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: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 383ea0c..a13c40c 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -393,6 +393,12 @@ 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
         * complete. So be sure to clear complete again when we start