But what if the updater must wait for the completion of code to be @@ -1862,11 +2174,26 @@ kthreads to be spawned. Therefore, invoking synchronize_rcu() during scheduler initialization can result in deadlock. -

    Quick Quiz 15: -So what happens with synchronize_rcu() during -scheduler initialization for CONFIG_PREEMPT=n -kernels? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + So what happens with synchronize_rcu() during + scheduler initialization for CONFIG_PREEMPT=n + kernels? +
    Answer:
    + In CONFIG_PREEMPT=n kernel, synchronize_rcu() + maps directly to synchronize_sched(). + Therefore, synchronize_rcu() works normally throughout + boot in CONFIG_PREEMPT=n kernels. + However, your code must also work in CONFIG_PREEMPT=y kernels, + so it is still necessary to avoid invoking synchronize_rcu() + during scheduler initialization. +
     

    I learned of these boot-time requirements as a result of a series of @@ -2571,10 +2898,23 @@ If you needed to wait on multiple different flavors of SRCU (but why???), you would need to create a wrapper function resembling call_my_srcu() for each SRCU flavor. -

    Quick Quiz 16: -But what if I need to wait for multiple RCU flavors, but I also need -the grace periods to be expedited? -
    Answer + + + + + + + +
     
    Quick Quiz:
    + But what if I need to wait for multiple RCU flavors, but I also need + the grace periods to be expedited? +
    Answer:
    + If you are using expedited grace periods, there should be less penalty + for waiting on them in succession. + But if that is nevertheless a problem, you can use workqueues + or multiple kthreads to wait on the various expedited grace + periods concurrently. +
     

    Again, it is usually better to adjust the RCU read-side critical sections @@ -2678,377 +3018,4 @@ and is provided under the terms of the Creative Commons Attribution-Share Alike 3.0 United States license. -

    -Answers to Quick Quizzes

    - - -

    Quick Quiz 1: -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -synchronize_rcu()!!! -Just who are you trying to fool??? - - -

    Answer: -First, if updaters do not wish to be blocked by readers, they can use -call_rcu() or kfree_rcu(), which will -be discussed later. -Second, even when using synchronize_rcu(), the other -update-side code does run concurrently with readers, whether pre-existing -or not. - - -

    Back to Quick Quiz 1. - - -

    Quick Quiz 2: -Why is the synchronize_rcu() on line 28 needed? - - -

    Answer: -Without that extra grace period, memory reordering could result in -do_something_dlm() executing do_something() -concurrently with the last bits of recovery(). - - -

    Back to Quick Quiz 2. - - -

    Quick Quiz 3: -But rcu_assign_pointer() does nothing to prevent the -two assignments to p->a and p->b -from being reordered. -Can't that also cause problems? - - -

    Answer: -No, it cannot. -The readers cannot see either of these two fields until -the assignment to gp, by which time both fields are -fully initialized. -So reordering the assignments -to p->a and p->b cannot possibly -cause any problems. - - -

    Back to Quick Quiz 3. - - -

    Quick Quiz 4: -Without the rcu_dereference() or the -rcu_access_pointer(), what destructive optimizations -might the compiler make use of? - - -

    Answer: -Let's start with what happens to do_something_gp() -if it fails to use rcu_dereference(). -It could reuse a value formerly fetched from this same pointer. -It could also fetch the pointer from gp in a byte-at-a-time -manner, resulting in load tearing, in turn resulting a bytewise -mash-up of two distince pointer values. -It might even use value-speculation optimizations, where it makes a wrong -guess, but by the time it gets around to checking the value, an update -has changed the pointer to match the wrong guess. -Too bad about any dereferences that returned pre-initialization garbage -in the meantime! - -

    -For remove_gp_synchronous(), as long as all modifications -to gp are carried out while holding gp_lock, -the above optimizations are harmless. -However, -with CONFIG_SPARSE_RCU_POINTER=y, -sparse will complain if you -define gp with __rcu and then -access it without using -either rcu_access_pointer() or rcu_dereference(). - - -

    Back to Quick Quiz 4. - - -

    Quick Quiz 5: -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of synchronize_rcu()? - - -

    Answer: -If RCU cannot tell whether or not a given -RCU read-side critical section starts before a -given instance of synchronize_rcu(), -then it must assume that the RCU read-side critical section -started first. -In other words, a given instance of synchronize_rcu() -can avoid waiting on a given RCU read-side critical section only -if it can prove that synchronize_rcu() started first. - - -

    Back to Quick Quiz 5. - - -

    Quick Quiz 6: -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers really required? - - -

    Answer: -Yes, they really are required. -To see why the first guarantee is required, consider the following -sequence of events: - -

      -
    1. CPU 1: rcu_read_lock() -
    2. CPU 1: q = rcu_dereference(gp); - /* Very likely to return p. */ -
    3. CPU 0: list_del_rcu(p); -
    4. CPU 0: synchronize_rcu() starts. -
    5. CPU 1: do_something_with(q->a); - /* No smp_mb(), so might happen after kfree(). */ -
    6. CPU 1: rcu_read_unlock() -
    7. CPU 0: synchronize_rcu() returns. -
    8. CPU 0: kfree(p); -
    - -

    -Therefore, there absolutely must be a full memory barrier between the -end of the RCU read-side critical section and the end of the -grace period. - -

    -The sequence of events demonstrating the necessity of the second rule -is roughly similar: - -

      -
    1. CPU 0: list_del_rcu(p); -
    2. CPU 0: synchronize_rcu() starts. -
    3. CPU 1: rcu_read_lock() -
    4. CPU 1: q = rcu_dereference(gp); - /* Might return p if no memory barrier. */ -
    5. CPU 0: synchronize_rcu() returns. -
    6. CPU 0: kfree(p); -
    7. CPU 1: do_something_with(q->a); /* Boom!!! */ -
    8. CPU 1: rcu_read_unlock() -
    - -

    -And similarly, without a memory barrier between the beginning of the -grace period and the beginning of the RCU read-side critical section, -CPU 1 might end up accessing the freelist. - -

    -The “as if” rule of course applies, so that any implementation -that acts as if the appropriate memory barriers were in place is a -correct implementation. -That said, it is much easier to fool yourself into believing that you have -adhered to the as-if rule than it is to actually adhere to it! - - -

    Back to Quick Quiz 6. - - -

    Quick Quiz 7: -You claim that rcu_read_lock() and rcu_read_unlock() -generate absolutely no code in some kernel builds. -This means that the compiler might arbitrarily rearrange consecutive -RCU read-side critical sections. -Given such rearrangement, if a given RCU read-side critical section -is done, how can you be sure that all prior RCU read-side critical -sections are done? -Won't the compiler rearrangements make that impossible to determine? - - -

    Answer: -In cases where rcu_read_lock() and rcu_read_unlock() -generate absolutely no code, RCU infers quiescent states only at -special locations, for example, within the scheduler. -Because calls to schedule() had better prevent calling-code -accesses to shared variables from being rearranged across the call to -schedule(), if RCU detects the end of a given RCU read-side -critical section, it will necessarily detect the end of all prior -RCU read-side critical sections, no matter how aggressively the -compiler scrambles the code. - -

    -Again, this all assumes that the compiler cannot scramble code across -calls to the scheduler, out of interrupt handlers, into the idle loop, -into user-mode code, and so on. -But if your kernel build allows that sort of scrambling, you have broken -far more than just RCU! - - -

    Back to Quick Quiz 7. - - -

    Quick Quiz 8: -But how does the upgrade-to-write operation exclude other readers? - - -

    Answer: -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. - - -

    Back to Quick Quiz 8. - - -

    Quick Quiz 9: -Can't the compiler also reorder this code? - - -

    Answer: -No, the volatile casts in READ_ONCE() and -WRITE_ONCE() prevent the compiler from reordering in -this particular case. - - -

    Back to Quick Quiz 9. - - -

    Quick Quiz 10: -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? - - -

    Answer: -No. -Even if synchronize_rcu() were to wait until -all readers had completed, a new reader might start immediately after -synchronize_rcu() completed. -Therefore, the code following -synchronize_rcu() cannot rely on there being no readers -in any case. - - -

    Back to Quick Quiz 10. - - -

    Quick Quiz 11: -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? - - -

    Answer: -In theory, an infinite number. -In practice, an unknown number that is sensitive to both implementation -details and timing considerations. -Therefore, even in practice, RCU users must abide by the theoretical rather -than the practical answer. - - -

    Back to Quick Quiz 11. - - -

    Quick Quiz 12: -What about sleeping locks? - - -

    Answer: -These are forbidden within Linux-kernel RCU read-side critical sections -because it is not legal to place a quiescent state (in this case, -voluntary context switch) within an RCU read-side critical section. -However, sleeping locks may be used within userspace RCU read-side critical -sections, and also within Linux-kernel sleepable RCU -(SRCU) -read-side critical sections. -In addition, the -rt patchset turns spinlocks into a sleeping locks so -that the corresponding critical sections can be preempted, which -also means that these sleeplockified spinlocks (but not other sleeping locks!) -may be acquire within -rt-Linux-kernel RCU read-side critical sections. - -

    -Note that it is legal for a normal RCU read-side critical section -to conditionally acquire a sleeping locks (as in mutex_trylock()), -but only as long as it does not loop indefinitely attempting to -conditionally acquire that sleeping locks. -The key point is that things like mutex_trylock() -either return with the mutex held, or return an error indication if -the mutex was not immediately available. -Either way, mutex_trylock() returns immediately without sleeping. - - -

    Back to Quick Quiz 12. - - -

    Quick Quiz 13: -Why does line 19 use rcu_access_pointer()? -After all, call_rcu() on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that rcu_dereference() is required? - - -

    Answer: -Presumably the ->gp_lock acquired on line 18 excludes -any changes, including any insertions that rcu_dereference() -would protect against. -Therefore, any insertions will be delayed until after ->gp_lock -is released on line 25, which in turn means that -rcu_access_pointer() suffices. - - -

    Back to Quick Quiz 13. - - -

    Quick Quiz 14: -Earlier it was claimed that call_rcu() and -kfree_rcu() allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? - - -

    Answer: -We could define things this way, but keep in mind that this sort of -definition would say that updates in garbage-collected languages -cannot complete until the next time the garbage collector runs, -which does not seem at all reasonable. -The key point is that in most cases, an updater using either -call_rcu() or kfree_rcu() can proceed to the -next update as soon as it has invoked call_rcu() or -kfree_rcu(), without having to wait for a subsequent -grace period. - - -

    Back to Quick Quiz 14. - - -

    Quick Quiz 15: -So what happens with synchronize_rcu() during -scheduler initialization for CONFIG_PREEMPT=n -kernels? - - -

    Answer: -In CONFIG_PREEMPT=n kernel, synchronize_rcu() -maps directly to synchronize_sched(). -Therefore, synchronize_rcu() works normally throughout -boot in CONFIG_PREEMPT=n kernels. -However, your code must also work in CONFIG_PREEMPT=y kernels, -so it is still necessary to avoid invoking synchronize_rcu() -during scheduler initialization. - - -

    Back to Quick Quiz 15. - - -

    Quick Quiz 16: -But what if I need to wait for multiple RCU flavors, but I also need -the grace periods to be expedited? - - -

    Answer: -If you are using expedited grace periods, there should be less penalty -for waiting on them in succession. -But if that is nevertheless a problem, you can use workqueues or multiple -kthreads to wait on the various expedited grace periods concurrently. - - -

    Back to Quick Quiz 16. - - diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx deleted file mode 100644 index d6a84f3e0451..000000000000 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ /dev/null @@ -1,2872 +0,0 @@ - - - A Tour Through RCU's Requirements [LWN.net] - - -

    A Tour Through RCU's Requirements

    - -

    Copyright IBM Corporation, 2015

    -

    Author: Paul E. McKenney

    -

    The initial version of this document appeared in the -LWN articles -here, -here, and -here.

    - -

    Introduction

    - -

    -Read-copy update (RCU) is a synchronization mechanism that is often -used as a replacement for reader-writer locking. -RCU is unusual in that updaters do not block readers, -which means that RCU's read-side primitives can be exceedingly fast -and scalable. -In addition, updaters can make useful forward progress concurrently -with readers. -However, all this concurrency between RCU readers and updaters does raise -the question of exactly what RCU readers are doing, which in turn -raises the question of exactly what RCU's requirements are. - -

    -This document therefore summarizes RCU's requirements, and can be thought -of as an informal, high-level specification for RCU. -It is important to understand that RCU's specification is primarily -empirical in nature; -in fact, I learned about many of these requirements the hard way. -This situation might cause some consternation, however, not only -has this learning process been a lot of fun, but it has also been -a great privilege to work with so many people willing to apply -technologies in interesting new ways. - -

    -All that aside, here are the categories of currently known RCU requirements: -

    - -
      -
    1. - Fundamental Requirements -
    2. Fundamental Non-Requirements -
    3. - Parallelism Facts of Life -
    4. - Quality-of-Implementation Requirements -
    5. - Linux Kernel Complications -
    6. - Software-Engineering Requirements -
    7. - Other RCU Flavors -
    8. - Possible Future Changes -
    - -

    -This is followed by a summary, -which is in turn followed by the inevitable -answers to the quick quizzes. - -

    Fundamental Requirements

    - -

    -RCU's fundamental requirements are the closest thing RCU has to hard -mathematical requirements. -These are: - -

      -
    1. - Grace-Period Guarantee -
    2. - Publish-Subscribe Guarantee -
    3. - Memory-Barrier Guarantees -
    4. - RCU Primitives Guaranteed to Execute Unconditionally -
    5. - Guaranteed Read-to-Write Upgrade -
    - -

    Grace-Period Guarantee

    - -

    -RCU's grace-period guarantee is unusual in being premeditated: -Jack Slingwine and I had this guarantee firmly in mind when we started -work on RCU (then called “rclock”) in the early 1990s. -That said, the past two decades of experience with RCU have produced -a much more detailed understanding of this guarantee. - -

    -RCU's grace-period guarantee allows updaters to wait for the completion -of all pre-existing RCU read-side critical sections. -An RCU read-side critical section -begins with the marker rcu_read_lock() and ends with -the marker rcu_read_unlock(). -These markers may be nested, and RCU treats a nested set as one -big RCU read-side critical section. -Production-quality implementations of rcu_read_lock() and -rcu_read_unlock() are extremely lightweight, and in -fact have exactly zero overhead in Linux kernels built for production -use with CONFIG_PREEMPT=n. - -

    -This guarantee allows ordering to be enforced with extremely low -overhead to readers, for example: - -

    -
    - 1 int x, y;
- 2
- 3 void thread0(void)
- 4 {
- 5   rcu_read_lock();
- 6   r1 = READ_ONCE(x);
- 7   r2 = READ_ONCE(y);
- 8   rcu_read_unlock();
- 9 }
-10
-11 void thread1(void)
-12 {
-13   WRITE_ONCE(x, 1);
-14   synchronize_rcu();
-15   WRITE_ONCE(y, 1);
-16 }
-
    -
    - -

    -Because the synchronize_rcu() on line 14 waits for -all pre-existing readers, any instance of thread0() that -loads a value of zero from x must complete before -thread1() stores to y, so that instance must -also load a value of zero from y. -Similarly, any instance of thread0() that loads a value of -one from y must have started after the -synchronize_rcu() started, and must therefore also load -a value of one from x. -Therefore, the outcome: -

    -
    -(r1 == 0 && r2 == 1)
-
    -
    -cannot happen. - -

    @@QQ@@ -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -synchronize_rcu()!!! -Just who are you trying to fool??? -

    @@QQA@@ -First, if updaters do not wish to be blocked by readers, they can use -call_rcu() or kfree_rcu(), which will -be discussed later. -Second, even when using synchronize_rcu(), the other -update-side code does run concurrently with readers, whether pre-existing -or not. -

    @@QQE@@ - -

    -This scenario resembles one of the first uses of RCU in -DYNIX/ptx, -which managed a distributed lock manager's transition into -a state suitable for handling recovery from node failure, -more or less as follows: - -

    -
    - 1 #define STATE_NORMAL        0
- 2 #define STATE_WANT_RECOVERY 1
- 3 #define STATE_RECOVERING    2
- 4 #define STATE_WANT_NORMAL   3
- 5
- 6 int state = STATE_NORMAL;
- 7
- 8 void do_something_dlm(void)
- 9 {
-10   int state_snap;
-11
-12   rcu_read_lock();
-13   state_snap = READ_ONCE(state);
-14   if (state_snap == STATE_NORMAL)
-15     do_something();
-16   else
-17     do_something_carefully();
-18   rcu_read_unlock();
-19 }
-20
-21 void start_recovery(void)
-22 {
-23   WRITE_ONCE(state, STATE_WANT_RECOVERY);
-24   synchronize_rcu();
-25   WRITE_ONCE(state, STATE_RECOVERING);
-26   recovery();
-27   WRITE_ONCE(state, STATE_WANT_NORMAL);
-28   synchronize_rcu();
-29   WRITE_ONCE(state, STATE_NORMAL);
-30 }
-
    -
    - -

    -The RCU read-side critical section in do_something_dlm() -works with the synchronize_rcu() in start_recovery() -to guarantee that do_something() never runs concurrently -with recovery(), but with little or no synchronization -overhead in do_something_dlm(). - -

    @@QQ@@ -Why is the synchronize_rcu() on line 28 needed? -

    @@QQA@@ -Without that extra grace period, memory reordering could result in -do_something_dlm() executing do_something() -concurrently with the last bits of recovery(). -

    @@QQE@@ - -

    -In order to avoid fatal problems such as deadlocks, -an RCU read-side critical section must not contain calls to -synchronize_rcu(). -Similarly, an RCU read-side critical section must not -contain anything that waits, directly or indirectly, on completion of -an invocation of synchronize_rcu(). - -

    -Although RCU's grace-period guarantee is useful in and of itself, with -quite a few use cases, -it would be good to be able to use RCU to coordinate read-side -access to linked data structures. -For this, the grace-period guarantee is not sufficient, as can -be seen in function add_gp_buggy() below. -We will look at the reader's code later, but in the meantime, just think of -the reader as locklessly picking up the gp pointer, -and, if the value loaded is non-NULL, locklessly accessing the -->a and ->b fields. - -

    -
    - 1 bool add_gp_buggy(int a, int b)
- 2 {
- 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
- 4   if (!p)
- 5     return -ENOMEM;
- 6   spin_lock(&gp_lock);
- 7   if (rcu_access_pointer(gp)) {
- 8     spin_unlock(&gp_lock);
- 9     return false;
-10   }
-11   p->a = a;
-12   p->b = a;
-13   gp = p; /* ORDERING BUG */
-14   spin_unlock(&gp_lock);
-15   return true;
-16 }
-
    -
    - -

    -The problem is that both the compiler and weakly ordered CPUs are within -their rights to reorder this code as follows: - -

    -
    - 1 bool add_gp_buggy_optimized(int a, int b)
- 2 {
- 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
- 4   if (!p)
- 5     return -ENOMEM;
- 6   spin_lock(&gp_lock);
- 7   if (rcu_access_pointer(gp)) {
- 8     spin_unlock(&gp_lock);
- 9     return false;
-10   }
-11   gp = p; /* ORDERING BUG */
-12   p->a = a;
-13   p->b = a;
-14   spin_unlock(&gp_lock);
-15   return true;
-16 }
-
    -
    - -

    -If an RCU reader fetches gp just after -add_gp_buggy_optimized executes line 11, -it will see garbage in the ->a and ->b -fields. -And this is but one of many ways in which compiler and hardware optimizations -could cause trouble. -Therefore, we clearly need some way to prevent the compiler and the CPU from -reordering in this manner, which brings us to the publish-subscribe -guarantee discussed in the next section. - -

    Publish/Subscribe Guarantee

    - -

    -RCU's publish-subscribe guarantee allows data to be inserted -into a linked data structure without disrupting RCU readers. -The updater uses rcu_assign_pointer() to insert the -new data, and readers use rcu_dereference() to -access data, whether new or old. -The following shows an example of insertion: - -

    -
    - 1 bool add_gp(int a, int b)
- 2 {
- 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
- 4   if (!p)
- 5     return -ENOMEM;
- 6   spin_lock(&gp_lock);
- 7   if (rcu_access_pointer(gp)) {
- 8     spin_unlock(&gp_lock);
- 9     return false;
-10   }
-11   p->a = a;
-12   p->b = a;
-13   rcu_assign_pointer(gp, p);
-14   spin_unlock(&gp_lock);
-15   return true;
-16 }
-
    -
    - -

    -The rcu_assign_pointer() on line 13 is conceptually -equivalent to a simple assignment statement, but also guarantees -that its assignment will -happen after the two assignments in lines 11 and 12, -similar to the C11 memory_order_release store operation. -It also prevents any number of “interesting” compiler -optimizations, for example, the use of gp as a scratch -location immediately preceding the assignment. - -

