io_uring: fix memory ordering when SQPOLL thread goes to sleep

Without a full memory barrier between the store to the flags and the
load of the SQ tail the two operations can be reordered and this can
lead to a situation where the SQPOLL thread goes to sleep while the
application writes to the SQ tail and doesn't see the wakeup flag.
This memory barrier pairs with a full memory barrier in the application
between its store to the SQ tail and its load of the flags.
Signed-off-by: Almog Khaikin <almogkh@gmail.com>
Link: https://lore.kernel.org/r/20220321090059.46313-1-almogkh@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8042,6 +8042,13 @@ static int io_sq_thread(void *data)
 					needs_sched = false;
 					break;
 				}
+
+				/*
+				 * Ensure the store of the wakeup flag is not
+				 * reordered with the load of the SQ tail
+				 */
+				smp_mb();
+
 				if (io_sqring_entries(ctx)) {
 					needs_sched = false;
 					break;