srcu: Remove Classic SRCU

Classic SRCU was only ever intended to be a fallback in case of issues
with Tree/Tiny SRCU, and the latter two are doing quite well in testing.
This commit therefore removes Classic SRCU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

4 files changed, 0 insertions, 702 deletions

diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
index 23803c7d5180..3945337c8ce4 100644
--- a/kernel/rcu/Makefile
+++ b/kernel/rcu/Makefile
@@ -3,7 +3,6 @@
 KCOV_INSTRUMENT := n
 
 obj-y += update.o sync.o
-obj-$(CONFIG_CLASSIC_SRCU) += srcu.o
 obj-$(CONFIG_TREE_SRCU) += srcutree.o
 obj-$(CONFIG_TINY_SRCU) += srcutiny.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 27f871c88e0a..d06c42deee0b 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -457,22 +457,6 @@ void srcutorture_get_gp_data(enum rcutorture_type test_type,
 			     struct srcu_struct *sp, int *flags,
 			     unsigned long *gpnum, unsigned long *completed);
 
-#elif defined(CONFIG_CLASSIC_SRCU)
-
-static inline void srcutorture_get_gp_data(enum rcutorture_type test_type,
-					   struct srcu_struct *sp, int *flags,
-					   unsigned long *gpnum,
-					   unsigned long *completed)
-{
-	if (test_type != SRCU_FLAVOR)
-		return;
-	*flags = 0;
-	*completed = sp->completed;
-	*gpnum = *completed;
-	if (sp->batch_queue.head || sp->batch_check0.head || sp->batch_check1.head)
-		(*gpnum)++;
-}
-
 #endif
 
 #ifdef CONFIG_TINY_RCU
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 03cdf79e73d4..b8f7f8ce8575 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -564,31 +564,19 @@ static void srcu_torture_stats(void)
 	int __maybe_unused cpu;
 	int idx;
 
-#if defined(CONFIG_TREE_SRCU) || defined(CONFIG_CLASSIC_SRCU)
 #ifdef CONFIG_TREE_SRCU
 	idx = srcu_ctlp->srcu_idx & 0x1;
-#else /* #ifdef CONFIG_TREE_SRCU */
-	idx = srcu_ctlp->completed & 0x1;
-#endif /* #else #ifdef CONFIG_TREE_SRCU */
 	pr_alert("%s%s Tree SRCU per-CPU(idx=%d):",
 		 torture_type, TORTURE_FLAG, idx);
 	for_each_possible_cpu(cpu) {
 		unsigned long l0, l1;
 		unsigned long u0, u1;
 		long c0, c1;
-#ifdef CONFIG_TREE_SRCU
 		struct srcu_data *counts;
 
 		counts = per_cpu_ptr(srcu_ctlp->sda, cpu);
 		u0 = counts->srcu_unlock_count[!idx];
 		u1 = counts->srcu_unlock_count[idx];
-#else /* #ifdef CONFIG_TREE_SRCU */
-		struct srcu_array *counts;
-
-		counts = per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
-		u0 = counts->unlock_count[!idx];
-		u1 = counts->unlock_count[idx];
-#endif /* #else #ifdef CONFIG_TREE_SRCU */
 
 		/*
 		 * Make sure that a lock is always counted if the corresponding
@@ -596,13 +584,8 @@ static void srcu_torture_stats(void)
 		 */
 		smp_rmb();
 
-#ifdef CONFIG_TREE_SRCU
 		l0 = counts->srcu_lock_count[!idx];
 		l1 = counts->srcu_lock_count[idx];
-#else /* #ifdef CONFIG_TREE_SRCU */
-		l0 = counts->lock_count[!idx];
-		l1 = counts->lock_count[idx];
-#endif /* #else #ifdef CONFIG_TREE_SRCU */
 