    @@QQ@@ -But rcu_assign_pointer() does nothing to prevent the -two assignments to p->a and p->b -from being reordered. -Can't that also cause problems? -

    @@QQA@@ -No, it cannot. -The readers cannot see either of these two fields until -the assignment to gp, by which time both fields are -fully initialized. -So reordering the assignments -to p->a and p->b cannot possibly -cause any problems. -

    @@QQE@@ - -

    -It is tempting to assume that the reader need not do anything special -to control its accesses to the RCU-protected data, -as shown in do_something_gp_buggy() below: - -

    -
    - 1 bool do_something_gp_buggy(void)
- 2 {
- 3   rcu_read_lock();
- 4   p = gp;  /* OPTIMIZATIONS GALORE!!! */
- 5   if (p) {
- 6     do_something(p->a, p->b);
- 7     rcu_read_unlock();
- 8     return true;
- 9   }
-10   rcu_read_unlock();
-11   return false;
-12 }
-
    -
    - -

    -However, this temptation must be resisted because there are a -surprisingly large number of ways that the compiler -(to say nothing of -DEC Alpha CPUs) -can trip this code up. -For but one example, if the compiler were short of registers, it -might choose to refetch from gp rather than keeping -a separate copy in p as follows: - -

    -
    - 1 bool do_something_gp_buggy_optimized(void)
- 2 {
- 3   rcu_read_lock();
- 4   if (gp) { /* OPTIMIZATIONS GALORE!!! */
- 5     do_something(gp->a, gp->b);
- 6     rcu_read_unlock();
- 7     return true;
- 8   }
- 9   rcu_read_unlock();
-10   return false;
-11 }
-
    -
    - -

    -If this function ran concurrently with a series of updates that -replaced the current structure with a new one, -the fetches of gp->a -and gp->b might well come from two different structures, -which could cause serious confusion. -To prevent this (and much else besides), do_something_gp() uses -rcu_dereference() to fetch from gp: - -

    -
    - 1 bool do_something_gp(void)
- 2 {
- 3   rcu_read_lock();
- 4   p = rcu_dereference(gp);
- 5   if (p) {
- 6     do_something(p->a, p->b);
- 7     rcu_read_unlock();
- 8     return true;
- 9   }
-10   rcu_read_unlock();
-11   return false;
-12 }
-
    -
    - -

    -The rcu_dereference() uses volatile casts and (for DEC Alpha) -memory barriers in the Linux kernel. -Should a -high-quality implementation of C11 memory_order_consume [PDF] -ever appear, then rcu_dereference() could be implemented -as a memory_order_consume load. -Regardless of the exact implementation, a pointer fetched by -rcu_dereference() may not be used outside of the -outermost RCU read-side critical section containing that -rcu_dereference(), unless protection of -the corresponding data element has been passed from RCU to some -other synchronization mechanism, most commonly locking or -reference counting. - -

    -In short, updaters use rcu_assign_pointer() and readers -use rcu_dereference(), and these two RCU API elements -work together to ensure that readers have a consistent view of -newly added data elements. - -

    -Of course, it is also necessary to remove elements from RCU-protected -data structures, for example, using the following process: - -

      -
    1. Remove the data element from the enclosing structure. -
    2. Wait for all pre-existing RCU read-side critical sections - to complete (because only pre-existing readers can possibly have - a reference to the newly removed data element). -
    3. At this point, only the updater has a reference to the - newly removed data element, so it can safely reclaim - the data element, for example, by passing it to kfree(). -
    - -This process is implemented by remove_gp_synchronous(): - -
    -
    - 1 bool remove_gp_synchronous(void)
- 2 {
- 3   struct foo *p;
- 4
- 5   spin_lock(&gp_lock);
- 6   p = rcu_access_pointer(gp);
- 7   if (!p) {
- 8     spin_unlock(&gp_lock);
- 9     return false;
-10   }
-11   rcu_assign_pointer(gp, NULL);
-12   spin_unlock(&gp_lock);
-13   synchronize_rcu();
-14   kfree(p);
-15   return true;
-16 }
-
    -
    - -

    -This function is straightforward, with line 13 waiting for a grace -period before line 14 frees the old data element. -This waiting ensures that readers will reach line 7 of -do_something_gp() before the data element referenced by -p is freed. -The rcu_access_pointer() on line 6 is similar to -rcu_dereference(), except that: - -

      -
    1. The value returned by rcu_access_pointer() - cannot be dereferenced. - If you want to access the value pointed to as well as - the pointer itself, use rcu_dereference() - instead of rcu_access_pointer(). -
    2. The call to rcu_access_pointer() need not be - protected. - In contrast, rcu_dereference() must either be - within an RCU read-side critical section or in a code - segment where the pointer cannot change, for example, in - code protected by the corresponding update-side lock. -
    - -

    @@QQ@@ -Without the rcu_dereference() or the -rcu_access_pointer(), what destructive optimizations -might the compiler make use of? -

    @@QQA@@ -Let's start with what happens to do_something_gp() -if it fails to use rcu_dereference(). -It could reuse a value formerly fetched from this same pointer. -It could also fetch the pointer from gp in a byte-at-a-time -manner, resulting in load tearing, in turn resulting a bytewise -mash-up of two distince pointer values. -It might even use value-speculation optimizations, where it makes a wrong -guess, but by the time it gets around to checking the value, an update -has changed the pointer to match the wrong guess. -Too bad about any dereferences that returned pre-initialization garbage -in the meantime! - -

    -For remove_gp_synchronous(), as long as all modifications -to gp are carried out while holding gp_lock, -the above optimizations are harmless. -However, -with CONFIG_SPARSE_RCU_POINTER=y, -sparse will complain if you -define gp with __rcu and then -access it without using -either rcu_access_pointer() or rcu_dereference(). -

    @@QQE@@ - -

    -In short, RCU's publish-subscribe guarantee is provided by the combination -of rcu_assign_pointer() and rcu_dereference(). -This guarantee allows data elements to be safely added to RCU-protected -linked data structures without disrupting RCU readers. -This guarantee can be used in combination with the grace-period -guarantee to also allow data elements to be removed from RCU-protected -linked data structures, again without disrupting RCU readers. - -

    -This guarantee was only partially premeditated. -DYNIX/ptx used an explicit memory barrier for publication, but had nothing -resembling rcu_dereference() for subscription, nor did it -have anything resembling the smp_read_barrier_depends() -that was later subsumed into rcu_dereference(). -The need for these operations made itself known quite suddenly at a -late-1990s meeting with the DEC Alpha architects, back in the days when -DEC was still a free-standing company. -It took the Alpha architects a good hour to convince me that any sort -of barrier would ever be needed, and it then took me a good two hours -to convince them that their documentation did not make this point clear. -More recent work with the C and C++ standards committees have provided -much education on tricks and traps from the compiler. -In short, compilers were much less tricky in the early 1990s, but in -2015, don't even think about omitting rcu_dereference()! - -

    Memory-Barrier Guarantees

    - -

    -The previous section's simple linked-data-structure scenario clearly -demonstrates the need for RCU's stringent memory-ordering guarantees on -systems with more than one CPU: - -

      -
    1. Each CPU that has an RCU read-side critical section that - begins before synchronize_rcu() starts is - guaranteed to execute a full memory barrier between the time - that the RCU read-side critical section ends and the time that - synchronize_rcu() returns. - Without this guarantee, a pre-existing RCU read-side critical section - might hold a reference to the newly removed struct foo - after the kfree() on line 14 of - remove_gp_synchronous(). -
    2. Each CPU that has an RCU read-side critical section that ends - after synchronize_rcu() returns is guaranteed - to execute a full memory barrier between the time that - synchronize_rcu() begins and the time that the RCU - read-side critical section begins. - Without this guarantee, a later RCU read-side critical section - running after the kfree() on line 14 of - remove_gp_synchronous() might - later run do_something_gp() and find the - newly deleted struct foo. -
    3. If the task invoking synchronize_rcu() remains - on a given CPU, then that CPU is guaranteed to execute a full - memory barrier sometime during the execution of - synchronize_rcu(). - This guarantee ensures that the kfree() on - line 14 of remove_gp_synchronous() really does - execute after the removal on line 11. -
    4. If the task invoking synchronize_rcu() migrates - among a group of CPUs during that invocation, then each of the - CPUs in that group is guaranteed to execute a full memory barrier - sometime during the execution of synchronize_rcu(). - This guarantee also ensures that the kfree() on - line 14 of remove_gp_synchronous() really does - execute after the removal on - line 11, but also in the case where the thread executing the - synchronize_rcu() migrates in the meantime. -
    - -

    @@QQ@@ -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of synchronize_rcu()? -

    @@QQA@@ -If RCU cannot tell whether or not a given -RCU read-side critical section starts before a -given instance of synchronize_rcu(), -then it must assume that the RCU read-side critical section -started first. -In other words, a given instance of synchronize_rcu() -can avoid waiting on a given RCU read-side critical section only -if it can prove that synchronize_rcu() started first. -

    @@QQE@@ - -

    @@QQ@@ -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers really required? -

    @@QQA@@ -Yes, they really are required. -To see why the first guarantee is required, consider the following -sequence of events: - -

      -
    1. CPU 1: rcu_read_lock() -
    2. CPU 1: q = rcu_dereference(gp); - /* Very likely to return p. */ -
    3. CPU 0: list_del_rcu(p); -
    4. CPU 0: synchronize_rcu() starts. -
    5. CPU 1: do_something_with(q->a); - /* No smp_mb(), so might happen after kfree(). */ -
    6. CPU 1: rcu_read_unlock() -
    7. CPU 0: synchronize_rcu() returns. -
    8. CPU 0: kfree(p); -
    - -

    -Therefore, there absolutely must be a full memory barrier between the -end of the RCU read-side critical section and the end of the -grace period. - -

    -The sequence of events demonstrating the necessity of the second rule -is roughly similar: - -

      -
    1. CPU 0: list_del_rcu(p); -
    2. CPU 0: synchronize_rcu() starts. -
    3. CPU 1: rcu_read_lock() -
    4. CPU 1: q = rcu_dereference(gp); - /* Might return p if no memory barrier. */ -
    5. CPU 0: synchronize_rcu() returns. -
    6. CPU 0: kfree(p); -
    7. CPU 1: do_something_with(q->a); /* Boom!!! */ -
    8. CPU 1: rcu_read_unlock() -
    - -

    -And similarly, without a memory barrier between the beginning of the -grace period and the beginning of the RCU read-side critical section, -CPU 1 might end up accessing the freelist. - -

    -The “as if” rule of course applies, so that any implementation -that acts as if the appropriate memory barriers were in place is a -correct implementation. -That said, it is much easier to fool yourself into believing that you have -adhered to the as-if rule than it is to actually adhere to it! -

    @@QQE@@ - -

    @@QQ@@ -You claim that rcu_read_lock() and rcu_read_unlock() -generate absolutely no code in some kernel builds. -This means that the compiler might arbitrarily rearrange consecutive -RCU read-side critical sections. -Given such rearrangement, if a given RCU read-side critical section -is done, how can you be sure that all prior RCU read-side critical -sections are done? -Won't the compiler rearrangements make that impossible to determine? -

    @@QQA@@ -In cases where rcu_read_lock() and rcu_read_unlock() -generate absolutely no code, RCU infers quiescent states only at -special locations, for example, within the scheduler. -Because calls to schedule() had better prevent calling-code -accesses to shared variables from being rearranged across the call to -schedule(), if RCU detects the end of a given RCU read-side -critical section, it will necessarily detect the end of all prior -RCU read-side critical sections, no matter how aggressively the -compiler scrambles the code. - -

    -Again, this all assumes that the compiler cannot scramble code across -calls to the scheduler, out of interrupt handlers, into the idle loop, -into user-mode code, and so on. -But if your kernel build allows that sort of scrambling, you have broken -far more than just RCU! -

    @@QQE@@ - -

    -Note that these memory-barrier requirements do not replace the fundamental -RCU requirement that a grace period wait for all pre-existing readers. -On the contrary, the memory barriers called out in this section must operate in -such a way as to enforce this fundamental requirement. -Of course, different implementations enforce this requirement in different -ways, but enforce it they must. - -

    RCU Primitives Guaranteed to Execute Unconditionally

    - -

    -The common-case RCU primitives are unconditional. -They are invoked, they do their job, and they return, with no possibility -of error, and no need to retry. -This is a key RCU design philosophy. - -

    -However, this philosophy is pragmatic rather than pigheaded. -If someone comes up with a good justification for a particular conditional -RCU primitive, it might well be implemented and added. -After all, this guarantee was reverse-engineered, not premeditated. -The unconditional nature of the RCU primitives was initially an -accident of implementation, and later experience with synchronization -primitives with conditional primitives caused me to elevate this -accident to a guarantee. -Therefore, the justification for adding a conditional primitive to -RCU would need to be based on detailed and compelling use cases. - -

    Guaranteed Read-to-Write Upgrade

    - -

    -As far as RCU is concerned, it is always possible to carry out an -update within an RCU read-side critical section. -For example, that RCU read-side critical section might search for -a given data element, and then might acquire the update-side -spinlock in order to update that element, all while remaining -in that RCU read-side critical section. -Of course, it is necessary to exit the RCU read-side critical section -before invoking synchronize_rcu(), however, this -inconvenience can be avoided through use of the -call_rcu() and kfree_rcu() API members -described later in this document. - -

    @@QQ@@ -But how does the upgrade-to-write operation exclude other readers? -

    @@QQA@@ -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. -

    @@QQE@@ - -

    -This guarantee allows lookup code to be shared between read-side -and update-side code, and was premeditated, appearing in the earliest -DYNIX/ptx RCU documentation. - -

    Fundamental Non-Requirements

    - -

    -RCU provides extremely lightweight readers, and its read-side guarantees, -though quite useful, are correspondingly lightweight. -It is therefore all too easy to assume that RCU is guaranteeing more -than it really is. -Of course, the list of things that RCU does not guarantee is infinitely -long, however, the following sections list a few non-guarantees that -have caused confusion. -Except where otherwise noted, these non-guarantees were premeditated. - -

      -
    1. - Readers Impose Minimal Ordering -
    2. - Readers Do Not Exclude Updaters -
    3. - Updaters Only Wait For Old Readers -
    4. - Grace Periods Don't Partition Read-Side Critical Sections -
    5. - Read-Side Critical Sections Don't Partition Grace Periods -
    6. - Disabling Preemption Does Not Block Grace Periods -
    - -

    Readers Impose Minimal Ordering

    - -

    -Reader-side markers such as rcu_read_lock() and -rcu_read_unlock() provide absolutely no ordering guarantees -except through their interaction with the grace-period APIs such as -synchronize_rcu(). -To see this, consider the following pair of threads: - -

    -
    - 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(x, 1);
- 5   rcu_read_unlock();
- 6   rcu_read_lock();
- 7   WRITE_ONCE(y, 1);
- 8   rcu_read_unlock();
- 9 }
-10
-11 void thread1(void)
-12 {
-13   rcu_read_lock();
-14   r1 = READ_ONCE(y);
-15   rcu_read_unlock();
-16   rcu_read_lock();
-17   r2 = READ_ONCE(x);
-18   rcu_read_unlock();
-19 }
-
    -
    - -

    -After thread0() and thread1() execute -concurrently, it is quite possible to have - -

    -
    -(r1 == 1 && r2 == 0)
-
    -
    - -(that is, y appears to have been assigned before x), -which would not be possible if rcu_read_lock() and -rcu_read_unlock() had much in the way of ordering -properties. -But they do not, so the CPU is within its rights -to do significant reordering. -This is by design: Any significant ordering constraints would slow down -these fast-path APIs. - -

    @@QQ@@ -Can't the compiler also reorder this code? -

    @@QQA@@ -No, the volatile casts in READ_ONCE() and -WRITE_ONCE() prevent the compiler from reordering in -this particular case. -

    @@QQE@@ - -

    Readers Do Not Exclude Updaters

    - -

    -Neither rcu_read_lock() nor rcu_read_unlock() -exclude updates. -All they do is to prevent grace periods from ending. -The following example illustrates this: - -

    -
    - 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   r1 = READ_ONCE(y);
- 5   if (r1) {
- 6     do_something_with_nonzero_x();
- 7     r2 = READ_ONCE(x);
- 8     WARN_ON(!r2); /* BUG!!! */
- 9   }
-10   rcu_read_unlock();
-11 }
-12
-13 void thread1(void)
-14 {
-15   spin_lock(&my_lock);
-16   WRITE_ONCE(x, 1);
-17   WRITE_ONCE(y, 1);
-18   spin_unlock(&my_lock);
-19 }
-
    -
    - -

    -If the thread0() function's rcu_read_lock() -excluded the thread1() function's update, -the WARN_ON() could never fire. -But the fact is that rcu_read_lock() does not exclude -much of anything aside from subsequent grace periods, of which -thread1() has none, so the -WARN_ON() can and does fire. - -

    Updaters Only Wait For Old Readers

    - -

    -It might be tempting to assume that after synchronize_rcu() -completes, there are no readers executing. -This temptation must be avoided because -new readers can start immediately after synchronize_rcu() -starts, and synchronize_rcu() is under no -obligation to wait for these new readers. - -

    @@QQ@@ -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? -

    @@QQA@@ -No. -Even if synchronize_rcu() were to wait until -all readers had completed, a new reader might start immediately after -synchronize_rcu() completed. -Therefore, the code following -synchronize_rcu() cannot rely on there being no readers -in any case. -

    @@QQE@@ - -

    -Grace Periods Don't Partition Read-Side Critical Sections

    - -

    -It is tempting to assume that if any part of one RCU read-side critical -section precedes a given grace period, and if any part of another RCU -read-side critical section follows that same grace period, then all of -the first RCU read-side critical section must precede all of the second. -However, this just isn't the case: A single grace period does not -partition the set of RCU read-side critical sections. -An example of this situation can be illustrated as follows, where -x, y, and z are initially all zero: - -

    -
    - 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(a, 1);
- 5   WRITE_ONCE(b, 1);
- 6   rcu_read_unlock();
- 7 }
- 8
- 9 void thread1(void)
-10 {
-11   r1 = READ_ONCE(a);
-12   synchronize_rcu();
-13   WRITE_ONCE(c, 1);
-14 }
-15
-16 void thread2(void)
-17 {
-18   rcu_read_lock();
-19   r2 = READ_ONCE(b);
-20   r3 = READ_ONCE(c);
-21   rcu_read_unlock();
-22 }
-
    -
    - -

    -It turns out that the outcome: - -

    -
    -(r1 == 1 && r2 == 0 && r3 == 1)
-
    -
    - -is entirely possible. -The following figure show how this can happen, with each circled -QS indicating the point at which RCU recorded a -quiescent state for each thread, that is, a state in which -RCU knows that the thread cannot be in the midst of an RCU read-side -critical section that started before the current grace period: - -

    GPpartitionReaders1.svg

    - -

    -If it is necessary to partition RCU read-side critical sections in this -manner, it is necessary to use two grace periods, where the first -grace period is known to end before the second grace period starts: - -

    -
    - 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(a, 1);
- 5   WRITE_ONCE(b, 1);
- 6   rcu_read_unlock();
- 7 }
- 8
- 9 void thread1(void)
-10 {
-11   r1 = READ_ONCE(a);
-12   synchronize_rcu();
-13   WRITE_ONCE(c, 1);
-14 }
-15
-16 void thread2(void)
-17 {
-18   r2 = READ_ONCE(c);
-19   synchronize_rcu();
-20   WRITE_ONCE(d, 1);
-21 }
-22
-23 void thread3(void)
-24 {
-25   rcu_read_lock();
-26   r3 = READ_ONCE(b);
-27   r4 = READ_ONCE(d);
-28   rcu_read_unlock();
-29 }
-
    -
    - -

    -Here, if (r1 == 1), then -thread0()'s write to b must happen -before the end of thread1()'s grace period. -If in addition (r4 == 1), then -thread3()'s read from b must happen -after the beginning of thread2()'s grace period. -If it is also the case that (r2 == 1), then the -end of thread1()'s grace period must precede the -beginning of thread2()'s grace period. -This mean that the two RCU read-side critical sections cannot overlap, -guaranteeing that (r3 == 1). -As a result, the outcome: - -

    -
    -(r1 == 1 && r2 == 1 && r3 == 0 && r4 == 1)
-
    -
    - -cannot happen. - -

    -This non-requirement was also non-premeditated, but became apparent -when studying RCU's interaction with memory ordering. - -

