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rwsem: skip initial trylock in rwsem_down_write_failed

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We can skip the initial trylock in rwsem_down_write_failed() if there
are known active lockers already, thus saving one likely-to-fail
cmpxchg.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michel Lespinasse 2013-05-07 06:45:57 -07:00 • committed by Linus Torvalds
commit 9b0fc9c09f
1 changed files with 9 additions and 8 deletions
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17
lib/rwsem.c
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@ -216,14 +216,15 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
/* wait until we successfully acquire the lock */ /* wait until we successfully acquire the lock */
set_task_state(tsk, TASK_UNINTERRUPTIBLE); set_task_state(tsk, TASK_UNINTERRUPTIBLE);
while (true) { while (true) {
if (!(count & RWSEM_ACTIVE_MASK)) {
/* Try acquiring the write lock. */ /* Try acquiring the write lock. */
count = RWSEM_ACTIVE_WRITE_BIAS; count = RWSEM_ACTIVE_WRITE_BIAS;
if (!list_is_singular(&sem->wait_list)) if (!list_is_singular(&sem->wait_list))
count += RWSEM_WAITING_BIAS; count += RWSEM_WAITING_BIAS;
if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
RWSEM_WAITING_BIAS) RWSEM_WAITING_BIAS)
break; break;
}
raw_spin_unlock_irq(&sem->wait_lock); raw_spin_unlock_irq(&sem->wait_lock);
@ -231,7 +232,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
do { do {
schedule(); schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE); set_task_state(tsk, TASK_UNINTERRUPTIBLE);
} while (sem->count & RWSEM_ACTIVE_MASK); } while ((count = sem->count) & RWSEM_ACTIVE_MASK);
raw_spin_lock_irq(&sem->wait_lock); raw_spin_lock_irq(&sem->wait_lock);
} }