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rwsem: use cmpxchg for trying to steal write lock

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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: 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 06:45:55 -07:00 committed by Linus Torvalds
parent ed00f64346
commit 5ede972df1
1 changed files with 6 additions and 20 deletions
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26
lib/rwsem.c
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@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
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
*/
@ -236,7 +217,12 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
while (true) {
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;
raw_spin_unlock_irq(&sem->wait_lock);