    -Read-Side Critical Sections Don't Partition Grace Periods

    - -

    -It is also tempting to assume that if an RCU read-side critical section -happens between a pair of grace periods, then those grace periods cannot -overlap. -However, this temptation leads nowhere good, as can be illustrated by -the following, with all variables initially zero: - -

    -
    - 1 void thread0(void)
- 2 {
- 3   rcu_read_lock();
- 4   WRITE_ONCE(a, 1);
- 5   WRITE_ONCE(b, 1);
- 6   rcu_read_unlock();
- 7 }
- 8
- 9 void thread1(void)
-10 {
-11   r1 = READ_ONCE(a);
-12   synchronize_rcu();
-13   WRITE_ONCE(c, 1);
-14 }
-15
-16 void thread2(void)
-17 {
-18   rcu_read_lock();
-19   WRITE_ONCE(d, 1);
-20   r2 = READ_ONCE(c);
-21   rcu_read_unlock();
-22 }
-23
-24 void thread3(void)
-25 {
-26   r3 = READ_ONCE(d);
-27   synchronize_rcu();
-28   WRITE_ONCE(e, 1);
-29 }
-30
-31 void thread4(void)
-32 {
-33   rcu_read_lock();
-34   r4 = READ_ONCE(b);
-35   r5 = READ_ONCE(e);
-36   rcu_read_unlock();
-37 }
-
    -
    - -

    -In this case, the outcome: - -

    -
    -(r1 == 1 && r2 == 1 && r3 == 1 && r4 == 0 && r5 == 1)
-
    -
    - -is entirely possible, as illustrated below: - -

    ReadersPartitionGP1.svg

    - -

    -Again, an RCU read-side critical section can overlap almost all of a -given grace period, just so long as it does not overlap the entire -grace period. -As a result, an RCU read-side critical section cannot partition a pair -of RCU grace periods. - -

    @@QQ@@ -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? -

    @@QQA@@ -In theory, an infinite number. -In practice, an unknown number that is sensitive to both implementation -details and timing considerations. -Therefore, even in practice, RCU users must abide by the theoretical rather -than the practical answer. -

    @@QQE@@ - -

    -Disabling Preemption Does Not Block Grace Periods

    - -

    -There was a time when disabling preemption on any given CPU would block -subsequent grace periods. -However, this was an accident of implementation and is not a requirement. -And in the current Linux-kernel implementation, disabling preemption -on a given CPU in fact does not block grace periods, as Oleg Nesterov -demonstrated. - -

    -If you need a preempt-disable region to block grace periods, you need to add -rcu_read_lock() and rcu_read_unlock(), for example -as follows: - -

    -
    - 1 preempt_disable();
- 2 rcu_read_lock();
- 3 do_something();
- 4 rcu_read_unlock();
- 5 preempt_enable();
- 6
- 7 /* Spinlocks implicitly disable preemption. */
- 8 spin_lock(&mylock);
- 9 rcu_read_lock();
-10 do_something();
-11 rcu_read_unlock();
-12 spin_unlock(&mylock);
-
    -
    - -

    -In theory, you could enter the RCU read-side critical section first, -but it is more efficient to keep the entire RCU read-side critical -section contained in the preempt-disable region as shown above. -Of course, RCU read-side critical sections that extend outside of -preempt-disable regions will work correctly, but such critical sections -can be preempted, which forces rcu_read_unlock() to do -more work. -And no, this is not an invitation to enclose all of your RCU -read-side critical sections within preempt-disable regions, because -doing so would degrade real-time response. - -

    -This non-requirement appeared with preemptible RCU. -If you need a grace period that waits on non-preemptible code regions, use -RCU-sched. - -

    Parallelism Facts of Life

    - -

    -These parallelism facts of life are by no means specific to RCU, but -the RCU implementation must abide by them. -They therefore bear repeating: - -

      -
    1. Any CPU or task may be delayed at any time, - and any attempts to avoid these delays by disabling - preemption, interrupts, or whatever are completely futile. - This is most obvious in preemptible user-level - environments and in virtualized environments (where - a given guest OS's VCPUs can be preempted at any time by - the underlying hypervisor), but can also happen in bare-metal - environments due to ECC errors, NMIs, and other hardware - events. - Although a delay of more than about 20 seconds can result - in splats, the RCU implementation is obligated to use - algorithms that can tolerate extremely long delays, but where - “extremely long” is not long enough to allow - wrap-around when incrementing a 64-bit counter. -
    2. Both the compiler and the CPU can reorder memory accesses. - Where it matters, RCU must use compiler directives and - memory-barrier instructions to preserve ordering. -
    3. Conflicting writes to memory locations in any given cache line - will result in expensive cache misses. - Greater numbers of concurrent writes and more-frequent - concurrent writes will result in more dramatic slowdowns. - RCU is therefore obligated to use algorithms that have - sufficient locality to avoid significant performance and - scalability problems. -
    4. As a rough rule of thumb, only one CPU's worth of processing - may be carried out under the protection of any given exclusive - lock. - RCU must therefore use scalable locking designs. -
    5. Counters are finite, especially on 32-bit systems. - RCU's use of counters must therefore tolerate counter wrap, - or be designed such that counter wrap would take way more - time than a single system is likely to run. - An uptime of ten years is quite possible, a runtime - of a century much less so. - As an example of the latter, RCU's dyntick-idle nesting counter - allows 54 bits for interrupt nesting level (this counter - is 64 bits even on a 32-bit system). - Overflowing this counter requires 254 - half-interrupts on a given CPU without that CPU ever going idle. - If a half-interrupt happened every microsecond, it would take - 570 years of runtime to overflow this counter, which is currently - believed to be an acceptably long time. -
    6. Linux systems can have thousands of CPUs running a single - Linux kernel in a single shared-memory environment. - RCU must therefore pay close attention to high-end scalability. -
    - -

    -This last parallelism fact of life means that RCU must pay special -attention to the preceding facts of life. -The idea that Linux might scale to systems with thousands of CPUs would -have been met with some skepticism in the 1990s, but these requirements -would have otherwise have been unsurprising, even in the early 1990s. - -

    Quality-of-Implementation Requirements

    - -

    -These sections list quality-of-implementation requirements. -Although an RCU implementation that ignores these requirements could -still be used, it would likely be subject to limitations that would -make it inappropriate for industrial-strength production use. -Classes of quality-of-implementation requirements are as follows: - -

      -
    1. Specialization -
    2. Performance and Scalability -
    3. Composability -
    4. Corner Cases -
    - -

    -These classes is covered in the following sections. - -

    Specialization

    - -

    -RCU is and always has been intended primarily for read-mostly situations, -which means that RCU's read-side primitives are optimized, often at the -expense of its update-side primitives. -Experience thus far is captured by the following list of situations: - -

      -
    1. Read-mostly data, where stale and inconsistent data is not - a problem: RCU works great! -
    2. Read-mostly data, where data must be consistent: - RCU works well. -
    3. Read-write data, where data must be consistent: - RCU might work OK. - Or not. -
    4. Write-mostly data, where data must be consistent: - RCU is very unlikely to be the right tool for the job, - with the following exceptions, where RCU can provide: -
        -
      1. Existence guarantees for update-friendly mechanisms. -
      2. Wait-free read-side primitives for real-time use. -
      -
    - -

    -This focus on read-mostly situations means that RCU must interoperate -with other synchronization primitives. -For example, the add_gp() and remove_gp_synchronous() -examples discussed earlier use RCU to protect readers and locking to -coordinate updaters. -However, the need extends much farther, requiring that a variety of -synchronization primitives be legal within RCU read-side critical sections, -including spinlocks, sequence locks, atomic operations, reference -counters, and memory barriers. - -

    @@QQ@@ -What about sleeping locks? -

    @@QQA@@ -These are forbidden within Linux-kernel RCU read-side critical sections -because it is not legal to place a quiescent state (in this case, -voluntary context switch) within an RCU read-side critical section. -However, sleeping locks may be used within userspace RCU read-side critical -sections, and also within Linux-kernel sleepable RCU -(SRCU) -read-side critical sections. -In addition, the -rt patchset turns spinlocks into a sleeping locks so -that the corresponding critical sections can be preempted, which -also means that these sleeplockified spinlocks (but not other sleeping locks!) -may be acquire within -rt-Linux-kernel RCU read-side critical sections. - -

    -Note that it is legal for a normal RCU read-side critical section -to conditionally acquire a sleeping locks (as in mutex_trylock()), -but only as long as it does not loop indefinitely attempting to -conditionally acquire that sleeping locks. -The key point is that things like mutex_trylock() -either return with the mutex held, or return an error indication if -the mutex was not immediately available. -Either way, mutex_trylock() returns immediately without sleeping. -

    @@QQE@@ - -

    -It often comes as a surprise that many algorithms do not require a -consistent view of data, but many can function in that mode, -with network routing being the poster child. -Internet routing algorithms take significant time to propagate -updates, so that by the time an update arrives at a given system, -that system has been sending network traffic the wrong way for -a considerable length of time. -Having a few threads continue to send traffic the wrong way for a -few more milliseconds is clearly not a problem: In the worst case, -TCP retransmissions will eventually get the data where it needs to go. -In general, when tracking the state of the universe outside of the -computer, some level of inconsistency must be tolerated due to -speed-of-light delays if nothing else. - -

    -Furthermore, uncertainty about external state is inherent in many cases. -For example, a pair of veternarians might use heartbeat to determine -whether or not a given cat was alive. -But how long should they wait after the last heartbeat to decide that -the cat is in fact dead? -Waiting less than 400 milliseconds makes no sense because this would -mean that a relaxed cat would be considered to cycle between death -and life more than 100 times per minute. -Moreover, just as with human beings, a cat's heart might stop for -some period of time, so the exact wait period is a judgment call. -One of our pair of veternarians might wait 30 seconds before pronouncing -the cat dead, while the other might insist on waiting a full minute. -The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat. - -

    -Interestingly enough, this same situation applies to hardware. -When push comes to shove, how do we tell whether or not some -external server has failed? -We send messages to it periodically, and declare it failed if we -don't receive a response within a given period of time. -Policy decisions can usually tolerate short -periods of inconsistency. -The policy was decided some time ago, and is only now being put into -effect, so a few milliseconds of delay is normally inconsequential. - -

    -However, there are algorithms that absolutely must see consistent data. -For example, the translation between a user-level SystemV semaphore -ID to the corresponding in-kernel data structure is protected by RCU, -but it is absolutely forbidden to update a semaphore that has just been -removed. -In the Linux kernel, this need for consistency is accommodated by acquiring -spinlocks located in the in-kernel data structure from within -the RCU read-side critical section, and this is indicated by the -green box in the figure above. -Many other techniques may be used, and are in fact used within the -Linux kernel. - -

    -In short, RCU is not required to maintain consistency, and other -mechanisms may be used in concert with RCU when consistency is required. -RCU's specialization allows it to do its job extremely well, and its -ability to interoperate with other synchronization mechanisms allows -the right mix of synchronization tools to be used for a given job. - -

    Performance and Scalability

    - -

    -Energy efficiency is a critical component of performance today, -and Linux-kernel RCU implementations must therefore avoid unnecessarily -awakening idle CPUs. -I cannot claim that this requirement was premeditated. -In fact, I learned of it during a telephone conversation in which I -was given “frank and open” feedback on the importance -of energy efficiency in battery-powered systems and on specific -energy-efficiency shortcomings of the Linux-kernel RCU implementation. -In my experience, the battery-powered embedded community will consider -any unnecessary wakeups to be extremely unfriendly acts. -So much so that mere Linux-kernel-mailing-list posts are -insufficient to vent their ire. - -

    -Memory consumption is not particularly important for in most -situations, and has become decreasingly -so as memory sizes have expanded and memory -costs have plummeted. -However, as I learned from Matt Mackall's -bloatwatch -efforts, memory footprint is critically important on single-CPU systems with -non-preemptible (CONFIG_PREEMPT=n) kernels, and thus -tiny RCU -was born. -Josh Triplett has since taken over the small-memory banner with his -Linux kernel tinification -project, which resulted in -SRCU -becoming optional for those kernels not needing it. - -

    -The remaining performance requirements are, for the most part, -unsurprising. -For example, in keeping with RCU's read-side specialization, -rcu_dereference() should have negligible overhead (for -example, suppression of a few minor compiler optimizations). -Similarly, in non-preemptible environments, rcu_read_lock() and -rcu_read_unlock() should have exactly zero overhead. - -

    -In preemptible environments, in the case where the RCU read-side -critical section was not preempted (as will be the case for the -highest-priority real-time process), rcu_read_lock() and -rcu_read_unlock() should have minimal overhead. -In particular, they should not contain atomic read-modify-write -operations, memory-barrier instructions, preemption disabling, -interrupt disabling, or backwards branches. -However, in the case where the RCU read-side critical section was preempted, -rcu_read_unlock() may acquire spinlocks and disable interrupts. -This is why it is better to nest an RCU read-side critical section -within a preempt-disable region than vice versa, at least in cases -where that critical section is short enough to avoid unduly degrading -real-time latencies. - -

    -The synchronize_rcu() grace-period-wait primitive is -optimized for throughput. -It may therefore incur several milliseconds of latency in addition to -the duration of the longest RCU read-side critical section. -On the other hand, multiple concurrent invocations of -synchronize_rcu() are required to use batching optimizations -so that they can be satisfied by a single underlying grace-period-wait -operation. -For example, in the Linux kernel, it is not unusual for a single -grace-period-wait operation to serve more than -1,000 separate invocations -of synchronize_rcu(), thus amortizing the per-invocation -overhead down to nearly zero. -However, the grace-period optimization is also required to avoid -measurable degradation of real-time scheduling and interrupt latencies. - -

    -In some cases, the multi-millisecond synchronize_rcu() -latencies are unacceptable. -In these cases, synchronize_rcu_expedited() may be used -instead, reducing the grace-period latency down to a few tens of -microseconds on small systems, at least in cases where the RCU read-side -critical sections are short. -There are currently no special latency requirements for -synchronize_rcu_expedited() on large systems, but, -consistent with the empirical nature of the RCU specification, -that is subject to change. -However, there most definitely are scalability requirements: -A storm of synchronize_rcu_expedited() invocations on 4096 -CPUs should at least make reasonable forward progress. -In return for its shorter latencies, synchronize_rcu_expedited() -is permitted to impose modest degradation of real-time latency -on non-idle online CPUs. -That said, it will likely be necessary to take further steps to reduce this -degradation, hopefully to roughly that of a scheduling-clock interrupt. - -

    -There are a number of situations where even -synchronize_rcu_expedited()'s reduced grace-period -latency is unacceptable. -In these situations, the asynchronous call_rcu() can be -used in place of synchronize_rcu() as follows: - -

    -
    - 1 struct foo {
- 2   int a;
- 3   int b;
- 4   struct rcu_head rh;
- 5 };
- 6
- 7 static void remove_gp_cb(struct rcu_head *rhp)
- 8 {
- 9   struct foo *p = container_of(rhp, struct foo, rh);
-10
-11   kfree(p);
-12 }
-13
-14 bool remove_gp_asynchronous(void)
-15 {
-16   struct foo *p;
-17
-18   spin_lock(&gp_lock);
-19   p = rcu_dereference(gp);
-20   if (!p) {
-21     spin_unlock(&gp_lock);
-22     return false;
-23   }
-24   rcu_assign_pointer(gp, NULL);
-25   call_rcu(&p->rh, remove_gp_cb);
-26   spin_unlock(&gp_lock);
-27   return true;
-28 }
-
    -
    - -

    -A definition of struct foo is finally needed, and appears -on lines 1-5. -The function remove_gp_cb() is passed to call_rcu() -on line 25, and will be invoked after the end of a subsequent -grace period. -This gets the same effect as remove_gp_synchronous(), -but without forcing the updater to wait for a grace period to elapse. -The call_rcu() function may be used in a number of -situations where neither synchronize_rcu() nor -synchronize_rcu_expedited() would be legal, -including within preempt-disable code, local_bh_disable() code, -interrupt-disable code, and interrupt handlers. -However, even call_rcu() is illegal within NMI handlers -and from offline CPUs. -The callback function (remove_gp_cb() in this case) will be -executed within softirq (software interrupt) environment within the -Linux kernel, -either within a real softirq handler or under the protection -of local_bh_disable(). -In both the Linux kernel and in userspace, it is bad practice to -write an RCU callback function that takes too long. -Long-running operations should be relegated to separate threads or -(in the Linux kernel) workqueues. - -

    @@QQ@@ -Why does line 19 use rcu_access_pointer()? -After all, call_rcu() on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that rcu_dereference() is required? -

    @@QQA@@ -Presumably the ->gp_lock acquired on line 18 excludes -any changes, including any insertions that rcu_dereference() -would protect against. -Therefore, any insertions will be delayed until after ->gp_lock -is released on line 25, which in turn means that -rcu_access_pointer() suffices. -

    @@QQE@@ - -

    -However, all that remove_gp_cb() is doing is -invoking kfree() on the data element. -This is a common idiom, and is supported by kfree_rcu(), -which allows “fire and forget” operation as shown below: - -

    -
    - 1 struct foo {
- 2   int a;
- 3   int b;
- 4   struct rcu_head rh;
- 5 };
- 6
- 7 bool remove_gp_faf(void)
- 8 {
- 9   struct foo *p;
-10
-11   spin_lock(&gp_lock);
-12   p = rcu_dereference(gp);
-13   if (!p) {
-14     spin_unlock(&gp_lock);
-15     return false;
-16   }
-17   rcu_assign_pointer(gp, NULL);
-18   kfree_rcu(p, rh);
-19   spin_unlock(&gp_lock);
-20   return true;
-21 }
-
    -
    - -

    -Note that remove_gp_faf() simply invokes -kfree_rcu() and proceeds, without any need to pay any -further attention to the subsequent grace period and kfree(). -It is permissible to invoke kfree_rcu() from the same -environments as for call_rcu(). -Interestingly enough, DYNIX/ptx had the equivalents of -call_rcu() and kfree_rcu(), but not -synchronize_rcu(). -This was due to the fact that RCU was not heavily used within DYNIX/ptx, -so the very few places that needed something like -synchronize_rcu() simply open-coded it. - -

    @@QQ@@ -Earlier it was claimed that call_rcu() and -kfree_rcu() allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? -

    @@QQA@@ -We could define things this way, but keep in mind that this sort of -definition would say that updates in garbage-collected languages -cannot complete until the next time the garbage collector runs, -which does not seem at all reasonable. -The key point is that in most cases, an updater using either -call_rcu() or kfree_rcu() can proceed to the -next update as soon as it has invoked call_rcu() or -kfree_rcu(), without having to wait for a subsequent -grace period. -

    @@QQE@@ - -

    -But what if the updater must wait for the completion of code to be -executed after the end of the grace period, but has other tasks -that can be carried out in the meantime? -The polling-style get_state_synchronize_rcu() and -cond_synchronize_rcu() functions may be used for this -purpose, as shown below: - -

    -
    - 1 bool remove_gp_poll(void)
- 2 {
- 3   struct foo *p;
- 4   unsigned long s;
- 5
- 6   spin_lock(&gp_lock);
- 7   p = rcu_access_pointer(gp);
- 8   if (!p) {
- 9     spin_unlock(&gp_lock);
-10     return false;
-11   }
-12   rcu_assign_pointer(gp, NULL);
-13   spin_unlock(&gp_lock);
-14   s = get_state_synchronize_rcu();
-15   do_something_while_waiting();
-16   cond_synchronize_rcu(s);
-17   kfree(p);
-18   return true;
-19 }
-
    -
    - -

    -On line 14, get_state_synchronize_rcu() obtains a -“cookie” from RCU, -then line 15 carries out other tasks, -and finally, line 16 returns immediately if a grace period has -elapsed in the meantime, but otherwise waits as required. -The need for get_state_synchronize_rcu and -cond_synchronize_rcu() has appeared quite recently, -so it is too early to tell whether they will stand the test of time. - -

    -RCU thus provides a range of tools to allow updaters to strike the -required tradeoff between latency, flexibility and CPU overhead. - -

    Composability

    - -

    -Composability has received much attention in recent years, perhaps in part -due to the collision of multicore hardware with object-oriented techniques -designed in single-threaded environments for single-threaded use. -And in theory, RCU read-side critical sections may be composed, and in -fact may be nested arbitrarily deeply. -In practice, as with all real-world implementations of composable -constructs, there are limitations. - -

    -Implementations of RCU for which rcu_read_lock() -and rcu_read_unlock() generate no code, such as -Linux-kernel RCU when CONFIG_PREEMPT=n, can be -nested arbitrarily deeply. -After all, there is no overhead. -Except that if all these instances of rcu_read_lock() -and rcu_read_unlock() are visible to the compiler, -compilation will eventually fail due to exhausting memory, -mass storage, or user patience, whichever comes first. -If the nesting is not visible to the compiler, as is the case with -mutually recursive functions each in its own translation unit, -stack overflow will result. -If the nesting takes the form of loops, either the control variable -will overflow or (in the Linux kernel) you will get an RCU CPU stall warning. -Nevertheless, this class of RCU implementations is one -of the most composable constructs in existence. - -

