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: 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

@@ -216,22 +216,23 @@ 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) {
-
-		/* 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) ==
+		if (!(count & RWSEM_ACTIVE_MASK)) {
+			/* 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);
 
 		/* Block until there are no active lockers. */
 		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);
 	}