 		c0 = l0 - u0;
 		c1 = l1 - u1;
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
deleted file mode 100644
index 4e3f558409a0..000000000000
--- a/kernel/rcu/srcu.c
+++ /dev/null
@@ -1,668 +0,0 @@
-/*
- * Sleepable Read-Copy Update mechanism for mutual exclusion.
- *
- * 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) IBM Corporation, 2006
- * Copyright (C) Fujitsu, 2012
- *
- * Author: Paul McKenney <paulmck@us.ibm.com>
- *	   Lai Jiangshan <laijs@cn.fujitsu.com>
- *
- * For detailed explanation of Read-Copy Update mechanism see -
- *		Documentation/RCU/ *.txt
- *
- */
-
-#include <linux/export.h>
-#include <linux/mutex.h>
-#include <linux/percpu.h>
-#include <linux/preempt.h>
-#include <linux/rcupdate_wait.h>
-#include <linux/sched.h>
-#include <linux/smp.h>
-#include <linux/delay.h>
-#include <linux/srcu.h>
-
-#include "rcu.h"
-
-/*
- * Initialize an rcu_batch structure to empty.
- */
-static inline void rcu_batch_init(struct rcu_batch *b)
-{
-	b->head = NULL;
-	b->tail = &b->head;
-}
-
-/*
- * Enqueue a callback onto the tail of the specified rcu_batch structure.
- */
-static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
-{
-	*b->tail = head;
-	b->tail = &head->next;
-}
-
-/*
- * Is the specified rcu_batch structure empty?
- */
-static inline bool rcu_batch_empty(struct rcu_batch *b)
-{
-	return b->tail == &b->head;
-}
-
-/*
- * Remove the callback at the head of the specified rcu_batch structure
- * and return a pointer to it, or return NULL if the structure is empty.
- */
-static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
-{
-	struct rcu_head *head;
-
-	if (rcu_batch_empty(b))
-		return NULL;
-
-	head = b->head;
-	b->head = head->next;
-	if (b->tail == &head->next)
-		rcu_batch_init(b);
-
-	return head;
-}
-
-/*
- * Move all callbacks from the rcu_batch structure specified by "from" to
- * the structure specified by "to".
- */
-static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
-{
-	if (!rcu_batch_empty(from)) {
-		*to->tail = from->head;
-		to->tail = from->tail;
-		rcu_batch_init(from);
-	}
-}
-
-static int init_srcu_struct_fields(struct srcu_struct *sp)
-{
-	sp->completed = 0;
-	spin_lock_init(&sp->queue_lock);
-	sp->running = false;
-	rcu_batch_init(&sp->batch_queue);
-	rcu_batch_init(&sp->batch_check0);
-	rcu_batch_init(&sp->batch_check1);
-	rcu_batch_init(&sp->batch_done);
-	INIT_DELAYED_WORK(&sp->work, process_srcu);
-	sp->per_cpu_ref = alloc_percpu(struct srcu_array);
-	return sp->per_cpu_ref ? 0 : -ENOMEM;
-}
-
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
-
-int __init_srcu_struct(struct srcu_struct *sp, const char *name,
-		       struct lock_class_key *key)
-{
-	/* Don't re-initialize a lock while it is held. */
-	debug_check_no_locks_freed((void *)sp, sizeof(*sp));
-	lockdep_init_map(&sp->dep_map, name, key, 0);
-	return init_srcu_struct_fields(sp);
-}
-EXPORT_SYMBOL_GPL(__init_srcu_struct);
-
-#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
-
-/**
- * init_srcu_struct - initialize a sleep-RCU structure
- * @sp: structure to initialize.
- *
- * Must invoke this on a given srcu_struct before passing that srcu_struct
- * to any other function.  Each srcu_struct represents a separate domain
- * of SRCU protection.
- */
-int init_srcu_struct(struct srcu_struct *sp)
-{
-	return init_srcu_struct_fields(sp);
-}
-EXPORT_SYMBOL_GPL(init_srcu_struct);
-
-#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
-
-/*
- * Returns approximate total of the readers' ->lock_count[] values for the
- * rank of per-CPU counters specified by idx.
- */
-static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
-{
-	int cpu;
-	unsigned long sum = 0;
-
-	for_each_possible_cpu(cpu) {
-		struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
-
-		sum += READ_ONCE(cpuc->lock_count[idx]);
-	}
-	return sum;
-}
-
-/*
- * Returns approximate total of the readers' ->unlock_count[] values for the
- * rank of per-CPU counters specified by idx.
- */
-static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
-{
-	int cpu;
-	unsigned long sum = 0;
-
-	for_each_possible_cpu(cpu) {
-		struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
-
-		sum += READ_ONCE(cpuc->unlock_count[idx]);
-	}
-	return sum;
-}
-
-/*
- * Return true if the number of pre-existing readers is determined to
- * be zero.
- */