    -RCU implementations that explicitly track nesting depth -are limited by the nesting-depth counter. -For example, the Linux kernel's preemptible RCU limits nesting to -INT_MAX. -This should suffice for almost all practical purposes. -That said, a consecutive pair of RCU read-side critical sections -between which there is an operation that waits for a grace period -cannot be enclosed in another RCU read-side critical section. -This is because it is not legal to wait for a grace period within -an RCU read-side critical section: To do so would result either -in deadlock or -in RCU implicitly splitting the enclosing RCU read-side critical -section, neither of which is conducive to a long-lived and prosperous -kernel. - -

    -It is worth noting that RCU is not alone in limiting composability. -For example, many transactional-memory implementations prohibit -composing a pair of transactions separated by an irrevocable -operation (for example, a network receive operation). -For another example, lock-based critical sections can be composed -surprisingly freely, but only if deadlock is avoided. - -

    -In short, although RCU read-side critical sections are highly composable, -care is required in some situations, just as is the case for any other -composable synchronization mechanism. - -

    Corner Cases

    - -

    -A given RCU workload might have an endless and intense stream of -RCU read-side critical sections, perhaps even so intense that there -was never a point in time during which there was not at least one -RCU read-side critical section in flight. -RCU cannot allow this situation to block grace periods: As long as -all the RCU read-side critical sections are finite, grace periods -must also be finite. - -

    -That said, preemptible RCU implementations could potentially result -in RCU read-side critical sections being preempted for long durations, -which has the effect of creating a long-duration RCU read-side -critical section. -This situation can arise only in heavily loaded systems, but systems using -real-time priorities are of course more vulnerable. -Therefore, RCU priority boosting is provided to help deal with this -case. -That said, the exact requirements on RCU priority boosting will likely -evolve as more experience accumulates. - -

    -Other workloads might have very high update rates. -Although one can argue that such workloads should instead use -something other than RCU, the fact remains that RCU must -handle such workloads gracefully. -This requirement is another factor driving batching of grace periods, -but it is also the driving force behind the checks for large numbers -of queued RCU callbacks in the call_rcu() code path. -Finally, high update rates should not delay RCU read-side critical -sections, although some read-side delays can occur when using -synchronize_rcu_expedited(), courtesy of this function's use -of try_stop_cpus(). -(In the future, synchronize_rcu_expedited() will be -converted to use lighter-weight inter-processor interrupts (IPIs), -but this will still disturb readers, though to a much smaller degree.) - -

    -Although all three of these corner cases were understood in the early -1990s, a simple user-level test consisting of close(open(path)) -in a tight loop -in the early 2000s suddenly provided a much deeper appreciation of the -high-update-rate corner case. -This test also motivated addition of some RCU code to react to high update -rates, for example, if a given CPU finds itself with more than 10,000 -RCU callbacks queued, it will cause RCU to take evasive action by -more aggressively starting grace periods and more aggressively forcing -completion of grace-period processing. -This evasive action causes the grace period to complete more quickly, -but at the cost of restricting RCU's batching optimizations, thus -increasing the CPU overhead incurred by that grace period. - -

    -Software-Engineering Requirements

    - -

    -Between Murphy's Law and “To err is human”, it is necessary to -guard against mishaps and misuse: - -

      -
    1. It is all too easy to forget to use rcu_read_lock() - everywhere that it is needed, so kernels built with - CONFIG_PROVE_RCU=y will spat if - rcu_dereference() is used outside of an - RCU read-side critical section. - Update-side code can use rcu_dereference_protected(), - which takes a - lockdep expression - to indicate what is providing the protection. - If the indicated protection is not provided, a lockdep splat - is emitted. - -

      - Code shared between readers and updaters can use - rcu_dereference_check(), which also takes a - lockdep expression, and emits a lockdep splat if neither - rcu_read_lock() nor the indicated protection - is in place. - In addition, rcu_dereference_raw() is used in those - (hopefully rare) cases where the required protection cannot - be easily described. - Finally, rcu_read_lock_held() is provided to - allow a function to verify that it has been invoked within - an RCU read-side critical section. - I was made aware of this set of requirements shortly after Thomas - Gleixner audited a number of RCU uses. -

    2. A given function might wish to check for RCU-related preconditions - upon entry, before using any other RCU API. - The rcu_lockdep_assert() does this job, - asserting the expression in kernels having lockdep enabled - and doing nothing otherwise. -
    3. It is also easy to forget to use rcu_assign_pointer() - and rcu_dereference(), perhaps (incorrectly) - substituting a simple assignment. - To catch this sort of error, a given RCU-protected pointer may be - tagged with __rcu, after which running sparse - with CONFIG_SPARSE_RCU_POINTER=y will complain - about simple-assignment accesses to that pointer. - Arnd Bergmann made me aware of this requirement, and also - supplied the needed - patch series. -
    4. Kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y - will splat if a data element is passed to call_rcu() - twice in a row, without a grace period in between. - (This error is similar to a double free.) - The corresponding rcu_head structures that are - dynamically allocated are automatically tracked, but - rcu_head structures allocated on the stack - must be initialized with init_rcu_head_on_stack() - and cleaned up with destroy_rcu_head_on_stack(). - Similarly, statically allocated non-stack rcu_head - structures must be initialized with init_rcu_head() - and cleaned up with destroy_rcu_head(). - Mathieu Desnoyers made me aware of this requirement, and also - supplied the needed - patch. -
    5. An infinite loop in an RCU read-side critical section will - eventually trigger an RCU CPU stall warning splat, with - the duration of “eventually” being controlled by the - RCU_CPU_STALL_TIMEOUT Kconfig option, or, - alternatively, by the - rcupdate.rcu_cpu_stall_timeout boot/sysfs - parameter. - However, RCU is not obligated to produce this splat - unless there is a grace period waiting on that particular - RCU read-side critical section. -

      - Some extreme workloads might intentionally delay - RCU grace periods, and systems running those workloads can - be booted with rcupdate.rcu_cpu_stall_suppress - to suppress the splats. - This kernel parameter may also be set via sysfs. - Furthermore, RCU CPU stall warnings are counter-productive - during sysrq dumps and during panics. - RCU therefore supplies the rcu_sysrq_start() and - rcu_sysrq_end() API members to be called before - and after long sysrq dumps. - RCU also supplies the rcu_panic() notifier that is - automatically invoked at the beginning of a panic to suppress - further RCU CPU stall warnings. - -

      - This requirement made itself known in the early 1990s, pretty - much the first time that it was necessary to debug a CPU stall. - That said, the initial implementation in DYNIX/ptx was quite - generic in comparison with that of Linux. -

    6. Although it would be very good to detect pointers leaking out - of RCU read-side critical sections, there is currently no - good way of doing this. - One complication is the need to distinguish between pointers - leaking and pointers that have been handed off from RCU to - some other synchronization mechanism, for example, reference - counting. -
    7. In kernels built with CONFIG_RCU_TRACE=y, RCU-related - information is provided via both debugfs and event tracing. -
    8. Open-coded use of rcu_assign_pointer() and - rcu_dereference() to create typical linked - data structures can be surprisingly error-prone. - Therefore, RCU-protected - linked lists - and, more recently, RCU-protected - hash tables - are available. - Many other special-purpose RCU-protected data structures are - available in the Linux kernel and the userspace RCU library. -
    9. Some linked structures are created at compile time, but still - require __rcu checking. - The RCU_POINTER_INITIALIZER() macro serves this - purpose. -
    10. It is not necessary to use rcu_assign_pointer() - when creating linked structures that are to be published via - a single external pointer. - The RCU_INIT_POINTER() macro is provided for - this task and also for assigning NULL pointers - at runtime. -
    - -

    -This not a hard-and-fast list: RCU's diagnostic capabilities will -continue to be guided by the number and type of usage bugs found -in real-world RCU usage. - -

    Linux Kernel Complications

    - -

    -The Linux kernel provides an interesting environment for all kinds of -software, including RCU. -Some of the relevant points of interest are as follows: - -

      -
    1. Configuration. -
    2. Firmware Interface. -
    3. Early Boot. -
    4. - Interrupts and non-maskable interrupts (NMIs). -
    5. Loadable Modules. -
    6. Hotplug CPU. -
    7. Scheduler and RCU. -
    8. Tracing and RCU. -
    9. Energy Efficiency. -
    10. Memory Efficiency. -
    11. - Performance, Scalability, Response Time, and Reliability. -
    - -

    -This list is probably incomplete, but it does give a feel for the -most notable Linux-kernel complications. -Each of the following sections covers one of the above topics. - -

    Configuration

    - -

    -RCU's goal is automatic configuration, so that almost nobody -needs to worry about RCU's Kconfig options. -And for almost all users, RCU does in fact work well -“out of the box.” - -

    -However, there are specialized use cases that are handled by -kernel boot parameters and Kconfig options. -Unfortunately, the Kconfig system will explicitly ask users -about new Kconfig options, which requires almost all of them -be hidden behind a CONFIG_RCU_EXPERT Kconfig option. - -

    -This all should be quite obvious, but the fact remains that -Linus Torvalds recently had to -remind -me of this requirement. - -

    Firmware Interface

    - -

    -In many cases, kernel obtains information about the system from the -firmware, and sometimes things are lost in translation. -Or the translation is accurate, but the original message is bogus. - -

    -For example, some systems' firmware overreports the number of CPUs, -sometimes by a large factor. -If RCU naively believed the firmware, as it used to do, -it would create too many per-CPU kthreads. -Although the resulting system will still run correctly, the extra -kthreads needlessly consume memory and can cause confusion -when they show up in ps listings. - -

    -RCU must therefore wait for a given CPU to actually come online before -it can allow itself to believe that the CPU actually exists. -The resulting “ghost CPUs” (which are never going to -come online) cause a number of -interesting complications. - -

    Early Boot

    - -

    -The Linux kernel's boot sequence is an interesting process, -and RCU is used early, even before rcu_init() -is invoked. -In fact, a number of RCU's primitives can be used as soon as the -initial task's task_struct is available and the -boot CPU's per-CPU variables are set up. -The read-side primitives (rcu_read_lock(), -rcu_read_unlock(), rcu_dereference(), -and rcu_access_pointer()) will operate normally very early on, -as will rcu_assign_pointer(). - -

    -Although call_rcu() may be invoked at any -time during boot, callbacks are not guaranteed to be invoked until after -the scheduler is fully up and running. -This delay in callback invocation is due to the fact that RCU does not -invoke callbacks until it is fully initialized, and this full initialization -cannot occur until after the scheduler has initialized itself to the -point where RCU can spawn and run its kthreads. -In theory, it would be possible to invoke callbacks earlier, -however, this is not a panacea because there would be severe restrictions -on what operations those callbacks could invoke. - -

    -Perhaps surprisingly, synchronize_rcu(), -synchronize_rcu_bh() -(discussed below), -and -synchronize_sched() -will all operate normally -during very early boot, the reason being that there is only one CPU -and preemption is disabled. -This means that the call synchronize_rcu() (or friends) -itself is a quiescent -state and thus a grace period, so the early-boot implementation can -be a no-op. - -

    -Both synchronize_rcu_bh() and synchronize_sched() -continue to operate normally through the remainder of boot, courtesy -of the fact that preemption is disabled across their RCU read-side -critical sections and also courtesy of the fact that there is still -only one CPU. -However, once the scheduler starts initializing, preemption is enabled. -There is still only a single CPU, but the fact that preemption is enabled -means that the no-op implementation of synchronize_rcu() no -longer works in CONFIG_PREEMPT=y kernels. -Therefore, as soon as the scheduler starts initializing, the early-boot -fastpath is disabled. -This means that synchronize_rcu() switches to its runtime -mode of operation where it posts callbacks, which in turn means that -any call to synchronize_rcu() will block until the corresponding -callback is invoked. -Unfortunately, the callback cannot be invoked until RCU's runtime -grace-period machinery is up and running, which cannot happen until -the scheduler has initialized itself sufficiently to allow RCU's -kthreads to be spawned. -Therefore, invoking synchronize_rcu() during scheduler -initialization can result in deadlock. - -

    @@QQ@@ -So what happens with synchronize_rcu() during -scheduler initialization for CONFIG_PREEMPT=n -kernels? -

    @@QQA@@ -In CONFIG_PREEMPT=n kernel, synchronize_rcu() -maps directly to synchronize_sched(). -Therefore, synchronize_rcu() works normally throughout -boot in CONFIG_PREEMPT=n kernels. -However, your code must also work in CONFIG_PREEMPT=y kernels, -so it is still necessary to avoid invoking synchronize_rcu() -during scheduler initialization. -

    @@QQE@@ - -

    -I learned of these boot-time requirements as a result of a series of -system hangs. - -

    Interrupts and NMIs

    - -

    -The Linux kernel has interrupts, and RCU read-side critical sections are -legal within interrupt handlers and within interrupt-disabled regions -of code, as are invocations of call_rcu(). - -

    -Some Linux-kernel architectures can enter an interrupt handler from -non-idle process context, and then just never leave it, instead stealthily -transitioning back to process context. -This trick is sometimes used to invoke system calls from inside the kernel. -These “half-interrupts” mean that RCU has to be very careful -about how it counts interrupt nesting levels. -I learned of this requirement the hard way during a rewrite -of RCU's dyntick-idle code. - -

    -The Linux kernel has non-maskable interrupts (NMIs), and -RCU read-side critical sections are legal within NMI handlers. -Thankfully, RCU update-side primitives, including -call_rcu(), are prohibited within NMI handlers. - -

    -The name notwithstanding, some Linux-kernel architectures -can have nested NMIs, which RCU must handle correctly. -Andy Lutomirski -surprised me -with this requirement; -he also kindly surprised me with -an algorithm -that meets this requirement. - -

    Loadable Modules

    - -

    -The Linux kernel has loadable modules, and these modules can -also be unloaded. -After a given module has been unloaded, any attempt to call -one of its functions results in a segmentation fault. -The module-unload functions must therefore cancel any -delayed calls to loadable-module functions, for example, -any outstanding mod_timer() must be dealt with -via del_timer_sync() or similar. - -

    -Unfortunately, there is no way to cancel an RCU callback; -once you invoke call_rcu(), the callback function is -going to eventually be invoked, unless the system goes down first. -Because it is normally considered socially irresponsible to crash the system -in response to a module unload request, we need some other way -to deal with in-flight RCU callbacks. - -

    -RCU therefore provides -rcu_barrier(), -which waits until all in-flight RCU callbacks have been invoked. -If a module uses call_rcu(), its exit function should therefore -prevent any future invocation of call_rcu(), then invoke -rcu_barrier(). -In theory, the underlying module-unload code could invoke -rcu_barrier() unconditionally, but in practice this would -incur unacceptable latencies. - -

    -Nikita Danilov noted this requirement for an analogous filesystem-unmount -situation, and Dipankar Sarma incorporated rcu_barrier() into RCU. -The need for rcu_barrier() for module unloading became -apparent later. - -

    Hotplug CPU

    - -

    -The Linux kernel supports CPU hotplug, which means that CPUs -can come and go. -It is of course illegal to use any RCU API member from an offline CPU. -This requirement was present from day one in DYNIX/ptx, but -on the other hand, the Linux kernel's CPU-hotplug implementation -is “interesting.” - -

    -The Linux-kernel CPU-hotplug implementation has notifiers that -are used to allow the various kernel subsystems (including RCU) -to respond appropriately to a given CPU-hotplug operation. -Most RCU operations may be invoked from CPU-hotplug notifiers, -including even normal synchronous grace-period operations -such as synchronize_rcu(). -However, expedited grace-period operations such as -synchronize_rcu_expedited() are not supported, -due to the fact that current implementations block CPU-hotplug -operations, which could result in deadlock. - -

    -In addition, all-callback-wait operations such as -rcu_barrier() are also not supported, due to the -fact that there are phases of CPU-hotplug operations where -the outgoing CPU's callbacks will not be invoked until after -the CPU-hotplug operation ends, which could also result in deadlock. - -

    Scheduler and RCU

    - -

    -RCU depends on the scheduler, and the scheduler uses RCU to -protect some of its data structures. -This means the scheduler is forbidden from acquiring -the runqueue locks and the priority-inheritance locks -in the middle of an outermost RCU read-side critical section unless either -(1) it releases them before exiting that same -RCU read-side critical section, or -(2) interrupts are disabled across -that entire RCU read-side critical section. -This same prohibition also applies (recursively!) to any lock that is acquired -while holding any lock to which this prohibition applies. -Adhering to this rule prevents preemptible RCU from invoking -rcu_read_unlock_special() while either runqueue or -priority-inheritance locks are held, thus avoiding deadlock. - -

    -Prior to v4.4, it was only necessary to disable preemption across -RCU read-side critical sections that acquired scheduler locks. -In v4.4, expedited grace periods started using IPIs, and these -IPIs could force a rcu_read_unlock() to take the slowpath. -Therefore, this expedited-grace-period change required disabling of -interrupts, not just preemption. - -

    -For RCU's part, the preemptible-RCU rcu_read_unlock() -implementation must be written carefully to avoid similar deadlocks. -In particular, rcu_read_unlock() must tolerate an -interrupt where the interrupt handler invokes both -rcu_read_lock() and rcu_read_unlock(). -This possibility requires rcu_read_unlock() to use -negative nesting levels to avoid destructive recursion via -interrupt handler's use of RCU. - -

    -This pair of mutual scheduler-RCU requirements came as a -complete surprise. - -

    -As noted above, RCU makes use of kthreads, and it is necessary to -avoid excessive CPU-time accumulation by these kthreads. -This requirement was no surprise, but RCU's violation of it -when running context-switch-heavy workloads when built with -CONFIG_NO_HZ_FULL=y -did come as a surprise [PDF]. -RCU has made good progress towards meeting this requirement, even -for context-switch-have CONFIG_NO_HZ_FULL=y workloads, -but there is room for further improvement. - -

    Tracing and RCU

    - -

    -It is possible to use tracing on RCU code, but tracing itself -uses RCU. -For this reason, rcu_dereference_raw_notrace() -is provided for use by tracing, which avoids the destructive -recursion that could otherwise ensue. -This API is also used by virtualization in some architectures, -where RCU readers execute in environments in which tracing -cannot be used. -The tracing folks both located the requirement and provided the -needed fix, so this surprise requirement was relatively painless. - -

    Energy Efficiency

    - -

    -Interrupting idle CPUs is considered socially unacceptable, -especially by people with battery-powered embedded systems. -RCU therefore conserves energy by detecting which CPUs are -idle, including tracking CPUs that have been interrupted from idle. -This is a large part of the energy-efficiency requirement, -so I learned of this via an irate phone call. - -

    -Because RCU avoids interrupting idle CPUs, it is illegal to -execute an RCU read-side critical section on an idle CPU. -(Kernels built with CONFIG_PROVE_RCU=y will splat -if you try it.) -The RCU_NONIDLE() macro and _rcuidle -event tracing is provided to work around this restriction. -In addition, rcu_is_watching() may be used to -test whether or not it is currently legal to run RCU read-side -critical sections on this CPU. -I learned of the need for diagnostics on the one hand -and RCU_NONIDLE() on the other while inspecting -idle-loop code. -Steven Rostedt supplied _rcuidle event tracing, -which is used quite heavily in the idle loop. - -

    -It is similarly socially unacceptable to interrupt an -nohz_full CPU running in userspace. -RCU must therefore track nohz_full userspace -execution. -And in -CONFIG_NO_HZ_FULL_SYSIDLE=y -kernels, RCU must separately track idle CPUs on the one hand and -CPUs that are either idle or executing in userspace on the other. -In both cases, RCU must be able to sample state at two points in -time, and be able to determine whether or not some other CPU spent -any time idle and/or executing in userspace. - -

    -These energy-efficiency requirements have proven quite difficult to -understand and to meet, for example, there have been more than five -clean-sheet rewrites of RCU's energy-efficiency code, the last of -which was finally able to demonstrate -real energy savings running on real hardware [PDF]. -As noted earlier, -I learned of many of these requirements via angry phone calls: -Flaming me on the Linux-kernel mailing list was apparently not -sufficient to fully vent their ire at RCU's energy-efficiency bugs! - -

    Memory Efficiency

    - -

    -Although small-memory non-realtime systems can simply use Tiny RCU, -code size is only one aspect of memory efficiency. -Another aspect is the size of the rcu_head structure -used by call_rcu() and kfree_rcu(). -Although this structure contains nothing more than a pair of pointers, -it does appear in many RCU-protected data structures, including -some that are size critical. -The page structure is a case in point, as evidenced by -the many occurrences of the union keyword within that structure. - -

