blk-mq: request deadline must be visible before marking rq as started

When we start the request, we set the deadline and flip the bits
marking the request as started and non-complete. However, it's
important that the deadline store is ordered before flipping the
bits, otherwise we could have a small window where the request is
marked started but with an invalid deadline. This can confuse the
timeout handling.
Suggested-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -392,6 +392,12 @@ static void blk_mq_start_request(struct request *rq, bool last)
 
 	blk_add_timer(rq);
 
+	/*
+	 * Ensure that ->deadline is visible before set the started
+	 * flag and clear the completed flag.
+	 */
+	smp_mb__before_atomic();
+
 	/*
 	 * Mark us as started and clear complete. Complete might have been
 	 * set if requeue raced with timeout, which then marked it as