-static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
-{
-	unsigned long unlocks;
-
-	unlocks = srcu_readers_unlock_idx(sp, idx);
-
-	/*
-	 * Make sure that a lock is always counted if the corresponding unlock
-	 * is counted. Needs to be a smp_mb() as the read side may contain a
-	 * read from a variable that is written to before the synchronize_srcu()
-	 * in the write side. In this case smp_mb()s A and B act like the store
-	 * buffering pattern.
-	 *
-	 * This smp_mb() also pairs with smp_mb() C to prevent accesses after the
-	 * synchronize_srcu() from being executed before the grace period ends.
-	 */
-	smp_mb(); /* A */
-
-	/*
-	 * If the locks are the same as the unlocks, then there must have
-	 * been no readers on this index at some time in between. This does not
-	 * mean that there are no more readers, as one could have read the
-	 * current index but not have incremented the lock counter yet.
-	 *
-	 * Possible bug: There is no guarantee that there haven't been ULONG_MAX
-	 * increments of ->lock_count[] since the unlocks were counted, meaning
-	 * that this could return true even if there are still active readers.
-	 * Since there are no memory barriers around srcu_flip(), the CPU is not
-	 * required to increment ->completed before running
-	 * srcu_readers_unlock_idx(), which means that there could be an
-	 * arbitrarily large number of critical sections that execute after
-	 * srcu_readers_unlock_idx() but use the old value of ->completed.
-	 */
-	return srcu_readers_lock_idx(sp, idx) == unlocks;
-}
-
-/**
- * srcu_readers_active - returns true if there are readers. and false
- *                       otherwise
- * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
- *
- * Note that this is not an atomic primitive, and can therefore suffer
- * severe errors when invoked on an active srcu_struct.  That said, it
- * can be useful as an error check at cleanup time.
- */
-static bool srcu_readers_active(struct srcu_struct *sp)
-{
-	int cpu;
-	unsigned long sum = 0;
-
-	for_each_possible_cpu(cpu) {
-		struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
-
-		sum += READ_ONCE(cpuc->lock_count[0]);
-		sum += READ_ONCE(cpuc->lock_count[1]);
-		sum -= READ_ONCE(cpuc->unlock_count[0]);
-		sum -= READ_ONCE(cpuc->unlock_count[1]);
-	}
-	return sum;
-}
-
-/**
- * cleanup_srcu_struct - deconstruct a sleep-RCU structure
- * @sp: structure to clean up.
- *
- * Must invoke this only after you are finished using a given srcu_struct
- * that was initialized via init_srcu_struct().  This code does some
- * probabalistic checking, spotting late uses of srcu_read_lock(),
- * synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
- * If any such late uses are detected, the per-CPU memory associated with
- * the srcu_struct is simply leaked and WARN_ON() is invoked.  If the
- * caller frees the srcu_struct itself, a use-after-free crash will likely
- * ensue, but at least there will be a warning printed.
- */
-void cleanup_srcu_struct(struct srcu_struct *sp)
-{
-	if (WARN_ON(srcu_readers_active(sp)))
-		return; /* Leakage unless caller handles error. */
-	free_percpu(sp->per_cpu_ref);
-	sp->per_cpu_ref = NULL;
-}
-EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
-
-/*
- * Counts the new reader in the appropriate per-CPU element of the
- * srcu_struct.
- * Returns an index that must be passed to the matching srcu_read_unlock().
- */
-int __srcu_read_lock(struct srcu_struct *sp)
-{
-	int idx;
-
-	idx = READ_ONCE(sp->completed) & 0x1;
-	this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
-	smp_mb(); /* B */  /* Avoid leaking the critical section. */
-	return idx;
-}
-EXPORT_SYMBOL_GPL(__srcu_read_lock);
-
-/*
- * Removes the count for the old reader from the appropriate per-CPU
- * element of the srcu_struct.  Note that this may well be a different
- * CPU than that which was incremented by the corresponding srcu_read_lock().
- */
-void __srcu_read_unlock(struct srcu_struct *sp, int idx)
-{
-	smp_mb(); /* C */  /* Avoid leaking the critical section. */
-	this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
-}
-EXPORT_SYMBOL_GPL(__srcu_read_unlock);
-
-/*
- * We use an adaptive strategy for synchronize_srcu() and especially for
- * synchronize_srcu_expedited().  We spin for a fixed time period
- * (defined below) to allow SRCU readers to exit their read-side critical