    -This need for memory efficiency is one reason that RCU uses hand-crafted -singly linked lists to track the rcu_head structures that -are waiting for a grace period to elapse. -It is also the reason why rcu_head structures do not contain -debug information, such as fields tracking the file and line of the -call_rcu() or kfree_rcu() that posted them. -Although this information might appear in debug-only kernel builds at some -point, in the meantime, the ->func field will often provide -the needed debug information. - -

    -However, in some cases, the need for memory efficiency leads to even -more extreme measures. -Returning to the page structure, the rcu_head field -shares storage with a great many other structures that are used at -various points in the corresponding page's lifetime. -In order to correctly resolve certain -race conditions, -the Linux kernel's memory-management subsystem needs a particular bit -to remain zero during all phases of grace-period processing, -and that bit happens to map to the bottom bit of the -rcu_head structure's ->next field. -RCU makes this guarantee as long as call_rcu() -is used to post the callback, as opposed to kfree_rcu() -or some future “lazy” -variant of call_rcu() that might one day be created for -energy-efficiency purposes. - -

    -Performance, Scalability, Response Time, and Reliability

    - -

    -Expanding on the -earlier discussion, -RCU is used heavily by hot code paths in performance-critical -portions of the Linux kernel's networking, security, virtualization, -and scheduling code paths. -RCU must therefore use efficient implementations, especially in its -read-side primitives. -To that end, it would be good if preemptible RCU's implementation -of rcu_read_lock() could be inlined, however, doing -this requires resolving #include issues with the -task_struct structure. - -

    -The Linux kernel supports hardware configurations with up to -4096 CPUs, which means that RCU must be extremely scalable. -Algorithms that involve frequent acquisitions of global locks or -frequent atomic operations on global variables simply cannot be -tolerated within the RCU implementation. -RCU therefore makes heavy use of a combining tree based on the -rcu_node structure. -RCU is required to tolerate all CPUs continuously invoking any -combination of RCU's runtime primitives with minimal per-operation -overhead. -In fact, in many cases, increasing load must decrease the -per-operation overhead, witness the batching optimizations for -synchronize_rcu(), call_rcu(), -synchronize_rcu_expedited(), and rcu_barrier(). -As a general rule, RCU must cheerfully accept whatever the -rest of the Linux kernel decides to throw at it. - -

    -The Linux kernel is used for real-time workloads, especially -in conjunction with the --rt patchset. -The real-time-latency response requirements are such that the -traditional approach of disabling preemption across RCU -read-side critical sections is inappropriate. -Kernels built with CONFIG_PREEMPT=y therefore -use an RCU implementation that allows RCU read-side critical -sections to be preempted. -This requirement made its presence known after users made it -clear that an earlier -real-time patch -did not meet their needs, in conjunction with some -RCU issues -encountered by a very early version of the -rt patchset. - -

    -In addition, RCU must make do with a sub-100-microsecond real-time latency -budget. -In fact, on smaller systems with the -rt patchset, the Linux kernel -provides sub-20-microsecond real-time latencies for the whole kernel, -including RCU. -RCU's scalability and latency must therefore be sufficient for -these sorts of configurations. -To my surprise, the sub-100-microsecond real-time latency budget - -applies to even the largest systems [PDF], -up to and including systems with 4096 CPUs. -This real-time requirement motivated the grace-period kthread, which -also simplified handling of a number of race conditions. - -

    -RCU must avoid degrading real-time response for CPU-bound threads, whether -executing in usermode (which is one use case for -CONFIG_NO_HZ_FULL=y) or in the kernel. -That said, CPU-bound loops in the kernel must execute -cond_resched_rcu_qs() at least once per few tens of milliseconds -in order to avoid receiving an IPI from RCU. - -

    -Finally, RCU's status as a synchronization primitive means that -any RCU failure can result in arbitrary memory corruption that can be -extremely difficult to debug. -This means that RCU must be extremely reliable, which in -practice also means that RCU must have an aggressive stress-test -suite. -This stress-test suite is called rcutorture. - -

    -Although the need for rcutorture was no surprise, -the current immense popularity of the Linux kernel is posing -interesting—and perhaps unprecedented—validation -challenges. -To see this, keep in mind that there are well over one billion -instances of the Linux kernel running today, given Android -smartphones, Linux-powered televisions, and servers. -This number can be expected to increase sharply with the advent of -the celebrated Internet of Things. - -

    -Suppose that RCU contains a race condition that manifests on average -once per million years of runtime. -This bug will be occurring about three times per day across -the installed base. -RCU could simply hide behind hardware error rates, given that no one -should really expect their smartphone to last for a million years. -However, anyone taking too much comfort from this thought should -consider the fact that in most jurisdictions, a successful multi-year -test of a given mechanism, which might include a Linux kernel, -suffices for a number of types of safety-critical certifications. -In fact, rumor has it that the Linux kernel is already being used -in production for safety-critical applications. -I don't know about you, but I would feel quite bad if a bug in RCU -killed someone. -Which might explain my recent focus on validation and verification. - -

    Other RCU Flavors

    - -

    -One of the more surprising things about RCU is that there are now -no fewer than five flavors, or API families. -In addition, the primary flavor that has been the sole focus up to -this point has two different implementations, non-preemptible and -preemptible. -The other four flavors are listed below, with requirements for each -described in a separate section. - -

      -
    1. Bottom-Half Flavor -
    2. Sched Flavor -
    3. Sleepable RCU -
    4. Tasks RCU -
    5. - Waiting for Multiple Grace Periods -
    - -

    Bottom-Half Flavor

    - -

    -The softirq-disable (AKA “bottom-half”, -hence the “_bh” abbreviations) -flavor of RCU, or RCU-bh, was developed by -Dipankar Sarma to provide a flavor of RCU that could withstand the -network-based denial-of-service attacks researched by Robert -Olsson. -These attacks placed so much networking load on the system -that some of the CPUs never exited softirq execution, -which in turn prevented those CPUs from ever executing a context switch, -which, in the RCU implementation of that time, prevented grace periods -from ever ending. -The result was an out-of-memory condition and a system hang. - -

    -The solution was the creation of RCU-bh, which does -local_bh_disable() -across its read-side critical sections, and which uses the transition -from one type of softirq processing to another as a quiescent state -in addition to context switch, idle, user mode, and offline. -This means that RCU-bh grace periods can complete even when some of -the CPUs execute in softirq indefinitely, thus allowing algorithms -based on RCU-bh to withstand network-based denial-of-service attacks. - -

    -Because -rcu_read_lock_bh() and rcu_read_unlock_bh() -disable and re-enable softirq handlers, any attempt to start a softirq -handlers during the -RCU-bh read-side critical section will be deferred. -In this case, rcu_read_unlock_bh() -will invoke softirq processing, which can take considerable time. -One can of course argue that this softirq overhead should be associated -with the code following the RCU-bh read-side critical section rather -than rcu_read_unlock_bh(), but the fact -is that most profiling tools cannot be expected to make this sort -of fine distinction. -For example, suppose that a three-millisecond-long RCU-bh read-side -critical section executes during a time of heavy networking load. -There will very likely be an attempt to invoke at least one softirq -handler during that three milliseconds, but any such invocation will -be delayed until the time of the rcu_read_unlock_bh(). -This can of course make it appear at first glance as if -rcu_read_unlock_bh() was executing very slowly. - -

    -The -RCU-bh API -includes -rcu_read_lock_bh(), -rcu_read_unlock_bh(), -rcu_dereference_bh(), -rcu_dereference_bh_check(), -synchronize_rcu_bh(), -synchronize_rcu_bh_expedited(), -call_rcu_bh(), -rcu_barrier_bh(), and -rcu_read_lock_bh_held(). - -

    Sched Flavor

    - -

    -Before preemptible RCU, waiting for an RCU grace period had the -side effect of also waiting for all pre-existing interrupt -and NMI handlers. -However, there are legitimate preemptible-RCU implementations that -do not have this property, given that any point in the code outside -of an RCU read-side critical section can be a quiescent state. -Therefore, RCU-sched was created, which follows “classic” -RCU in that an RCU-sched grace period waits for for pre-existing -interrupt and NMI handlers. -In kernels built with CONFIG_PREEMPT=n, the RCU and RCU-sched -APIs have identical implementations, while kernels built with -CONFIG_PREEMPT=y provide a separate implementation for each. - -

    -Note well that in CONFIG_PREEMPT=y kernels, -rcu_read_lock_sched() and rcu_read_unlock_sched() -disable and re-enable preemption, respectively. -This means that if there was a preemption attempt during the -RCU-sched read-side critical section, rcu_read_unlock_sched() -will enter the scheduler, with all the latency and overhead entailed. -Just as with rcu_read_unlock_bh(), this can make it look -as if rcu_read_unlock_sched() was executing very slowly. -However, the highest-priority task won't be preempted, so that task -will enjoy low-overhead rcu_read_unlock_sched() invocations. - -

    -The -RCU-sched API -includes -rcu_read_lock_sched(), -rcu_read_unlock_sched(), -rcu_read_lock_sched_notrace(), -rcu_read_unlock_sched_notrace(), -rcu_dereference_sched(), -rcu_dereference_sched_check(), -synchronize_sched(), -synchronize_rcu_sched_expedited(), -call_rcu_sched(), -rcu_barrier_sched(), and -rcu_read_lock_sched_held(). -However, anything that disables preemption also marks an RCU-sched -read-side critical section, including -preempt_disable() and preempt_enable(), -local_irq_save() and local_irq_restore(), -and so on. - -

    Sleepable RCU

    - -

    -For well over a decade, someone saying “I need to block within -an RCU read-side critical section” was a reliable indication -that this someone did not understand RCU. -After all, if you are always blocking in an RCU read-side critical -section, you can probably afford to use a higher-overhead synchronization -mechanism. -However, that changed with the advent of the Linux kernel's notifiers, -whose RCU read-side critical -sections almost never sleep, but sometimes need to. -This resulted in the introduction of -sleepable RCU, -or SRCU. - -

    -SRCU allows different domains to be defined, with each such domain -defined by an instance of an srcu_struct structure. -A pointer to this structure must be passed in to each SRCU function, -for example, synchronize_srcu(&ss), where -ss is the srcu_struct structure. -The key benefit of these domains is that a slow SRCU reader in one -domain does not delay an SRCU grace period in some other domain. -That said, one consequence of these domains is that read-side code -must pass a “cookie” from srcu_read_lock() -to srcu_read_unlock(), for example, as follows: - -

    -
    - 1 int idx;
- 2
- 3 idx = srcu_read_lock(&ss);
- 4 do_something();
- 5 srcu_read_unlock(&ss, idx);
-
    -
    - -

    -As noted above, it is legal to block within SRCU read-side critical sections, -however, with great power comes great responsibility. -If you block forever in one of a given domain's SRCU read-side critical -sections, then that domain's grace periods will also be blocked forever. -Of course, one good way to block forever is to deadlock, which can -happen if any operation in a given domain's SRCU read-side critical -section can block waiting, either directly or indirectly, for that domain's -grace period to elapse. -For example, this results in a self-deadlock: - -

    -
    - 1 int idx;
- 2
- 3 idx = srcu_read_lock(&ss);
- 4 do_something();
- 5 synchronize_srcu(&ss);
- 6 srcu_read_unlock(&ss, idx);
-
    -
    - -

    -However, if line 5 acquired a mutex that was held across -a synchronize_srcu() for domain ss, -deadlock would still be possible. -Furthermore, if line 5 acquired a mutex that was held across -a synchronize_srcu() for some other domain ss1, -and if an ss1-domain SRCU read-side critical section -acquired another mutex that was held across as ss-domain -synchronize_srcu(), -deadlock would again be possible. -Such a deadlock cycle could extend across an arbitrarily large number -of different SRCU domains. -Again, with great power comes great responsibility. - -

    -Unlike the other RCU flavors, SRCU read-side critical sections can -run on idle and even offline CPUs. -This ability requires that srcu_read_lock() and -srcu_read_unlock() contain memory barriers, which means -that SRCU readers will run a bit slower than would RCU readers. -It also motivates the smp_mb__after_srcu_read_unlock() -API, which, in combination with srcu_read_unlock(), -guarantees a full memory barrier. - -

    -The -SRCU API -includes -srcu_read_lock(), -srcu_read_unlock(), -srcu_dereference(), -srcu_dereference_check(), -synchronize_srcu(), -synchronize_srcu_expedited(), -call_srcu(), -srcu_barrier(), and -srcu_read_lock_held(). -It also includes -DEFINE_SRCU(), -DEFINE_STATIC_SRCU(), and -init_srcu_struct() -APIs for defining and initializing srcu_struct structures. - -

    Tasks RCU

    - -

    -Some forms of tracing use “tramopolines” to handle the -binary rewriting required to install different types of probes. -It would be good to be able to free old trampolines, which sounds -like a job for some form of RCU. -However, because it is necessary to be able to install a trace -anywhere in the code, it is not possible to use read-side markers -such as rcu_read_lock() and rcu_read_unlock(). -In addition, it does not work to have these markers in the trampoline -itself, because there would need to be instructions following -rcu_read_unlock(). -Although synchronize_rcu() would guarantee that execution -reached the rcu_read_unlock(), it would not be able to -guarantee that execution had completely left the trampoline. - -

    -The solution, in the form of -Tasks RCU, -is to have implicit -read-side critical sections that are delimited by voluntary context -switches, that is, calls to schedule(), -cond_resched_rcu_qs(), and -synchronize_rcu_tasks(). -In addition, transitions to and from userspace execution also delimit -tasks-RCU read-side critical sections. - -

    -The tasks-RCU API is quite compact, consisting only of -call_rcu_tasks(), -synchronize_rcu_tasks(), and -rcu_barrier_tasks(). - -

    -Waiting for Multiple Grace Periods

    - -

    -Perhaps you have an RCU protected data structure that is accessed from -RCU read-side critical sections, from softirq handlers, and from -hardware interrupt handlers. -That is three flavors of RCU, the normal flavor, the bottom-half flavor, -and the sched flavor. -How to wait for a compound grace period? - -

    -The best approach is usually to “just say no!” and -insert rcu_read_lock() and rcu_read_unlock() -around each RCU read-side critical section, regardless of what -environment it happens to be in. -But suppose that some of the RCU read-side critical sections are -on extremely hot code paths, and that use of CONFIG_PREEMPT=n -is not a viable option, so that rcu_read_lock() and -rcu_read_unlock() are not free. -What then? - -

    -You could wait on all three grace periods in succession, as follows: - -

    -
    - 1 synchronize_rcu();
- 2 synchronize_rcu_bh();
- 3 synchronize_sched();
-
    -
    - -

    -This works, but triples the update-side latency penalty. -In cases where this is not acceptable, synchronize_rcu_mult() -may be used to wait on all three flavors of grace period concurrently: - -

    -
    - 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
-
    -
    - -

    -But what if it is necessary to also wait on SRCU? -This can be done as follows: - -

    -
    - 1 static void call_my_srcu(struct rcu_head *head,
- 2        void (*func)(struct rcu_head *head))
- 3 {
- 4   call_srcu(&my_srcu, head, func);
- 5 }
- 6
- 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
-
    -
    - -

    -If you needed to wait on multiple different flavors of SRCU -(but why???), you would need to create a wrapper function resembling -call_my_srcu() for each SRCU flavor. - -

    @@QQ@@ -But what if I need to wait for multiple RCU flavors, but I also need -the grace periods to be expedited? -

    @@QQA@@ -If you are using expedited grace periods, there should be less penalty -for waiting on them in succession. -But if that is nevertheless a problem, you can use workqueues or multiple -kthreads to wait on the various expedited grace periods concurrently. -

    @@QQE@@ - -

    -Again, it is usually better to adjust the RCU read-side critical sections -to use a single flavor of RCU, but when this is not feasible, you can use -synchronize_rcu_mult(). - -

    Possible Future Changes

    - -

    -One of the tricks that RCU uses to attain update-side scalability is -to increase grace-period latency with increasing numbers of CPUs. -If this becomes a serious problem, it will be necessary to rework the -grace-period state machine so as to avoid the need for the additional -latency. - -

    -Expedited grace periods scan the CPUs, so their latency and overhead -increases with increasing numbers of CPUs. -If this becomes a serious problem on large systems, it will be necessary -to do some redesign to avoid this scalability problem. - -

    -RCU disables CPU hotplug in a few places, perhaps most notably in the -expedited grace-period and rcu_barrier() operations. -If there is a strong reason to use expedited grace periods in CPU-hotplug -notifiers, it will be necessary to avoid disabling CPU hotplug. -This would introduce some complexity, so there had better be a very -good reason. - -

    -The tradeoff between grace-period latency on the one hand and interruptions -of other CPUs on the other hand may need to be re-examined. -The desire is of course for zero grace-period latency as well as zero -interprocessor interrupts undertaken during an expedited grace period -operation. -While this ideal is unlikely to be achievable, it is quite possible that -further improvements can be made. - -

    -The multiprocessor implementations of RCU use a combining tree that -groups CPUs so as to reduce lock contention and increase cache locality. -However, this combining tree does not spread its memory across NUMA -nodes nor does it align the CPU groups with hardware features such -as sockets or cores. -Such spreading and alignment is currently believed to be unnecessary -because the hotpath read-side primitives do not access the combining -tree, nor does call_rcu() in the common case. -If you believe that your architecture needs such spreading and alignment, -then your architecture should also benefit from the -rcutree.rcu_fanout_leaf boot parameter, which can be set -to the number of CPUs in a socket, NUMA node, or whatever. -If the number of CPUs is too large, use a fraction of the number of -CPUs. -If the number of CPUs is a large prime number, well, that certainly -is an “interesting” architectural choice! -More flexible arrangements might be considered, but only if -rcutree.rcu_fanout_leaf has proven inadequate, and only -if the inadequacy has been demonstrated by a carefully run and -realistic system-level workload. - -

    -Please note that arrangements that require RCU to remap CPU numbers will -require extremely good demonstration of need and full exploration of -alternatives. - -

    -There is an embarrassingly large number of flavors of RCU, and this -number has been increasing over time. -Perhaps it will be possible to combine some at some future date. - -

    -RCU's various kthreads are reasonably recent additions. -It is quite likely that adjustments will be required to more gracefully -handle extreme loads. -It might also be necessary to be able to relate CPU utilization by -RCU's kthreads and softirq handlers to the code that instigated this -CPU utilization. -For example, RCU callback overhead might be charged back to the -originating call_rcu() instance, though probably not -in production kernels. - -

    Summary

    - -

    -This document has presented more than two decade's worth of RCU -requirements. -Given that the requirements keep changing, this will not be the last -word on this subject, but at least it serves to get an important -subset of the requirements set forth. - -

    Acknowledgments

    - -I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar, -Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and -Andy Lutomirski for their help in rendering -this article human readable, and to Michelle Rankin for her support -of this effort. -Other contributions are acknowledged in the Linux kernel's git archive. -The cartoon is copyright (c) 2013 by Melissa Broussard, -and is provided -under the terms of the Creative Commons Attribution-Share Alike 3.0 -United States license. - -

    @@QQAL@@ - - diff --git a/Documentation/RCU/Design/htmlqqz.sh b/Documentation/RCU/Design/htmlqqz.sh deleted file mode 100755 index d354f069559b..000000000000 --- a/Documentation/RCU/Design/htmlqqz.sh +++ /dev/null @@ -1,108 +0,0 @@ -#!/bin/sh -# -# Usage: sh htmlqqz.sh file -# -# Extracts and converts quick quizzes in a proto-HTML document file.htmlx. -# Commands, all of which must be on a line by themselves: -# -# "

    @@QQ@@": Start of a quick quiz. -# "

    @@QQA@@": Start of a quick-quiz answer. -# "

    @@QQE@@": End of a quick-quiz answer, and thus of the quick quiz. -# "

    @@QQAL@@": Place to put quick-quiz answer list. -# -# Places the result in file.html. -# -# This program is free software; you can redistribute it and/or modify -# it under the terms of the GNU General Public License as published by -# the Free Software Foundation; either version 2 of the License, or -# (at your option) any later version. -# -# This program is distributed in the hope that it will be useful, -# but WITHOUT ANY WARRANTY; without even the implied warranty of -# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the -# GNU General Public License for more details. -# -# You should have received a copy of the GNU General Public License -# along with this program; if not, you can access it online at -# http://www.gnu.org/licenses/gpl-2.0.html. -# -# Copyright (c) 2013 Paul E. McKenney, IBM Corporation. - -fn=$1 -if test ! -r $fn.htmlx -then - echo "Error: $fn.htmlx unreadable." - exit 1 -fi - -echo "" > $fn.html -echo "" >> $fn.html -awk < $fn.htmlx >> $fn.html ' - -state == "" && $1 != "

