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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 <[email protected]>
Reviewed-by: Peter Hurley <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Michel Lespinasse authored and Linus Torvalds committed 2013-05-07 07:20:16 -07:00
1 parent a7d2c573ae
commit 9b0fc9c09f
1 file changed
+9 -8
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lib/rwsem.c
+9 -8
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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 */
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
while (true) {
/* Try acquiring the write lock. */
count = RWSEM_ACTIVE_WRITE_BIAS;
if (!list_is_singular(&sem->wait_list))
count += RWSEM_WAITING_BIAS;
if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
if (!(count & RWSEM_ACTIVE_MASK)) {
/* Try acquiring the write lock. */
count = RWSEM_ACTIVE_WRITE_BIAS;
if (!list_is_singular(&sem->wait_list))
count += RWSEM_WAITING_BIAS;
if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
RWSEM_WAITING_BIAS)
break;
break;
}
raw_spin_unlock_irq(&sem->wait_lock);
@@ -231,7 +232,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
do {
schedule();
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);
}