rwsem: skip initial trylock in rwsem_down_write_failed (9b0fc9c0) · Commits · 戴 / test

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 <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>

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);
		}

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