    @@QQ@@" && $1 != "

    @@QQAL@@" { - print $0; - if ($0 ~ /^

    @@QQ/) - print "Bad Quick Quiz command: " NR " (expected

    @@QQ@@ or

    @@QQAL@@)." > "/dev/stderr" - next; -} - -state == "" && $1 == "

    @@QQ@@" { - qqn++; - qqlineno = NR; - haveqq = 1; - state = "qq"; - print "

    Quick Quiz " qqn ":" - next; -} - -state == "qq" && $1 != "

    @@QQA@@" { - qq[qqn] = qq[qqn] $0 "\n"; - print $0 - if ($0 ~ /^

    @@QQ/) - print "Bad Quick Quiz command: " NR ". (expected

    @@QQA@@)" > "/dev/stderr" - next; -} - -state == "qq" && $1 == "

    @@QQA@@" { - state = "qqa"; - print "
    Answer" - next; -} - -state == "qqa" && $1 != "

    @@QQE@@" { - qqa[qqn] = qqa[qqn] $0 "\n"; - if ($0 ~ /^

    @@QQ/) - print "Bad Quick Quiz command: " NR " (expected

    @@QQE@@)." > "/dev/stderr" - next; -} - -state == "qqa" && $1 == "

    @@QQE@@" { - state = ""; - next; -} - -state == "" && $1 == "

    @@QQAL@@" { - haveqq = ""; - print "

    " - print "Answers to Quick Quizzes

    " - print ""; - for (i = 1; i <= qqn; i++) { - print "" - print "

    Quick Quiz " i ":" - print qq[i]; - print ""; - print "

    Answer:" - print qqa[i]; - print ""; - print "

    Back to Quick Quiz " i "." - print ""; - } - next; -} - -END { - if (state != "") - print "Unterminated Quick Quiz: " qqlineno "." > "/dev/stderr" - else if (haveqq) - print "Missing \"

    @@QQAL@@\", no Quick Quiz." > "/dev/stderr" -}' -- cgit 1.4.1 From 5413e24c943da33306047fc091fa34fa4f261b3b Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Mar 2016 13:40:28 -0700 Subject: documentation: Sharpen up the no-readers quick quiz Signed-off-by: Paul E. McKenney --- Documentation/RCU/Design/Requirements/Requirements.html | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index acdad96f78e9..85cf2238fd08 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1002,18 +1002,21 @@ obligation to wait for these new readers.   Quick Quiz: - Suppose that synchronize_rcu() did wait until all readers had completed. - Would the updater be able to rely on this? + Suppose that synchronize_rcu() did wait until all + readers had completed instead of waiting only on + pre-existing readers. + For how long would the updater be able to rely on there + being no readers? Answer: - No. + For no time at all. Even if synchronize_rcu() were to wait until all readers had completed, a new reader might start immediately after synchronize_rcu() completed. Therefore, the code following - synchronize_rcu() cannot rely on there being no readers - in any case. + synchronize_rcu() can never rely on there being + no readers.   -- cgit 1.4.1 From 0c7d10e4b998b2f751cebf98435f1ec2dd312c87 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 31 Mar 2016 11:00:08 -0700 Subject: documentation: Emphasize the call_rcu() is illegal from idle Although call_rcu()'s fastpath works just fine on an idle CPU, some branches of the slowpath invoke the scheduler, which uses RCU. Therefore, this commit emphasizes the fact that call_rcu() must not be invoked from an idle CPU. Signed-off-by: Paul E. McKenney --- Documentation/RCU/Design/Requirements/Requirements.html | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 85cf2238fd08..e7e24b3e86e2 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1650,7 +1650,7 @@ situations where neither synchronize_rcu() nor including within preempt-disable code, local_bh_disable() code, interrupt-disable code, and interrupt handlers. However, even call_rcu() is illegal within NMI handlers -and from offline CPUs. +and from idle and offline CPUs. The callback function (remove_gp_cb() in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, -- cgit 1.4.1 From e2fd9d35847d1936398d44c4df68dceb3d7f64e7 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sat, 30 Jan 2016 17:23:19 -0800 Subject: rcu: Remove expedited GP funnel-lock bypass Commit #cdacbe1f91264 ("rcu: Add fastpath bypassing funnel locking") turns out to be a pessimization at high load because it forces a tree full of tasks to wait for an expedited grace period that they probably do not need. This commit therefore removes this optimization. Signed-off-by: Paul E. McKenney --- Documentation/RCU/trace.txt | 10 +++++----- kernel/rcu/tree.c | 19 ------------------- kernel/rcu/tree.h | 1 - kernel/rcu/tree_trace.c | 7 +++---- 4 files changed, 8 insertions(+), 29 deletions(-) (limited to 'Documentation') diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt index ec6998b1b6d0..00a3a38b375a 100644 --- a/Documentation/RCU/trace.txt +++ b/Documentation/RCU/trace.txt @@ -237,17 +237,17 @@ o "ktl" is the low-order 16 bits (in hexadecimal) of the count of The output of "cat rcu/rcu_preempt/rcuexp" looks as follows: -s=21872 wd0=0 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872 +s=21872 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872 These fields are as follows: o "s" is the sequence number, with an odd number indicating that an expedited grace period is in progress. -o "wd0", "wd1", "wd2", and "wd3" are the number of times that an - attempt to start an expedited grace period found that someone - else had completed an expedited grace period that satisfies the - attempted request. "Our work is done." +o "wd1", "wd2", and "wd3" are the number of times that an attempt + to start an expedited grace period found that someone else had + completed an expedited grace period that satisfies the attempted + request. "Our work is done." o "n" is number of times that a concurrent CPU-hotplug operation forced a fallback to a normal grace period. diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 524026fd9dd7..62e73e0a929f 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3616,25 +3616,6 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s) struct rcu_node *rnp0; struct rcu_node *rnp1 = NULL; - /* - * First try directly acquiring the root lock in order to reduce - * latency in the common case where expedited grace periods are - * rare. We check mutex_is_locked() to avoid pathological levels of - * memory contention on ->exp_funnel_mutex in the heavy-load case. - */ - rnp0 = rcu_get_root(rsp); - if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) { - if (mutex_trylock(&rnp0->exp_funnel_mutex)) { - trace_rcu_exp_funnel_lock(rsp->name, rnp0->level, - rnp0->grplo, rnp0->grphi, - TPS("acq")); - if (sync_exp_work_done(rsp, rnp0, NULL, - &rdp->expedited_workdone0, s)) - return NULL; - return rnp0; - } - } - /* * Each pass through the following loop works its way * up the rcu_node tree, returning if others have done the diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index df668c0f9e64..ac9a7b0c36ae 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -388,7 +388,6 @@ struct rcu_data { struct rcu_head oom_head; #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */ struct mutex exp_funnel_mutex; - atomic_long_t expedited_workdone0; /* # done by others #0. */ atomic_long_t expedited_workdone1; /* # done by others #1. */ atomic_long_t expedited_workdone2; /* # done by others #2. */ atomic_long_t expedited_workdone3; /* # done by others #3. */ diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c index 1088e64f01ad..d149c412a4e5 100644 --- a/kernel/rcu/tree_trace.c +++ b/kernel/rcu/tree_trace.c @@ -185,17 +185,16 @@ static int show_rcuexp(struct seq_file *m, void *v) int cpu; struct rcu_state *rsp = (struct rcu_state *)m->private; struct rcu_data *rdp; - unsigned long s0 = 0, s1 = 0, s2 = 0, s3 = 0; + unsigned long s1 = 0, s2 = 0, s3 = 0; for_each_possible_cpu(cpu) { rdp = per_cpu_ptr(rsp->rda, cpu); - s0 += atomic_long_read(&rdp->expedited_workdone0); s1 += atomic_long_read(&rdp->expedited_workdone1); s2 += atomic_long_read(&rdp->expedited_workdone2); s3 += atomic_long_read(&rdp->expedited_workdone3); } - seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n", - rsp->expedited_sequence, s0, s1, s2, s3, + seq_printf(m, "s=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n", + rsp->expedited_sequence, s1, s2, s3, atomic_long_read(&rsp->expedited_normal), atomic_read(&rsp->expedited_need_qs), rsp->expedited_sequence / 2); -- cgit 1.4.1 From bdea9e347783c2724997db7c5d5b45a301e2dc90 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 1 Jan 2016 13:47:19 -0800 Subject: rcutorture: Documentation for rcuperf kernel parameters This commit adds documentation for the new rcuperf module's kernel boot parameters. Signed-off-by: Paul E. McKenney --- Documentation/kernel-parameters.txt | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) (limited to 'Documentation') diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index ecc74fa4bfde..951af481da5a 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -3284,6 +3284,38 @@ bytes respectively. Such letter suffixes can also be entirely omitted. Lazy RCU callbacks are those which RCU can prove do nothing more than free memory. + rcuperf.gp_exp= [KNL] + Measure performance of expedited synchronous + grace-period primitives. + + rcuperf.nreaders= [KNL] + Set number of RCU readers. The value -1 selects + N, where N is the number of CPUs. A value + "n" less than -1 selects N-n+1, where N is again + the number of CPUs. For example, -2 selects N + (the number of CPUs), -3 selects N+1, and so on. + A value of "n" less than or equal to -N selects + a single reader. + + rcuperf.nwriters= [KNL] + Set number of RCU writers. The values operate + the same as for rcuperf.nreaders. + N, where N is the number of CPUs + + rcuperf.perf_runnable= [BOOT] + Start rcuperf running at boot time. + + rcuperf.shutdown= [KNL] + Shut the system down after performance tests + complete. This is useful for hands-off automated + testing. + + rcuperf.perf_type= [KNL] + Specify the RCU implementation to test. + + rcuperf.verbose= [KNL] + Enable additional printk() statements. + rcutorture.cbflood_inter_holdoff= [KNL] Set holdoff time (jiffies) between successive callback-flood tests. -- cgit 1.4.1 From df37e66bfdbb57e8cae7dbf39a0c66b1b8701338 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sat, 30 Jan 2016 20:56:38 -0800 Subject: rcutorture: Add rcuperf holdoff boot parameter to reduce interference Boot-time activity can legitimately grab CPUs for extended time periods, so the commit adds a boot parameter to delay the start of the performance test until boot has completed. Defaults to 10 seconds. Signed-off-by: Paul E. McKenney --- Documentation/kernel-parameters.txt | 6 ++++++ kernel/rcu/rcuperf.c | 5 +++++ 2 files changed, 11 insertions(+) (limited to 'Documentation') diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 951af481da5a..da9ee466789b 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -3288,6 +3288,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted. Measure performance of expedited synchronous grace-period primitives. + rcuperf.holdoff= [KNL] + Set test-start holdoff period. The purpose of + this parameter is to delay the start of the + test until boot completes in order to avoid + interference. + rcuperf.nreaders= [KNL] Set number of RCU readers. The value -1 selects N, where N is the number of CPUs. A value diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c index 12561f96f0a2..278600143bb6 100644 --- a/kernel/rcu/rcuperf.c +++ b/kernel/rcu/rcuperf.c @@ -59,6 +59,7 @@ MODULE_AUTHOR("Paul E. McKenney "); do { if (verbose) pr_alert("%s" PERF_FLAG "!!! %s\n", perf_type, s); } while (0) torture_param(bool, gp_exp, true, "Use expedited GP wait primitives"); +torture_param(int, holdoff, 10, "Holdoff time before test start (s)"); torture_param(int, nreaders, -1, "Number of RCU reader threads"); torture_param(int, nwriters, -1, "Number of RCU updater threads"); torture_param(bool, shutdown, false, "Shutdown at end of performance tests."); @@ -368,6 +369,10 @@ rcu_perf_writer(void *arg) set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids)); sp.sched_priority = 1; sched_setscheduler_nocheck(current, SCHED_FIFO, &sp); + + if (holdoff) + schedule_timeout_uninterruptible(holdoff * HZ); + t = ktime_get_mono_fast_ns(); if (atomic_inc_return(&n_rcu_perf_writer_started) >= nrealwriters) { t_rcu_perf_writer_started = t; -- cgit 1.4.1 From 5c1458478c49b905652fc002708d09369763f58f Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 28 Jan 2016 16:49:24 -0800 Subject: documentation: Add documentation for RCU's major data structures This commit adds documentation for RCU's major data structures, including rcu_state, rcu_node, rcu_data, rcu_dynticks, and rcu_head. Signed-off-by: Paul E. McKenney --- .../Design/Data-Structures/BigTreeClassicRCU.svg | 474 +++++++ .../Design/Data-Structures/BigTreeClassicRCUBH.svg | 499 ++++++++ .../Data-Structures/BigTreeClassicRCUBHdyntick.svg | 695 ++++++++++ .../Data-Structures/BigTreePreemptRCUBHdyntick.svg | 741 +++++++++++ .../BigTreePreemptRCUBHdyntickCB.svg | 858 +++++++++++++ .../Design/Data-Structures/Data-Structures.html | 1333 ++++++++++++++++++++ .../Design/Data-Structures/HugeTreeClassicRCU.svg | 939 ++++++++++++++ .../RCU/Design/Data-Structures/TreeLevel.svg | 828 ++++++++++++ .../RCU/Design/Data-Structures/TreeMapping.svg | 305 +++++ .../Design/Data-Structures/TreeMappingLevel.svg | 380 ++++++ .../RCU/Design/Data-Structures/blkd_task.svg | 843 +++++++++++++ .../RCU/Design/Data-Structures/nxtlist.svg | 396 ++++++ 12 files changed, 8291 insertions(+) create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBH.svg create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntick.svg create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntickCB.svg create mode 100644 Documentation/RCU/Design/Data-Structures/Data-Structures.html create mode 100644 Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg create mode 100644 Documentation/RCU/Design/Data-Structures/TreeLevel.svg create mode 100644 Documentation/RCU/Design/Data-Structures/TreeMapping.svg create mode 100644 Documentation/RCU/Design/Data-Structures/TreeMappingLevel.svg create mode 100644 Documentation/RCU/Design/Data-Structures/blkd_task.svg create mode 100644 Documentation/RCU/Design/Data-Structures/nxtlist.svg (limited to 'Documentation') diff --git a/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg new file mode 100644 index 000000000000..727e270b11e4 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg @@ -0,0 +1,474 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + struct + + rcu_data + + CPU 0 + + struct + + rcu_data + + CPU 15 + + struct + + rcu_data + + CPU 1007 + + struct + + rcu_data + + CPU 1023 + + struct rcu_state + + struct + + rcu_node + + rcu_node + + struct + + struct + + rcu_node + + + + + + + + diff --git a/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBH.svg b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBH.svg new file mode 100644 index 000000000000..9bbb1944f962 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBH.svg @@ -0,0 +1,499 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + rcu_bh + + struct + + rcu_node + + struct + + rcu_node + + rcu_node + + struct + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct rcu_state + + rcu_sched + + + + + + + + + + + diff --git a/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg new file mode 100644 index 000000000000..21ba7823479d --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg @@ -0,0 +1,695 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + rcu_bh + + struct + + rcu_node + + struct + + rcu_node + + rcu_node + + struct + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct rcu_state + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + rcu_sched + + + + + diff --git a/Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntick.svg b/Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntick.svg new file mode 100644 index 000000000000..15adcac036c7 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntick.svg @@ -0,0 +1,741 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + rcu_bh + + struct + + rcu_node + + struct + + rcu_node + + rcu_node + + struct + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct rcu_state + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + rcu_preempt + + rcu_sched + + + + + diff --git a/Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntickCB.svg b/Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntickCB.svg new file mode 100644 index 000000000000..bbc3801470d0 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntickCB.svg @@ -0,0 +1,858 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + struct + + rcu_head + + struct + + rcu_head + + struct + + rcu_head + + rcu_sched + + rcu_bh + + struct + + rcu_node + + struct + + rcu_node + + rcu_node + + struct + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct rcu_state + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + rcu_preempt + + + + + diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html new file mode 100644 index 000000000000..7eb47ac25ad7 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -0,0 +1,1333 @@ + + + A Tour Through TREE_RCU's Data Structures [LWN.net] + + +

    January 27, 2016

    +

    This article was contributed by Paul E. McKenney

    + +

    Introduction

    + +This document describes RCU's major data structures and their relationship +to each other. + +
      +
    1. + Data-Structure Relationships +
    2. + The rcu_state Structure +
    3. + The rcu_node Structure +
    4. + The rcu_data Structure +
    5. + The rcu_dynticks Structure +
    6. + The rcu_head Structure +
    7. + RCU-Specific Fields in the task_struct Structure +
    8. + Accessor Functions +
    + +At the end we have the +answers to the quick quizzes. + +

    Data-Structure Relationships

    + +

    RCU is for all intents and purposes a large state machine, and its +data structures maintain the state in such a way as to allow RCU readers +to execute extremely quickly, while also processing the RCU grace periods +requested by updaters in an efficient and extremely scalable fashion. +The efficiency and scalability of RCU updaters is provided primarily +by a combining tree, as shown below: + +

    BigTreeClassicRCU.svg + +

    This diagram shows an enclosing rcu_state structure +containing a tree of rcu_node structures. +Each leaf node of the rcu_node tree has up to 16 +rcu_data structures associated with it, so that there +are NR_CPUS number of rcu_data structures, +one for each possible CPU. +This structure is adjusted at boot time, if needed, to handle the +common case where nr_cpu_ids is much less than +NR_CPUs. +For example, a number of Linux distributions set NR_CPUs=4096, +which results in a three-level rcu_node tree. +If the actual hardware has only 16 CPUs, RCU will adjust itself +at boot time, resulting in an rcu_node tree with only a single node. + +

    The purpose of this combining tree is to allow per-CPU events +such as quiescent states, dyntick-idle transitions, +and CPU hotplug operations to be processed efficiently +and scalably. +Quiescent states are recorded by the per-CPU rcu_data structures, +and other events are recorded by the leaf-level rcu_node +structures. +All of these events are combined at each level of the tree until finally +grace periods are completed at the tree's root rcu_node +structure. +A grace period can be completed at the root once every CPU +(or, in the case of CONFIG_PREEMPT_RCU, task) +has passed through a quiescent state. +Once a grace period has completed, record of that fact is propagated +back down the tree. + +

    As can be seen from the diagram, on a 64-bit system +a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout +of 64 at the root and a fanout of 16 at the leaves. + + + + + + + + +
     
    Quick Quiz:
    + Why isn't the fanout at the leaves also 64? +
    Answer:
    + Because there are more types of events that affect the leaf-level + rcu_node structures than further up the tree. + Therefore, if the leaf rcu_node structures have fanout of + 64, the contention on these structures' ->structures + becomes excessive. + Experimentation on a wide variety of systems has shown that a fanout + of 16 works well for the leaves of the rcu_node tree. + + +

    Of course, further experience with + systems having hundreds or thousands of CPUs may demonstrate + that the fanout for the non-leaf rcu_node structures + must also be reduced. + Such reduction can be easily carried out when and if it proves + necessary. + In the meantime, if you are using such a system and running into + contention problems on the non-leaf rcu_node structures, + you may use the CONFIG_RCU_FANOUT kernel configuration + parameter to reduce the non-leaf fanout as needed. + + +

    Kernels built for systems with + strong NUMA characteristics might also need to adjust + CONFIG_RCU_FANOUT so that the domains of the + rcu_node structures align with hardware boundaries. + However, there has thus far been no need for this. +

     
    + +

    If your system has more than 1,024 CPUs (or more than 512 CPUs on +a 32-bit system), then RCU will automatically add more levels to the +tree. +For example, if you are crazy enough to build a 64-bit system with 65,536 +CPUs, RCU would configure the rcu_node tree as follows: + +

    HugeTreeClassicRCU.svg + +

    RCU currently permits up to a four-level tree, which on a 64-bit system +accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for +32-bit systems. +On the other hand, you can set CONFIG_RCU_FANOUT to be +as small as 2 if you wish, which would permit only 16 CPUs, which +is useful for testing. + +

    This multi-level combining tree allows us to get most of the +performance and scalability +benefits of partitioning, even though RCU grace-period detection is +inherently a global operation. +The trick here is that only the last CPU to report a quiescent state +into a given rcu_node structure need advance to the rcu_node +structure at the next level up the tree. +This means that at the leaf-level rcu_node structure, only +one access out of sixteen will progress up the tree. +For the internal rcu_node structures, the situation is even +more extreme: Only one access out of sixty-four will progress up +the tree. +Because the vast majority of the CPUs do not progress up the tree, +the lock contention remains roughly constant up the tree. +No matter how many CPUs there are in the system, at most 64 quiescent-state +reports per grace period will progress all the way to the root +rcu_node structure, thus ensuring that the lock contention +on that root rcu_node structure remains acceptably low. + +

