Commit 5ede972d authored by Michel Lespinasse's avatar Michel Lespinasse Committed by Linus Torvalds

rwsem: use cmpxchg for trying to steal write lock

Using rwsem_atomic_update to try stealing the write lock forced us to
undo the adjustment in the failure path.  We can have simpler and faster
code by using cmpxchg instead.
Signed-off-by: default avatarMichel Lespinasse <walken@google.com>
Reviewed-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Acked-by: default avatarDavidlohr Bueso <davidlohr.bueso@hp.com>
Acked-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ed00f643
...@@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) ...@@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
return sem; return sem;
} }
/* Try to get write sem, caller holds sem->wait_lock: */
static int try_get_writer_sem(struct rw_semaphore *sem)
{
long oldcount, adjustment;
adjustment = RWSEM_ACTIVE_WRITE_BIAS;
if (list_is_singular(&sem->wait_list))
adjustment -= RWSEM_WAITING_BIAS;
try_again_write:
oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
if (!(oldcount & RWSEM_ACTIVE_MASK))
return 1;
/* some one grabbed the sem already */
if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
return 0;
goto try_again_write;
}
/* /*
* wait for the read lock to be granted * wait for the read lock to be granted
*/ */
...@@ -236,7 +217,12 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) ...@@ -236,7 +217,12 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
while (true) { while (true) {
set_task_state(tsk, TASK_UNINTERRUPTIBLE); set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (try_get_writer_sem(sem)) /* 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); raw_spin_unlock_irq(&sem->wait_lock);
......
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