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

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

lib/rwsem.c

+6 −20
Original line number Diff line number Diff line
@@ -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);