    In effect, the combining tree acts like a big shock absorber, +keeping lock contention under control at all tree levels regardless +of the level of loading on the system. + +

    The Linux kernel actually supports multiple flavors of RCU +running concurrently, so RCU builds separate data structures for each +flavor. +For example, for CONFIG_TREE_RCU=y kernels, RCU provides +rcu_sched and rcu_bh, as shown below: + +

    BigTreeClassicRCUBH.svg + +

    Energy efficiency is increasingly important, and for that +reason the Linux kernel provides CONFIG_NO_HZ_IDLE, which +turns off the scheduling-clock interrupts on idle CPUs, which in +turn allows those CPUs to attain deeper sleep states and to consume +less energy. +CPUs whose scheduling-clock interrupts have been turned off are +said to be in dyntick-idle mode. +RCU must handle dyntick-idle CPUs specially +because RCU would otherwise wake up each CPU on every grace period, +which would defeat the whole purpose of CONFIG_NO_HZ_IDLE. +RCU uses the rcu_dynticks structure to track +which CPUs are in dyntick idle mode, as shown below: + +

    BigTreeClassicRCUBHdyntick.svg + +

    However, if a CPU is in dyntick-idle mode, it is in that mode +for all flavors of RCU. +Therefore, a single rcu_dynticks structure is allocated per +CPU, and all of a given CPU's rcu_data structures share +that rcu_dynticks, as shown in the figure. + +

    Kernels built with CONFIG_PREEMPT_RCU support +rcu_preempt in addition to rcu_sched and rcu_bh, as shown below: + +

    BigTreePreemptRCUBHdyntick.svg + +

    RCU updaters wait for normal grace periods by registering +RCU callbacks, either directly via call_rcu() and +friends (namely call_rcu_bh() and call_rcu_sched()), +there being a separate interface per flavor of RCU) +or indirectly via synchronize_rcu() and friends. +RCU callbacks are represented by rcu_head structures, +which are queued on rcu_data structures while they are +waiting for a grace period to elapse, as shown in the following figure: + +

    BigTreePreemptRCUBHdyntickCB.svg + +

    This figure shows how TREE_RCU's and +PREEMPT_RCU's major data structures are related. +Lesser data structures will be introduced with the algorithms that +make use of them. + +

    Note that each of the data structures in the above figure has +its own synchronization: + +

      +
    1. Each rcu_state structures has a lock and a mutex, + and some fields are protected by the corresponding root + rcu_node structure's lock. +
    2. Each rcu_node structure has a spinlock. +
    3. The fields in rcu_data are private to the corresponding + CPU, although a few can be read and written by other CPUs. +
    4. Similarly, the fields in rcu_dynticks are private + to the corresponding CPU, although a few can be read by + other CPUs. +
    + +

    It is important to note that different data structures can have +very different ideas about the state of RCU at any given time. +For but one example, awareness of the start or end of a given RCU +grace period propagates slowly through the data structures. +This slow propagation is absolutely necessary for RCU to have good +read-side performance. +If this balkanized implementation seems foreign to you, one useful +trick is to consider each instance of these data structures to be +a different person, each having the usual slightly different +view of reality. + +

    The general role of each of these data structures is as +follows: + +

      +
    1. rcu_state: + This structure forms the interconnection between the + rcu_node and rcu_data structures, + tracks grace periods, serves as short-term repository + for callbacks orphaned by CPU-hotplug events, + maintains rcu_barrier() state, + tracks expedited grace-period state, + and maintains state used to force quiescent states when + grace periods extend too long, +
    2. rcu_node: This structure forms the combining + tree that propagates quiescent-state + information from the leaves to the root, and also propagates + grace-period information from the root to the leaves. + It provides local copies of the grace-period state in order + to allow this information to be accessed in a synchronized + manner without suffering the scalability limitations that + would otherwise be imposed by global locking. + In CONFIG_PREEMPT_RCU kernels, it manages the lists + of tasks that have blocked while in their current + RCU read-side critical section. + In CONFIG_PREEMPT_RCU with + CONFIG_RCU_BOOST, it manages the + per-rcu_node priority-boosting + kernel threads (kthreads) and state. + Finally, it records CPU-hotplug state in order to determine + which CPUs should be ignored during a given grace period. +
    3. rcu_data: This per-CPU structure is the + focus of quiescent-state detection and RCU callback queuing. + It also tracks its relationship to the corresponding leaf + rcu_node structure to allow more-efficient + propagation of quiescent states up the rcu_node + combining tree. + Like the rcu_node structure, it provides a local + copy of the grace-period information to allow for-free + synchronized + access to this information from the corresponding CPU. + Finally, this structure records past dyntick-idle state + for the corresponding CPU and also tracks statistics. +
    4. rcu_dynticks: + This per-CPU structure tracks the current dyntick-idle + state for the corresponding CPU. + Unlike the other three structures, the rcu_dynticks + structure is not replicated per RCU flavor. +
    5. rcu_head: + This structure represents RCU callbacks, and is the + only structure allocated and managed by RCU users. + The rcu_head structure is normally embedded + within the RCU-protected data structure. +
    + +

    If all you wanted from this article was a general notion of how +RCU's data structures are related, you are done. +Otherwise, each of the following sections give more details on +the rcu_state, rcu_node, rcu_data, +and rcu_dynticks data structures. + +

    +The rcu_state Structure

    + +

    The rcu_state structure is the base structure that +represents a flavor of RCU. +This structure forms the interconnection between the +rcu_node and rcu_data structures, +tracks grace periods, contains the lock used to +synchronize with CPU-hotplug events, +and maintains state used to force quiescent states when +grace periods extend too long, + +

    A few of the rcu_state structure's fields are discussed, +singly and in groups, in the following sections. +The more specialized fields are covered in the discussion of their +use. + +

    Relationship to rcu_node and rcu_data Structures
    + +This portion of the rcu_state structure is declared +as follows: + +
    +  1   struct rcu_node node[NUM_RCU_NODES];
+  2   struct rcu_node *level[NUM_RCU_LVLS + 1];
+  3   struct rcu_data __percpu *rda;
+
    + + + + + + + + +
     
    Quick Quiz:
    + Wait a minute! + You said that the rcu_node structures formed a tree, + but they are declared as a flat array! + What gives? +
    Answer:
    + The tree is laid out in the array. + The first node In the array is the head, the next set of nodes in the + array are children of the head node, and so on until the last set of + nodes in the array are the leaves. + + +

    See the following diagrams to see how + this works. +

     
    + +

    The rcu_node tree is embedded into the +->node[] array as shown in the following figure: + +

    TreeMapping.svg + +

    One interesting consequence of this mapping is that a +breadth-first traversal of the tree is implemented as a simple +linear scan of the array, which is in fact what the +rcu_for_each_node_breadth_first() macro does. +This macro is used at the beginning and ends of grace periods. + +

    Each entry of the ->level array references +the first rcu_node structure on the corresponding level +of the tree, for example, as shown below: + +

    TreeMappingLevel.svg + +

    The zeroth element of the array references the root +rcu_node structure, the first element references the +first child of the root rcu_node, and finally the second +element references the first leaf rcu_node structure. + +

    For whatever it is worth, if you draw the tree to be tree-shaped +rather than array-shaped, it is easy to draw a planar representation: + +

    TreeLevel.svg + +

    Finally, the ->rda field references a per-CPU +pointer to the corresponding CPU's rcu_data structure. + +

    All of these fields are constant once initialization is complete, +and therefore need no protection. + +

    Grace-Period Tracking
    + +

    This portion of the rcu_state structure is declared +as follows: + +

    +  1   unsigned long gpnum;
+  2   unsigned long completed;
+
    + +

    RCU grace periods are numbered, and +the ->gpnum field contains the number of the grace +period that started most recently. +The ->completed field contains the number of the +grace period that completed most recently. +If the two fields are equal, the RCU grace period that most recently +started has already completed, and therefore the corresponding +flavor of RCU is idle. +If ->gpnum is one greater than ->completed, +then ->gpnum gives the number of the current RCU +grace period, which has not yet completed. +Any other combination of values indicates that something is broken. +These two fields are protected by the root rcu_node's +->lock field. + +

    There are ->gpnum and ->completed fields +in the rcu_node and rcu_data structures +as well. +The fields in the rcu_state structure represent the +most current values, and those of the other structures are compared +in order to detect the start of a new grace period in a distributed +fashion. +The values flow from rcu_state to rcu_node +(down the tree from the root to the leaves) to rcu_data. + +

    Miscellaneous
    + +

    This portion of the rcu_state structure is declared +as follows: + +

    +  1   unsigned long gp_max;
+  2   char abbr;
+  3   char *name;
+
    + +

    The ->gp_max field tracks the duration of the longest +grace period in jiffies. +It is protected by the root rcu_node's ->lock. + +

    The ->name field points to the name of the RCU flavor +(for example, “rcu_sched”), and is constant. +The ->abbr field contains a one-character abbreviation, +for example, “s” for RCU-sched. + +

    +The rcu_node Structure

    + +

    The rcu_node structures form the combining +tree that propagates quiescent-state +information from the leaves to the root and also that propagates +grace-period information from the root down to the leaves. +They provides local copies of the grace-period state in order +to allow this information to be accessed in a synchronized +manner without suffering the scalability limitations that +would otherwise be imposed by global locking. +In CONFIG_PREEMPT_RCU kernels, they manage the lists +of tasks that have blocked while in their current +RCU read-side critical section. +In CONFIG_PREEMPT_RCU with +CONFIG_RCU_BOOST, they manage the +per-rcu_node priority-boosting +kernel threads (kthreads) and state. +Finally, they record CPU-hotplug state in order to determine +which CPUs should be ignored during a given grace period. + +

    The rcu_node structure's fields are discussed, +singly and in groups, in the following sections. + +

    Connection to Combining Tree
    + +

    This portion of the rcu_node structure is declared +as follows: + +

    +  1   struct rcu_node *parent;
+  2   u8 level;
+  3   u8 grpnum;
+  4   unsigned long grpmask;
+  5   int grplo;
+  6   int grphi;
+
    + +

    The ->parent pointer references the rcu_node +one level up in the tree, and is NULL for the root +rcu_node. +The RCU implementation makes heavy use of this field to push quiescent +states up the tree. +The ->level field gives the level in the tree, with +the root being at level zero, its children at level one, and so on. +The ->grpnum field gives this node's position within +the children of its parent, so this number can range between 0 and 31 +on 32-bit systems and between 0 and 63 on 64-bit systems. +The ->level and ->grpnum fields are +used only during initialization and for tracing. +The ->grpmask field is the bitmask counterpart of +->grpnum, and therefore always has exactly one bit set. +This mask is used to clear the bit corresponding to this rcu_node +structure in its parent's bitmasks, which are described later. +Finally, the ->grplo and ->grphi fields +contain the lowest and highest numbered CPU served by this +rcu_node structure, respectively. + +

    All of these fields are constant, and thus do not require any +synchronization. + +

    Synchronization
    + +

    This field of the rcu_node structure is declared +as follows: + +

    +  1   raw_spinlock_t lock;
+
    + +

    This field is used to protect the remaining fields in this structure, +unless otherwise stated. +That said, all of the fields in this structure can be accessed without +locking for tracing purposes. +Yes, this can result in confusing traces, but better some tracing confusion +than to be heisenbugged out of existence. + +

    Grace-Period Tracking
    + +

    This portion of the rcu_node structure is declared +as follows: + +

    +  1   unsigned long gpnum;
+  2   unsigned long completed;
+
    + +

    These fields are the counterparts of the fields of the same name in +the rcu_state structure. +They each may lag up to one behind their rcu_state +counterparts. +If a given rcu_node structure's ->gpnum and +->complete fields are equal, then this rcu_node +structure believes that RCU is idle. +Otherwise, as with the rcu_state structure, +the ->gpnum field will be one greater than the +->complete fields, with ->gpnum +indicating which grace period this rcu_node believes +is still being waited for. + +

    The >gpnum field of each rcu_node +structure is updated at the beginning +of each grace period, and the ->completed fields are +updated at the end of each grace period. + +

    Quiescent-State Tracking
    + +

    These fields manage the propagation of quiescent states up the +combining tree. + +

    This portion of the rcu_node structure has fields +as follows: + +

    +  1   unsigned long qsmask;
+  2   unsigned long expmask;
+  3   unsigned long qsmaskinit;
+  4   unsigned long expmaskinit;
+
    + +

    The ->qsmask field tracks which of this +rcu_node structure's children still need to report +quiescent states for the current normal grace period. +Such children will have a value of 1 in their corresponding bit. +Note that the leaf rcu_node structures should be +thought of as having rcu_data structures as their +children. +Similarly, the ->expmask field tracks which +of this rcu_node structure's children still need to report +quiescent states for the current expedited grace period. +An expedited grace period has +the same conceptual properties as a normal grace period, but the +expedited implementation accepts extreme CPU overhead to obtain +much lower grace-period latency, for example, consuming a few +tens of microseconds worth of CPU time to reduce grace-period +duration from milliseconds to tens of microseconds. +The ->qsmaskinit field tracks which of this +rcu_node structure's children cover for at least +one online CPU. +This mask is used to initialize ->qsmask, +and ->expmaskinit is used to initialize +->expmask and the beginning of the +normal and expedited grace periods, respectively. + + + + + + + + +
     
    Quick Quiz:
    + Why are these bitmasks protected by locking? + Come on, haven't you heard of atomic instructions??? +
    Answer:
    + Lockless grace-period computation! Such a tantalizing possibility! + + +

    But consider the following sequence of events: + + +

      +
    1. CPU 0 has been in dyntick-idle + mode for quite some time. + When it wakes up, it notices that the current RCU + grace period needs it to report in, so it sets a + flag where the scheduling clock interrupt will find it. +

      +

    2. Meanwhile, CPU 1 is running + force_quiescent_state(), + and notices that CPU 0 has been in dyntick idle mode, + which qualifies as an extended quiescent state. +

      +

    3. CPU 0's scheduling clock + interrupt fires in the + middle of an RCU read-side critical section, and notices + that the RCU core needs something, so commences RCU softirq + processing. + +

      +

    4. CPU 0's softirq handler + executes and is just about ready + to report its quiescent state up the rcu_node + tree. +

      +

    5. But CPU 1 beats it to the punch, + completing the current + grace period and starting a new one. +

      +

    6. CPU 0 now reports its quiescent + state for the wrong + grace period. + That grace period might now end before the RCU read-side + critical section. + If that happens, disaster will ensue. + +
    + +

    So the locking is absolutely required in + order to coordinate + clearing of the bits with the grace-period numbers in + ->gpnum and ->completed. +

     
    + +

    Blocked-Task Management
    + +

    PREEMPT_RCU allows tasks to be preempted in the +midst of their RCU read-side critical sections, and these tasks +must be tracked explicitly. +The details of exactly why and how they are tracked will be covered +in a separate article on RCU read-side processing. +For now, it is enough to know that the rcu_node +structure tracks them. + +

    +  1   struct list_head blkd_tasks;
+  2   struct list_head *gp_tasks;
+  3   struct list_head *exp_tasks;
+  4   bool wait_blkd_tasks;
+
    + +

    The ->blkd_tasks field is a list header for +the list of blocked and preempted tasks. +As tasks undergo context switches within RCU read-side critical +sections, their task_struct structures are enqueued +(via the task_struct's ->rcu_node_entry +field) onto the head of the ->blkd_tasks list for the +leaf rcu_node structure corresponding to the CPU +on which the outgoing context switch executed. +As these tasks later exit their RCU read-side critical sections, +they remove themselves from the list. +This list is therefore in reverse time order, so that if one of the tasks +is blocking the current grace period, all subsequent tasks must +also be blocking that same grace period. +Therefore, a single pointer into this list suffices to track +all tasks blocking a given grace period. +That pointer is stored in ->gp_tasks for normal +grace periods and in ->exp_tasks for expedited +grace periods. +These last two fields are NULL if either there is +no grace period in flight or if there are no blocked tasks +preventing that grace period from completing. +If either of these two pointers is referencing a task that +removes itself from the ->blkd_tasks list, +then that task must advance the pointer to the next task on +the list, or set the pointer to NULL if there +are no subsequent tasks on the list. + +

    For example, suppose that tasks T1, T2, and T3 are +all hard-affinitied to the largest-numbered CPU in the system. +Then if task T1 blocked in an RCU read-side +critical section, then an expedited grace period started, +then task T2 blocked in an RCU read-side critical section, +then a normal grace period started, and finally task 3 blocked +in an RCU read-side critical section, then the state of the +last leaf rcu_node structure's blocked-task list +would be as shown below: + +

    blkd_task.svg + +

    Task T1 is blocking both grace periods, task T2 is +blocking only the normal grace period, and task T3 is blocking +neither grace period. +Note that these tasks will not remove themselves from this list +immediately upon resuming execution. +They will instead remain on the list until they execute the outermost +rcu_read_unlock() that ends their RCU read-side critical +section. + +

    +The ->wait_blkd_tasks field indicates whether or not +the current grace period is waiting on a blocked task. + +

    Sizing the rcu_node Array
    + +

    The rcu_node array is sized via a series of +C-preprocessor expressions as follows: + +

    + 1 #ifdef CONFIG_RCU_FANOUT
+ 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
+ 3 #else
+ 4 # ifdef CONFIG_64BIT
+ 5 # define RCU_FANOUT 64
+ 6 # else
+ 7 # define RCU_FANOUT 32
+ 8 # endif
+ 9 #endif
+10
+11 #ifdef CONFIG_RCU_FANOUT_LEAF
+12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
+13 #else
+14 # ifdef CONFIG_64BIT
+15 # define RCU_FANOUT_LEAF 64
+16 # else
+17 # define RCU_FANOUT_LEAF 32
+18 # endif
+19 #endif
+20
+21 #define RCU_FANOUT_1        (RCU_FANOUT_LEAF)
+22 #define RCU_FANOUT_2        (RCU_FANOUT_1 * RCU_FANOUT)
+23 #define RCU_FANOUT_3        (RCU_FANOUT_2 * RCU_FANOUT)
+24 #define RCU_FANOUT_4        (RCU_FANOUT_3 * RCU_FANOUT)
+25
+26 #if NR_CPUS <= RCU_FANOUT_1
+27 #  define RCU_NUM_LVLS        1
+28 #  define NUM_RCU_LVL_0        1
+29 #  define NUM_RCU_NODES        NUM_RCU_LVL_0
+30 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
+31 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
+32 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
+33 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0" }
+34 #elif NR_CPUS <= RCU_FANOUT_2
+35 #  define RCU_NUM_LVLS        2
+36 #  define NUM_RCU_LVL_0        1
+37 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+38 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
+39 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+40 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
+41 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+42 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1" }
+43 #elif NR_CPUS <= RCU_FANOUT_3
+44 #  define RCU_NUM_LVLS        3
+45 #  define NUM_RCU_LVL_0        1
+46 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+47 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+48 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
+49 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+50 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+51 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+52 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
+53 #elif NR_CPUS <= RCU_FANOUT_4
+54 #  define RCU_NUM_LVLS        4
+55 #  define NUM_RCU_LVL_0        1
+56 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
+57 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+58 #  define NUM_RCU_LVL_3        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+59 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+60 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+61 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+62 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+63 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
+64 #else
+65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+66 #endif
+
    + +

    The maximum number of levels in the rcu_node structure +is currently limited to four, as specified by lines 21-24 +and the structure of the subsequent “if” statement. +For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which +should be sufficient for the next few years at least. +For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which +should see us through the next decade or so. +This four-level tree also allows kernels built with +CONFIG_RCU_FANOUT=8 to support up to 4096 CPUs, +which might be useful in very large systems having eight CPUs per +socket (but please note that no one has yet shown any measurable +performance degradation due to misaligned socket and rcu_node +boundaries). +In addition, building kernels with a full four levels of rcu_node +tree permits better testing of RCU's combining-tree code. + +

    The RCU_FANOUT symbol controls how many children +are permitted at each non-leaf level of the rcu_node tree. +If the CONFIG_RCU_FANOUT Kconfig option is not specified, +it is set based on the word size of the system, which is also +the Kconfig default. + +

    The RCU_FANOUT_LEAF symbol controls how many CPUs are +handled by each leaf rcu_node structure. +Experience has shown that allowing a given leaf rcu_node +structure to handle 64 CPUs, as permitted by the number of bits in +the ->qsmask field on a 64-bit system, results in +excessive contention for the leaf rcu_node structures' +->lock fields. +The number of CPUs per leaf rcu_node structure is therefore +limited to 16 given the default value of CONFIG_RCU_FANOUT_LEAF. +If CONFIG_RCU_FANOUT_LEAF is unspecified, the value +selected is based on the word size of the system, just as for +CONFIG_RCU_FANOUT. +Lines 11-19 perform this computation. + +

