diff options
Diffstat (limited to 'kernel/locking/rwsem-xadd.c')
| -rw-r--r-- | kernel/locking/rwsem-xadd.c | 194 |
1 files changed, 120 insertions, 74 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 09e30c6225e5..447e08de1fab 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -80,7 +80,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name, debug_check_no_locks_freed((void *)sem, sizeof(*sem)); lockdep_init_map(&sem->dep_map, name, key, 0); #endif - sem->count = RWSEM_UNLOCKED_VALUE; + atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE); raw_spin_lock_init(&sem->wait_lock); INIT_LIST_HEAD(&sem->wait_list); #ifdef CONFIG_RWSEM_SPIN_ON_OWNER @@ -114,12 +114,16 @@ enum rwsem_wake_type { * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) * - there must be someone on the queue - * - the spinlock must be held by the caller + * - the wait_lock must be held by the caller + * - tasks are marked for wakeup, the caller must later invoke wake_up_q() + * to actually wakeup the blocked task(s) and drop the reference count, + * preferably when the wait_lock is released * - woken process blocks are discarded from the list after having task zeroed - * - writers are only woken if downgrading is false + * - writers are only marked woken if downgrading is false */ static struct rw_semaphore * -__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) +__rwsem_mark_wake(struct rw_semaphore *sem, + enum rwsem_wake_type wake_type, struct wake_q_head *wake_q) { struct rwsem_waiter *waiter; struct task_struct *tsk; @@ -128,13 +132,16 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); if (waiter->type == RWSEM_WAITING_FOR_WRITE) { - if (wake_type == RWSEM_WAKE_ANY) - /* Wake writer at the front of the queue, but do not - * grant it the lock yet as we want other writers - * to be able to steal it. Readers, on the other hand, - * will block as they will notice the queued writer. + if (wake_type == RWSEM_WAKE_ANY) { + /* + * Mark writer at the front of the queue for wakeup. + * Until the task is actually later awoken later by + * the caller, other writers are able to steal it. + * Readers, on the other hand, will block as they + * will notice the queued writer. */ - wake_up_process(waiter->task); + wake_q_add(wake_q, waiter->task); + } goto out; } @@ -146,15 +153,27 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) if (wake_type != RWSEM_WAKE_READ_OWNED) { adjustment = RWSEM_ACTIVE_READ_BIAS; try_reader_grant: - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; + oldcount = atomic_long_fetch_add(adjustment, &sem->count); + if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { - /* A writer stole the lock. Undo our reader grant. */ - if (rwsem_atomic_update(-adjustment, sem) & - RWSEM_ACTIVE_MASK) + /* + * If the count is still less than RWSEM_WAITING_BIAS + * after removing the adjustment, it is assumed that + * a writer has stolen the lock. We have to undo our + * reader grant. + */ + if (atomic_long_add_return(-adjustment, &sem->count) < + RWSEM_WAITING_BIAS) goto out; /* Last active locker left. Retry waking readers. */ goto try_reader_grant; } + /* + * It is not really necessary to set it to reader-owned here, + * but it gives the spinners an early indication that the + * readers now have the lock. + */ + rwsem_set_reader_owned(sem); } /* Grant an infinite number of read locks to the readers at the front @@ -179,7 +198,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) adjustment -= RWSEM_WAITING_BIAS; if (adjustment) - rwsem_atomic_add(adjustment, sem); + atomic_long_add(adjustment, &sem->count); next = sem->wait_list.next; loop = woken; @@ -187,17 +206,15 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) waiter = list_entry(next, struct rwsem_waiter, list); next = waiter->list.next; tsk = waiter->task; + + wake_q_add(wake_q, tsk); /* - * Make sure we do not wakeup the next reader before - * setting the nil condition to grant the next reader; - * otherwise we could miss the wakeup on the other - * side and end up sleeping again. See the pairing - * in rwsem_down_read_failed(). + * Ensure that the last operation is setting the reader + * waiter to nil such that rwsem_down_read_failed() cannot + * race with do_exit() by always holding a reference count + * to the task to wakeup. */ - smp_mb(); - waiter->task = NULL; - wake_up_process(tsk); - put_task_struct(tsk); + smp_store_release(&waiter->task, NULL); } while (--loop); sem->wait_list.next = next; @@ -216,11 +233,11 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) long count, adjustment = -RWSEM_ACTIVE_READ_BIAS; struct rwsem_waiter waiter; struct task_struct *tsk = current; + WAKE_Q(wake_q); /* set up my own style of waitqueue */ waiter.task = tsk; waiter.type = RWSEM_WAITING_FOR_READ; - get_task_struct(tsk); raw_spin_lock_irq(&sem->wait_lock); if (list_empty(&sem->wait_list)) @@ -228,7 +245,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) list_add_tail(&waiter.list, &sem->wait_list); /* we're now waiting on the lock, but no longer actively locking */ - count = rwsem_atomic_update(adjustment, sem); + count = atomic_long_add_return(adjustment, &sem->count); /* If there are no active locks, wake the front queued process(es). * @@ -238,9 +255,10 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) if (count == RWSEM_WAITING_BIAS || (count > RWSEM_WAITING_BIAS && adjustment != -RWSEM_ACTIVE_READ_BIAS)) - sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); + sem = __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); /* wait to be given the lock */ while (true) { @@ -255,17 +273,29 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) } EXPORT_SYMBOL(rwsem_down_read_failed); +/* + * This function must be called with the sem->wait_lock held to prevent + * race conditions between checking the rwsem wait list and setting the + * sem->count accordingly. + */ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) { /* - * Try acquiring the write lock. Check count first in order - * to reduce unnecessary expensive cmpxchg() operations. + * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS. */ - if (count == RWSEM_WAITING_BIAS && - cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, - RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { - if (!list_is_singular(&sem->wait_list)) - rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); + if (count != RWSEM_WAITING_BIAS) + return false; + + /* + * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there + * are other tasks on the wait list, we need to add on WAITING_BIAS. + */ + count = list_is_singular(&sem->wait_list) ? + RWSEM_ACTIVE_WRITE_BIAS : + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS; + + if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) + == RWSEM_WAITING_BIAS) { rwsem_set_owner(sem); return true; } @@ -279,13 +309,13 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) */ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) { - long old, count = READ_ONCE(sem->count); + long old, count = atomic_long_read(&sem->count); while (true) { if (!(count == 0 || count == RWSEM_WAITING_BIAS)) return false; - old = cmpxchg_acquire(&sem->count, count, + old = atomic_long_cmpxchg_acquire(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS); if (old == count) { rwsem_set_owner(sem); @@ -306,16 +336,11 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) rcu_read_lock(); owner = READ_ONCE(sem->owner); - if (!owner) { - long count = READ_ONCE(sem->count); + if (!rwsem_owner_is_writer(owner)) { /* - * If sem->owner is not set, yet we have just recently entered the - * slowpath with the lock being active, then there is a possibility - * reader(s) may have the lock. To be safe, bail spinning in these - * situations. + * Don't spin if the rwsem is readers owned. */ - if (count & RWSEM_ACTIVE_MASK) - ret = false; + ret = !rwsem_owner_is_reader(owner); goto done; } @@ -325,10 +350,15 @@ done: return ret; } -static noinline -bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) +/* + * Return true only if we can still spin on the owner field of the rwsem. + */ +static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) { - long count; + struct task_struct *owner = READ_ONCE(sem->owner); + + if (!rwsem_owner_is_writer(owner)) + goto out; rcu_read_lock(); while (sem->owner == owner) { @@ -349,22 +379,16 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) cpu_relax_lowlatency(); } rcu_read_unlock(); - - if (READ_ONCE(sem->owner)) - return true; /* new owner, continue spinning */ - +out: /* - * When the owner is not set, the lock could be free or - * held by readers. Check the counter to verify the - * state. + * If there is a new owner or the owner is not set, we continue + * spinning. */ - count = READ_ONCE(sem->count); - return (count == 0 || count == RWSEM_WAITING_BIAS); + return !rwsem_owner_is_reader(READ_ONCE(sem->owner)); } static bool rwsem_optimistic_spin(struct rw_semaphore *sem) { - struct task_struct *owner; bool taken = false; preempt_disable(); @@ -376,12 +400,17 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) if (!osq_lock(&sem->osq)) goto done; - while (true) { - owner = READ_ONCE(sem->owner); - if (owner && !rwsem_spin_on_owner(sem, owner)) - break; - - /* wait_lock will be acquired if write_lock is obtained */ + /* + * Optimistically spin on the owner field and attempt to acquire the + * lock whenever the owner changes. Spinning will be stopped when: + * 1) the owning writer isn't running; or + * 2) readers own the lock as we can't determine if they are + * actively running or not. + */ + while (rwsem_spin_on_owner(sem)) { + /* + * Try to acquire the lock + */ if (rwsem_try_write_lock_unqueued(sem)) { taken = true; break; @@ -393,7 +422,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) * we're an RT task that will live-lock because we won't let * the owner complete. */ - if (!owner && (need_resched() || rt_task(current))) + if (!sem->owner && (need_resched() || rt_task(current))) break; /* @@ -440,9 +469,10 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) bool waiting = true; /* any queued threads before us */ struct rwsem_waiter waiter; struct rw_semaphore *ret = sem; + WAKE_Q(wake_q); /* undo write bias from down_write operation, stop active locking */ - count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem); + count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count); /* do optimistic spinning and steal lock if possible */ if (rwsem_optimistic_spin(sem)) @@ -465,18 +495,29 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) /* we're now waiting on the lock, but no longer actively locking */ if (waiting) { - count = READ_ONCE(sem->count); + count = atomic_long_read(&sem->count); /* * If there were already threads queued before us and there are * no active writers, the lock must be read owned; so we try to * wake any read locks that were queued ahead of us. */ - if (count > RWSEM_WAITING_BIAS) - sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS); + if (count > RWSEM_WAITING_BIAS) { + WAKE_Q(wake_q); + + sem = __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q); + /* + * The wakeup is normally called _after_ the wait_lock + * is released, but given that we are proactively waking + * readers we can deal with the wake_q overhead as it is + * similar to releasing and taking the wait_lock again + * for attempting rwsem_try_write_lock(). + */ + wake_up_q(&wake_q); + } } else - count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); + count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count); /* wait until we successfully acquire the lock */ set_current_state(state); @@ -492,7 +533,7 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) schedule(); set_current_state(state); - } while ((count = sem->count) & RWSEM_ACTIVE_MASK); + } while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK); raw_spin_lock_irq(&sem->wait_lock); } @@ -507,10 +548,11 @@ out_nolock: raw_spin_lock_irq(&sem->wait_lock); list_del(&waiter.list); if (list_empty(&sem->wait_list)) - rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem); + atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count); else - __rwsem_do_wake(sem, RWSEM_WAKE_ANY); + __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); return ERR_PTR(-EINTR); } @@ -537,6 +579,7 @@ __visible struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) { unsigned long flags; + WAKE_Q(wake_q); /* * If a spinner is present, it is not necessary to do the wakeup. @@ -573,9 +616,10 @@ locked: /* do nothing if list empty */ if (!list_empty(&sem->wait_list)) - sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); + sem = __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); + wake_up_q(&wake_q); return sem; } @@ -590,14 +634,16 @@ __visible struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) { unsigned long flags; + WAKE_Q(wake_q); raw_spin_lock_irqsave(&sem->wait_lock, flags); /* do nothing if list empty */ if (!list_empty(&sem->wait_list)) - sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); + sem = __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); + wake_up_q(&wake_q); return sem; } |