- * sections.  If there are still some readers after 10 microseconds,
- * we repeatedly block for 1-millisecond time periods.  This approach
- * has done well in testing, so there is no need for a config parameter.
- */
-#define SRCU_RETRY_CHECK_DELAY		5
-#define SYNCHRONIZE_SRCU_TRYCOUNT	2
-#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT	12
-
-/*
- * @@@ Wait until all pre-existing readers complete.  Such readers
- * will have used the index specified by "idx".
- * the caller should ensures the ->completed is not changed while checking
- * and idx = (->completed & 1) ^ 1
- */
-static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
-{
-	for (;;) {
-		if (srcu_readers_active_idx_check(sp, idx))
-			return true;
-		if (--trycount <= 0)
-			return false;
-		udelay(SRCU_RETRY_CHECK_DELAY);
-	}
-}
-
-/*
- * Increment the ->completed counter so that future SRCU readers will
- * use the other rank of the ->(un)lock_count[] arrays.  This allows
- * us to wait for pre-existing readers in a starvation-free manner.
- */
-static void srcu_flip(struct srcu_struct *sp)
-{
-	WRITE_ONCE(sp->completed, sp->completed + 1);
-
-	/*
-	 * Ensure that if the updater misses an __srcu_read_unlock()
-	 * increment, that task's next __srcu_read_lock() will see the
-	 * above counter update.  Note that both this memory barrier
-	 * and the one in srcu_readers_active_idx_check() provide the
-	 * guarantee for __srcu_read_lock().
-	 */
-	smp_mb(); /* D */  /* Pairs with C. */
-}
-
-/*
- * Enqueue an SRCU callback on the specified srcu_struct structure,
- * initiating grace-period processing if it is not already running.
- *
- * Note that all CPUs must agree that the grace period extended beyond
- * all pre-existing SRCU read-side critical section.  On systems with
- * more than one CPU, this means that when "func()" is invoked, each CPU
- * is guaranteed to have executed a full memory barrier since the end of
- * its last corresponding SRCU read-side critical section whose beginning
- * preceded the call to call_rcu().  It also means that each CPU executing
- * an SRCU read-side critical section that continues beyond the start of
- * "func()" must have executed a memory barrier after the call_rcu()
- * but before the beginning of that SRCU read-side critical section.
- * Note that these guarantees include CPUs that are offline, idle, or
- * executing in user mode, as well as CPUs that are executing in the kernel.
- *
- * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
- * resulting SRCU callback function "func()", then both CPU A and CPU
- * B are guaranteed to execute a full memory barrier during the time
- * interval between the call to call_rcu() and the invocation of "func()".
- * This guarantee applies even if CPU A and CPU B are the same CPU (but
- * again only if the system has more than one CPU).
- *
- * Of course, these guarantees apply only for invocations of call_srcu(),
- * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
- * srcu_struct structure.
- */
-void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
-	       rcu_callback_t func)
-{
-	unsigned long flags;
-
-	head->next = NULL;
-	head->func = func;
-	spin_lock_irqsave(&sp->queue_lock, flags);
-	smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
-	rcu_batch_queue(&sp->batch_queue, head);
-	if (!sp->running) {
-		sp->running = true;
-		queue_delayed_work(system_power_efficient_wq, &sp->work, 0);
-	}
-	spin_unlock_irqrestore(&sp->queue_lock, flags);
-}
-EXPORT_SYMBOL_GPL(call_srcu);
-
-static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
-static void srcu_reschedule(struct srcu_struct *sp);
-
-/*
- * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
- */
-static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
-{
-	struct rcu_synchronize rcu;
-	struct rcu_head *head = &rcu.head;
-	bool done = false;
-
-	RCU_LOCKDEP_WARN(lock_is_held(&sp->dep_map) ||
-			 lock_is_held(&rcu_bh_lock_map) ||
-			 lock_is_held(&rcu_lock_map) ||
-			 lock_is_held(&rcu_sched_lock_map),
-			 "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
-
-	might_sleep();
-	init_completion(&rcu.completion);
-
-	head->next = NULL;
-	head->func = wakeme_after_rcu;
-	spin_lock_irq(&sp->queue_lock);
-	smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
-	if (!sp->running) {
-		/* steal the processing owner */
