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Commit 9b0fc9c0 authored by Michel Lespinasse's avatar Michel Lespinasse Committed by Linus Torvalds
Browse files

rwsem: skip initial trylock in rwsem_down_write_failed 



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: 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 a7d2c573

lib/rwsem.c

+9 −8
Original line number Diff line number Diff line
@@ -216,7 +216,7 @@ 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) {



		if (!(count & RWSEM_ACTIVE_MASK)) {

			/* Try acquiring the write lock. */

			count = RWSEM_ACTIVE_WRITE_BIAS;

			if (!list_is_singular(&sem->wait_list))
@@ -224,6 +224,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) 
			if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==

							RWSEM_WAITING_BIAS)

				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);

	}