rwsem: shorter spinlocked section in rwsem_down_failed_common()

This change reduces the size of the spinlocked and TASK_UNINTERRUPTIBLE
sections in rwsem_down_failed_common():

- We only need the sem->wait_lock to insert ourselves on the wait_list;
  the waiter node can be prepared outside of the wait_lock.

- The task state only needs to be set to TASK_UNINTERRUPTIBLE immediately
  before checking if we actually need to sleep; it doesn't need to protect
  the entire function.
Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/rwsem.c

@@ -188,14 +188,12 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 	struct task_struct *tsk = current;
 	signed long count;
 
-	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
-
 	/* set up my own style of waitqueue */
-	raw_spin_lock_irq(&sem->wait_lock);
 	waiter.task = tsk;
 	waiter.type = type;
 	get_task_struct(tsk);
 
+	raw_spin_lock_irq(&sem->wait_lock);
 	if (list_empty(&sem->wait_list))
 		adjustment += RWSEM_WAITING_BIAS;
 	list_add_tail(&waiter.list, &sem->wait_list);
@@ -218,7 +216,8 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 	raw_spin_unlock_irq(&sem->wait_lock);
 
 	/* wait to be given the lock */
-	for (;;) {
+	while (true) {
+		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 		if (!waiter.task)
 			break;
 
@@ -231,7 +230,6 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 			}
 		raw_spin_unlock_irq(&sem->wait_lock);
 		schedule();
-		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 	}
 
 	tsk->state = TASK_RUNNING;