-		sp->running = true;
-		rcu_batch_queue(&sp->batch_check0, head);
-		spin_unlock_irq(&sp->queue_lock);
-
-		srcu_advance_batches(sp, trycount);
-		if (!rcu_batch_empty(&sp->batch_done)) {
-			BUG_ON(sp->batch_done.head != head);
-			rcu_batch_dequeue(&sp->batch_done);
-			done = true;
-		}
-		/* give the processing owner to work_struct */
-		srcu_reschedule(sp);
-	} else {
-		rcu_batch_queue(&sp->batch_queue, head);
-		spin_unlock_irq(&sp->queue_lock);
-	}
-
-	if (!done) {
-		wait_for_completion(&rcu.completion);
-		smp_mb(); /* Caller's later accesses after GP. */
-	}
-
-}
-
-/**
- * synchronize_srcu - wait for prior SRCU read-side critical-section completion
- * @sp: srcu_struct with which to synchronize.
- *
- * Wait for the count to drain to zero of both indexes. To avoid the
- * possible starvation of synchronize_srcu(), it waits for the count of
- * the index=((->completed & 1) ^ 1) to drain to zero at first,
- * and then flip the completed and wait for the count of the other index.
- *
- * Can block; must be called from process context.
- *
- * Note that it is illegal to call synchronize_srcu() from the corresponding
- * SRCU read-side critical section; doing so will result in deadlock.
- * However, it is perfectly legal to call synchronize_srcu() on one
- * srcu_struct from some other srcu_struct's read-side critical section,
- * as long as the resulting graph of srcu_structs is acyclic.
- *
- * There are memory-ordering constraints implied by synchronize_srcu().
- * On systems with more than one CPU, when synchronize_srcu() returns,
- * each CPU is guaranteed to have executed a full memory barrier since
- * the end of its last corresponding SRCU-sched read-side critical section
- * whose beginning preceded the call to synchronize_srcu().  In addition,
- * each CPU having an SRCU read-side critical section that extends beyond
- * the return from synchronize_srcu() is guaranteed to have executed a
- * full memory barrier after the beginning of synchronize_srcu() and before
- * the beginning of that SRCU read-side critical section.  Note that these
- * guarantees include CPUs that are offline, idle, or executing in user mode,
- * as well as CPUs that are executing in the kernel.
- *
- * Furthermore, if CPU A invoked synchronize_srcu(), which returned
- * to its caller on CPU B, then both CPU A and CPU B are guaranteed
- * to have executed a full memory barrier during the execution of
- * synchronize_srcu().  This guarantee applies even if CPU A and CPU B
- * are the same CPU, but again only if the system has more than one CPU.
- *
- * Of course, these memory-ordering guarantees apply only when
- * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
- * passed the same srcu_struct structure.
- */
-void synchronize_srcu(struct srcu_struct *sp)
-{
-	__synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
-			   ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
-			   : SYNCHRONIZE_SRCU_TRYCOUNT);
-}
-EXPORT_SYMBOL_GPL(synchronize_srcu);
-
-/**
- * synchronize_srcu_expedited - Brute-force SRCU grace period
- * @sp: srcu_struct with which to synchronize.
- *
- * Wait for an SRCU grace period to elapse, but be more aggressive about
- * spinning rather than blocking when waiting.
- *
- * Note that synchronize_srcu_expedited() has the same deadlock and
- * memory-ordering properties as does synchronize_srcu().
- */
-void synchronize_srcu_expedited(struct srcu_struct *sp)
-{
-	__synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
-}
-EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
-
-/**
- * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
- * @sp: srcu_struct on which to wait for in-flight callbacks.
- */
-void srcu_barrier(struct srcu_struct *sp)
-{
-	synchronize_srcu(sp);
-}
-EXPORT_SYMBOL_GPL(srcu_barrier);
-
-/**
- * srcu_batches_completed - return batches completed.
- * @sp: srcu_struct on which to report batch completion.
- *
- * Report the number of batches, correlated with, but not necessarily
- * precisely the same as, the number of grace periods that have elapsed.
- */
-unsigned long srcu_batches_completed(struct srcu_struct *sp)
-{
-	return sp->completed;
-}
-EXPORT_SYMBOL_GPL(srcu_batches_completed);
-
-#define SRCU_CALLBACK_BATCH	10
-#define SRCU_INTERVAL		1
-
-/*
- * Move any new SRCU callbacks to the first stage of the SRCU grace
