io_uring: improve same wq polling

Move earlier the check for whether __io_queue_proc() tries to poll
already polled waitqueue, and do the same for the second poll entry, if
any. Shouldn't really matter, but at least it would have a more
predictable behaviour.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8cb428cfe8ade0fd055859fabb878db8777d4c2f.1629228203.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5062,8 +5062,13 @@ static void __io_queue_proc(struct io_poll_iocb *poll, struct io_poll_table *pt,
 	if (unlikely(pt->nr_entries)) {
 		struct io_poll_iocb *poll_one = poll;
 
+		/* double add on the same waitqueue head, ignore */
+		if (poll_one->head == head)
+			return;
 		/* already have a 2nd entry, fail a third attempt */
 		if (*poll_ptr) {
+			if ((*poll_ptr)->head == head)
+				return;
 			pt->error = -EINVAL;
 			return;
 		}
@@ -5073,9 +5078,6 @@ static void __io_queue_proc(struct io_poll_iocb *poll, struct io_poll_table *pt,
 		 */
 		if (!(poll_one->events & EPOLLONESHOT))
 			poll_one->events |= EPOLLONESHOT;
-		/* double add on the same waitqueue head, ignore */
-		if (poll_one->head == head)
-			return;
 		poll = kmalloc(sizeof(*poll), GFP_ATOMIC);
 		if (!poll) {
 			pt->error = -ENOMEM;