    Lines 21-24 compute the maximum number of CPUs supported by +a single-level (which contains a single rcu_node structure), +two-level, three-level, and four-level rcu_node tree, +respectively, given the fanout specified by RCU_FANOUT +and RCU_FANOUT_LEAF. +These numbers of CPUs are retained in the +RCU_FANOUT_1, +RCU_FANOUT_2, +RCU_FANOUT_3, and +RCU_FANOUT_4 +C-preprocessor variables, respectively. + +

    These variables are used to control the C-preprocessor #if +statement spanning lines 26-66 that computes the number of +rcu_node structures required for each level of the tree, +as well as the number of levels required. +The number of levels is placed in the NUM_RCU_LVLS +C-preprocessor variable by lines 27, 35, 44, and 54. +The number of rcu_node structures for the topmost level +of the tree is always exactly one, and this value is unconditionally +placed into NUM_RCU_LVL_0 by lines 28, 36, 45, and 55. +The rest of the levels (if any) of the rcu_node tree +are computed by dividing the maximum number of CPUs by the +fanout supported by the number of levels from the current level down, +rounding up. This computation is performed by lines 37, +46-47, and 56-58. +Lines 31-33, 40-42, 50-52, and 62-63 create initializers +for lockdep lock-class names. +Finally, lines 64-66 produce an error if the maximum number of +CPUs is too large for the specified fanout. + +

    +The rcu_data Structure

    + +

    The rcu_data maintains the per-CPU state for the +corresponding flavor of RCU. +The fields in this structure may be accessed only from the corresponding +CPU (and from tracing) unless otherwise stated. +This structure is the +focus of quiescent-state detection and RCU callback queuing. +It also tracks its relationship to the corresponding leaf +rcu_node structure to allow more-efficient +propagation of quiescent states up the rcu_node +combining tree. +Like the rcu_node structure, it provides a local +copy of the grace-period information to allow for-free +synchronized +access to this information from the corresponding CPU. +Finally, this structure records past dyntick-idle state +for the corresponding CPU and also tracks statistics. + +

    The rcu_data structure's fields are discussed, +singly and in groups, in the following sections. + +

    Connection to Other Data Structures
    + +

    This portion of the rcu_data structure is declared +as follows: + +

    +  1   int cpu;
+  2   struct rcu_state *rsp;
+  3   struct rcu_node *mynode;
+  4   struct rcu_dynticks *dynticks;
+  5   unsigned long grpmask;
+  6   bool beenonline;
+
    + +

    The ->cpu field contains the number of the +corresponding CPU, the ->rsp pointer references +the corresponding rcu_state structure (and is most frequently +used to locate the name of the corresponding flavor of RCU for tracing), +and the ->mynode field references the corresponding +rcu_node structure. +The ->mynode is used to propagate quiescent states +up the combining tree. +

    The ->dynticks pointer references the +rcu_dynticks structure corresponding to this +CPU. +Recall that a single per-CPU instance of the rcu_dynticks +structure is shared among all flavors of RCU. +These first four fields are constant and therefore require not +synchronization. + +

    The ->grpmask field indicates the bit in +the ->mynode->qsmask corresponding to this +rcu_data structure, and is also used when propagating +quiescent states. +The ->beenonline flag is set whenever the corresponding +CPU comes online, which means that the debugfs tracing need not dump +out any rcu_data structure for which this flag is not set. + +

    Quiescent-State and Grace-Period Tracking
    + +

    This portion of the rcu_data structure is declared +as follows: + +

    +  1   unsigned long completed;
+  2   unsigned long gpnum;
+  3   bool cpu_no_qs;
+  4   bool core_needs_qs;
+  5   bool gpwrap;
+  6   unsigned long rcu_qs_ctr_snap;
+
    + +

    The completed and gpnum +fields are the counterparts of the fields of the same name +in the rcu_state and rcu_node structures. +They may each lag up to one behind their rcu_node +counterparts, but in CONFIG_NO_HZ_IDLE and +CONFIG_NO_HZ_FULL kernels can lag +arbitrarily far behind for CPUs in dyntick-idle mode (but these counters +will catch up upon exit from dyntick-idle mode). +If a given rcu_data structure's ->gpnum and +->complete fields are equal, then this rcu_data +structure believes that RCU is idle. +Otherwise, as with the rcu_state and rcu_node +structure, +the ->gpnum field will be one greater than the +->complete fields, with ->gpnum +indicating which grace period this rcu_data believes +is still being waited for. + + + + + + + + +
     
    Quick Quiz:
    + All this replication of the grace period numbers can only cause + massive confusion. + Why not just keep a global pair of counters and be done with it??? +
    Answer:
    + Because if there was only a single global pair of grace-period + numbers, there would need to be a single global lock to allow + safely accessing and updating them. + And if we are not going to have a single global lock, we need + to carefully manage the numbers on a per-node basis. + Recall from the answer to a previous Quick Quiz that the consequences + of applying a previously sampled quiescent state to the wrong + grace period are quite severe. +
     
    + +

    The ->cpu_no_qs flag indicates that the +CPU has not yet passed through a quiescent state, +while the ->core_needs_qs flag indicates that the +RCU core needs a quiescent state from the corresponding CPU. +The ->gpwrap field indicates that the corresponding +CPU has remained idle for so long that the completed +and gpnum counters are in danger of overflow, which +will cause the CPU to disregard the values of its counters on +its next exit from idle. +Finally, the rcu_qs_ctr_snap field is used to detect +cases where a given operation has resulted in a quiescent state +for all flavors of RCU, for example, cond_resched_rcu_qs(). + +

    RCU Callback Handling
    + +

    In the absence of CPU-hotplug events, RCU callbacks are invoked by +the same CPU that registered them. +This is strictly a cache-locality optimization: callbacks can and +do get invoked on CPUs other than the one that registered them. +After all, if the CPU that registered a given callback has gone +offline before the callback can be invoked, there really is no other +choice. + +

    This portion of the rcu_data structure is declared +as follows: + +

    + 1 struct rcu_head *nxtlist;
+ 2 struct rcu_head **nxttail[RCU_NEXT_SIZE];
+ 3 unsigned long nxtcompleted[RCU_NEXT_SIZE];
+ 4 long qlen_lazy;
+ 5 long qlen;
+ 6 long qlen_last_fqs_check;
+ 7 unsigned long n_force_qs_snap;
+ 8 unsigned long n_cbs_invoked;
+ 9 unsigned long n_cbs_orphaned;
+10 unsigned long n_cbs_adopted;
+11 long blimit;
+
    + +

    The ->nxtlist pointer and the +->nxttail[] array form a four-segment list with +older callbacks near the head and newer ones near the tail. +Each segment contains callbacks with the corresponding relationship +to the current grace period. +The pointer out of the end of each of the four segments is referenced +by the element of the ->nxttail[] array indexed by +RCU_DONE_TAIL (for callbacks handled by a prior grace period), +RCU_WAIT_TAIL (for callbacks waiting on the current grace period), +RCU_NEXT_READY_TAIL (for callbacks that will wait on the next +grace period), and +RCU_NEXT_TAIL (for callbacks that are not yet associated +with a specific grace period) +respectively, as shown in the following figure. + +

    nxtlist.svg + +

    In this figure, the ->nxtlist pointer references the +first +RCU callback in the list. +The ->nxttail[RCU_DONE_TAIL] array element references +the ->nxtlist pointer itself, indicating that none +of the callbacks is ready to invoke. +The ->nxttail[RCU_WAIT_TAIL] array element references callback +CB 2's ->next pointer, which indicates that +CB 1 and CB 2 are both waiting on the current grace period. +The ->nxttail[RCU_NEXT_READY_TAIL] array element +references the same RCU callback that ->nxttail[RCU_WAIT_TAIL] +does, which indicates that there are no callbacks waiting on the next +RCU grace period. +The ->nxttail[RCU_NEXT_TAIL] array element references +CB 4's ->next pointer, indicating that all the +remaining RCU callbacks have not yet been assigned to an RCU grace +period. +Note that the ->nxttail[RCU_NEXT_TAIL] array element +always references the last RCU callback's ->next pointer +unless the callback list is empty, in which case it references +the ->nxtlist pointer. + +

    CPUs advance their callbacks from the +RCU_NEXT_TAIL to the RCU_NEXT_READY_TAIL to the +RCU_WAIT_TAIL to the RCU_DONE_TAIL list segments +as grace periods advance. +The CPU advances the callbacks in its rcu_data structure +whenever it notices that another RCU grace period has completed. +The CPU detects the completion of an RCU grace period by noticing +that the value of its rcu_data structure's +->completed field differs from that of its leaf +rcu_node structure. +Recall that each rcu_node structure's +->completed field is updated at the end of each +grace period. + +

    The ->nxtcompleted[] array records grace-period +numbers corresponding to the list segments. +This allows CPUs that go idle for extended periods to determine +which of their callbacks are ready to be invoked after reawakening. + +

    The ->qlen counter contains the number of +callbacks in ->nxtlist, and the +->qlen_lazy contains the number of those callbacks that +are known to only free memory, and whose invocation can therefore +be safely deferred. +The ->qlen_last_fqs_check and +->n_force_qs_snap coordinate the forcing of quiescent +states from call_rcu() and friends when callback +lists grow excessively long. + +

    The ->n_cbs_invoked, +->n_cbs_orphaned, and ->n_cbs_adopted +fields count the number of callbacks invoked, +sent to other CPUs when this CPU goes offline, +and received from other CPUs when those other CPUs go offline. +Finally, the ->blimit counter is the maximum number of +RCU callbacks that may be invoked at a given time. + +

    Dyntick-Idle Handling
    + +

    This portion of the rcu_data structure is declared +as follows: + +

    +  1   int dynticks_snap;
+  2   unsigned long dynticks_fqs;
+
    + +The ->dynticks_snap field is used to take a snapshot +of the corresponding CPU's dyntick-idle state when forcing +quiescent states, and is therefore accessed from other CPUs. +Finally, the ->dynticks_fqs field is used to +count the number of times this CPU is determined to be in +dyntick-idle state, and is used for tracing and debugging purposes. + +

    +The rcu_dynticks Structure

    + +

    The rcu_dynticks maintains the per-CPU dyntick-idle state +for the corresponding CPU. +Unlike the other structures, rcu_dynticks is not +replicated over the different flavors of RCU. +The fields in this structure may be accessed only from the corresponding +CPU (and from tracing) unless otherwise stated. +Its fields are as follows: + +

    +  1   int dynticks_nesting;
+  2   int dynticks_nmi_nesting;
+  3   atomic_t dynticks;
+
    + +

    The ->dynticks_nesting field counts the +nesting depth of normal interrupts. +In addition, this counter is incremented when exiting dyntick-idle +mode and decremented when entering it. +This counter can therefore be thought of as counting the number +of reasons why this CPU cannot be permitted to enter dyntick-idle +mode, aside from non-maskable interrupts (NMIs). +NMIs are counted by the ->dynticks_nmi_nesting +field, except that NMIs that interrupt non-dyntick-idle execution +are not counted. + +

    Finally, the ->dynticks field counts the corresponding +CPU's transitions to and from dyntick-idle mode, so that this counter +has an even value when the CPU is in dyntick-idle mode and an odd +value otherwise. + + + + + + + + +
     
    Quick Quiz:
    + Why not just count all NMIs? + Wouldn't that be simpler and less error prone? +
    Answer:
    + It seems simpler only until you think hard about how to go about + updating the rcu_dynticks structure's + ->dynticks field. +
     
    + +

    Additional fields are present for some special-purpose +builds, and are discussed separately. + +

    +The rcu_head Structure

    + +

    Each rcu_head structure represents an RCU callback. +These structures are normally embedded within RCU-protected data +structures whose algorithms use asynchronous grace periods. +In contrast, when using algorithms that block waiting for RCU grace periods, +RCU users need not provide rcu_head structures. + +

    The rcu_head structure has fields as follows: + +

    +  1   struct rcu_head *next;
+  2   void (*func)(struct rcu_head *head);
+
    + +

    The ->next field is used +to link the rcu_head structures together in the +lists within the rcu_data structures. +The ->func field is a pointer to the function +to be called when the callback is ready to be invoked, and +this function is passed a pointer to the rcu_head +structure. +However, kfree_rcu() uses the ->func +field to record the offset of the rcu_head +structure within the enclosing RCU-protected data structure. + +

    Both of these fields are used internally by RCU. +From the viewpoint of RCU users, this structure is an +opaque “cookie”. + + + + + + + + +
     
    Quick Quiz:
    + Given that the callback function ->func + is passed a pointer to the rcu_head structure, + how is that function supposed to find the beginning of the + enclosing RCU-protected data structure? +
    Answer:
    + In actual practice, there is a separate callback function per + type of RCU-protected data structure. + The callback function can therefore use the container_of() + macro in the Linux kernel (or other pointer-manipulation facilities + in other software environments) to find the beginning of the + enclosing structure. +
     
    + +

    +RCU-Specific Fields in the task_struct Structure

    + +

    The CONFIG_PREEMPT_RCU implementation uses some +additional fields in the task_struct structure: + +

    + 1 #ifdef CONFIG_PREEMPT_RCU
+ 2   int rcu_read_lock_nesting;
+ 3   union rcu_special rcu_read_unlock_special;
+ 4   struct list_head rcu_node_entry;
+ 5   struct rcu_node *rcu_blocked_node;
+ 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
+ 7 #ifdef CONFIG_TASKS_RCU
+ 8   unsigned long rcu_tasks_nvcsw;
+ 9   bool rcu_tasks_holdout;
+10   struct list_head rcu_tasks_holdout_list;
+11   int rcu_tasks_idle_cpu;
+12 #endif /* #ifdef CONFIG_TASKS_RCU */
+
    + +

    The ->rcu_read_lock_nesting field records the +nesting level for RCU read-side critical sections, and +the ->rcu_read_unlock_special field is a bitmask +that records special conditions that require rcu_read_unlock() +to do additional work. +The ->rcu_node_entry field is used to form lists of +tasks that have blocked within preemptible-RCU read-side critical +sections and the ->rcu_blocked_node field references +the rcu_node structure whose list this task is a member of, +or NULL if it is not blocked within a preemptible-RCU +read-side critical section. + +

    The ->rcu_tasks_nvcsw field tracks the number of +voluntary context switches that this task had undergone at the +beginning of the current tasks-RCU grace period, +->rcu_tasks_holdout is set if the current tasks-RCU +grace period is waiting on this task, ->rcu_tasks_holdout_list +is a list element enqueuing this task on the holdout list, +and ->rcu_tasks_idle_cpu tracks which CPU this +idle task is running, but only if the task is currently running, +that is, if the CPU is currently idle. + +

    +Accessor Functions

    + +

    The following listing shows the +rcu_get_root(), rcu_for_each_node_breadth_first, +rcu_for_each_nonleaf_node_breadth_first(), and +rcu_for_each_leaf_node() function and macros: + +

    +  1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+  2 {
+  3   return &rsp->node[0];
+  4 }
+  5
+  6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
+  7   for ((rnp) = &(rsp)->node[0]; \
+  8        (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+  9
+ 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
+ 11   for ((rnp) = &(rsp)->node[0]; \
+ 12        (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++)
+ 13
+ 14 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 15   for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
+ 16        (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+
    + +

    The rcu_get_root() simply returns a pointer to the +first element of the specified rcu_state structure's +->node[] array, which is the root rcu_node +structure. + +

    As noted earlier, the rcu_for_each_node_breadth_first() +macro takes advantage of the layout of the rcu_node +structures in the rcu_state structure's +->node[] array, performing a breadth-first traversal by +simply traversing the array in order. +The rcu_for_each_nonleaf_node_breadth_first() macro operates +similarly, but traverses only the first part of the array, thus excluding +the leaf rcu_node structures. +Finally, the rcu_for_each_leaf_node() macro traverses only +the last part of the array, thus traversing only the leaf +rcu_node structures. + + + + + + + + +
     
    Quick Quiz:
    + What do rcu_for_each_nonleaf_node_breadth_first() and + rcu_for_each_leaf_node() do if the rcu_node tree + contains only a single node? +
    Answer:
    + In the single-node case, + rcu_for_each_nonleaf_node_breadth_first() is a no-op + and rcu_for_each_leaf_node() traverses the single node. +
     
    + +

    +Summary

    + +So each flavor of RCU is represented by an rcu_state structure, +which contains a combining tree of rcu_node and +rcu_data structures. +Finally, in CONFIG_NO_HZ_IDLE kernels, each CPU's dyntick-idle +state is tracked by an rcu_dynticks structure. + +If you made it this far, you are well prepared to read the code +walkthroughs in the other articles in this series. + +

    +Acknowledgments

    + +I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul +Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn +for helping me get this document into a more human-readable state. + +

    +Legal Statement

    + +

    This work represents the view of the author and does not necessarily +represent the view of IBM. + +

    Linux is a registered trademark of Linus Torvalds. + +

    Other company, product, and service names may be trademarks or +service marks of others. + + diff --git a/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg b/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg new file mode 100644 index 000000000000..2bf12b468206 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg @@ -0,0 +1,939 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + rcu_node + + struct + + struct + + rcu_node + + struct + + rcu_node + + rcu_node + + struct + + rcu_node + + struct + + struct + + rcu_node + + CPU 0 + + struct + + rcu_data + + CPU 15 + + struct + + rcu_data + + struct + + rcu_data + + CPU 21823 + + CPU 21839 + + rcu_data + + struct + + struct + + rcu_data + + CPU 43679 + + CPU 43695 + + rcu_data + + struct + + struct + + rcu_data + + CPU 65519 + + CPU 65535 + + rcu_data + + struct + + struct rcu_state + + struct + + rcu_node + + diff --git a/Documentation/RCU/Design/Data-Structures/TreeLevel.svg b/Documentation/RCU/Design/Data-Structures/TreeLevel.svg new file mode 100644 index 000000000000..7a7eb3bac95c --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/TreeLevel.svg @@ -0,0 +1,828 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + rcu_node + + struct + + struct + + rcu_node + + struct + + rcu_node + + rcu_node + + struct + + rcu_node + + struct + + struct + + rcu_node + + ->level[0] + + ->level[1] + + ->level[2] + + struct + + rcu_node + + CPU 15 + + CPU 0 + + CPU 65535 + + CPU 65519 + + CPU 43695 + + CPU 43679 + + CPU 21839 + + CPU 21823 + + struct rcu_state + + diff --git a/Documentation/RCU/Design/Data-Structures/TreeMapping.svg b/Documentation/RCU/Design/Data-Structures/TreeMapping.svg new file mode 100644 index 000000000000..729cfa9e6cdb --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/TreeMapping.svg @@ -0,0 +1,305 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 0:7 + + 4:7 + + 0:1 + + 2:3 + + 4:5 + + 6:7 + + 0:3 + + struct rcu_state + + diff --git a/Documentation/RCU/Design/Data-Structures/TreeMappingLevel.svg b/Documentation/RCU/Design/Data-Structures/TreeMappingLevel.svg new file mode 100644 index 000000000000..5b416a4b8453 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/TreeMappingLevel.svg @@ -0,0 +1,380 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ->level[0] + + ->level[1] + + ->level[2] + + 0:7 + + 4:7 + + 0:1 + + 2:3 + + 4:5 + + 6:7 + + 0:3 + + struct rcu_state + + + + + + + + + diff --git a/Documentation/RCU/Design/Data-Structures/blkd_task.svg b/Documentation/RCU/Design/Data-Structures/blkd_task.svg new file mode 100644 index 000000000000..00e810bb8419 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/blkd_task.svg @@ -0,0 +1,843 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + rcu_bh + + struct + + rcu_node + + struct + + rcu_node + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct + + rcu_data + + struct rcu_state + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + struct + + rcu_dynticks + + rcu_sched + + T3 + + T2 + + T1 + + + + + + + + + + + + + rcu_node + + struct + + blkd_tasks + + gp_tasks + + exp_tasks + + diff --git a/Documentation/RCU/Design/Data-Structures/nxtlist.svg b/Documentation/RCU/Design/Data-Structures/nxtlist.svg new file mode 100644 index 000000000000..abc4cc73a097 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/nxtlist.svg @@ -0,0 +1,396 @@ + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + nxtlist + + nxttail[RCU_DONE_TAIL] + + nxttail[RCU_WAIT_TAIL] + + nxttail[RCU_NEXT_READY_TAIL] + + nxttail[RCU_NEXT_TAIL] + + CB 1 + + next + + CB 3 + + next + + CB 4 + + next + + CB 2 + + next + + -- cgit 1.4.1