- * period pipeline.
- */
-static void srcu_collect_new(struct srcu_struct *sp)
-{
-	if (!rcu_batch_empty(&sp->batch_queue)) {
-		spin_lock_irq(&sp->queue_lock);
-		rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
-		spin_unlock_irq(&sp->queue_lock);
-	}
-}
-
-/*
- * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
- * ->batch_check1 and then to ->batch_done as readers drain.
- */
-static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
-{
-	int idx = 1 ^ (sp->completed & 1);
-
-	/*
-	 * Because readers might be delayed for an extended period after
-	 * fetching ->completed for their index, at any point in time there
-	 * might well be readers using both idx=0 and idx=1.  We therefore
-	 * need to wait for readers to clear from both index values before
-	 * invoking a callback.
-	 */
-
-	if (rcu_batch_empty(&sp->batch_check0) &&
-	    rcu_batch_empty(&sp->batch_check1))
-		return; /* no callbacks need to be advanced */
-
-	if (!try_check_zero(sp, idx, trycount))
-		return; /* failed to advance, will try after SRCU_INTERVAL */
-
-	/*
-	 * The callbacks in ->batch_check1 have already done with their
-	 * first zero check and flip back when they were enqueued on
-	 * ->batch_check0 in a previous invocation of srcu_advance_batches().
-	 * (Presumably try_check_zero() returned false during that
-	 * invocation, leaving the callbacks stranded on ->batch_check1.)
-	 * They are therefore ready to invoke, so move them to ->batch_done.
-	 */
-	rcu_batch_move(&sp->batch_done, &sp->batch_check1);
-
-	if (rcu_batch_empty(&sp->batch_check0))
-		return; /* no callbacks need to be advanced */
-	srcu_flip(sp);
-
-	/*
-	 * The callbacks in ->batch_check0 just finished their
-	 * first check zero and flip, so move them to ->batch_check1
-	 * for future checking on the other idx.
-	 */
-	rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
-
-	/*
-	 * SRCU read-side critical sections are normally short, so check
-	 * at least twice in quick succession after a flip.
-	 */
-	trycount = trycount < 2 ? 2 : trycount;
-	if (!try_check_zero(sp, idx^1, trycount))
-		return; /* failed to advance, will try after SRCU_INTERVAL */
-
-	/*
-	 * The callbacks in ->batch_check1 have now waited for all
-	 * pre-existing readers using both idx values.  They are therefore
-	 * ready to invoke, so move them to ->batch_done.
-	 */
-	rcu_batch_move(&sp->batch_done, &sp->batch_check1);
-}
-
-/*
- * Invoke a limited number of SRCU callbacks that have passed through
- * their grace period.  If there are more to do, SRCU will reschedule
- * the workqueue.  Note that needed memory barriers have been executed
- * in this task's context by srcu_readers_active_idx_check().
- */
-static void srcu_invoke_callbacks(struct srcu_struct *sp)
-{
-	int i;
-	struct rcu_head *head;
-
-	for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
-		head = rcu_batch_dequeue(&sp->batch_done);
-		if (!head)
-			break;
-		local_bh_disable();
-		head->func(head);
-		local_bh_enable();
-	}
-}
-
-/*
- * Finished one round of SRCU grace period.  Start another if there are
- * more SRCU callbacks queued, otherwise put SRCU into not-running state.
- */
-static void srcu_reschedule(struct srcu_struct *sp)
-{
-	bool pending = true;
-
-	if (rcu_batch_empty(&sp->batch_done) &&
-	    rcu_batch_empty(&sp->batch_check1) &&
-	    rcu_batch_empty(&sp->batch_check0) &&
-	    rcu_batch_empty(&sp->batch_queue)) {
-		spin_lock_irq(&sp->queue_lock);
-		if (rcu_batch_empty(&sp->batch_done) &&
-		    rcu_batch_empty(&sp->batch_check1) &&
-		    rcu_batch_empty(&sp->batch_check0) &&
-		    rcu_batch_empty(&sp->batch_queue)) {
-			sp->running = false;
-			pending = false;
-		}
-		spin_unlock_irq(&sp->queue_lock);
-	}
-
-	if (pending)
-		queue_delayed_work(system_power_efficient_wq,
-				   &sp->work, SRCU_INTERVAL);
-}
-
-/*
- * This is the work-queue function that handles SRCU grace periods.
- */
-void process_srcu(struct work_struct *work)
-{
-	struct srcu_struct *sp;
-
-	sp = container_of(work, struct srcu_struct, work.work);
-
-	srcu_collect_new(sp);
-	srcu_advance_batches(sp, 1);
-	srcu_invoke_callbacks(sp);
-	srcu_reschedule(sp);
-}
-EXPORT_SYMBOL_GPL(process_srcu);
-
-static int __init srcu_bootup_announce(void)
-{
-	pr_info("Classic SRCU implementation.\n");
-	return 0;
-}
-early_initcall(srcu_bootup_announce);