Re: [PATCH v1 2/3] blk-mq: comment on races related with timeout handler
From: Bart Van Assche <hidden>
Date: 2017-03-17 17:40:20
Also in:
lkml
On Fri, 2017-03-17 at 17:57 +0800, Ming Lei wrote:
quoted hunk ↗ jump to hunk
+/* + * When we reach here because queue is busy, REQ_ATOM_COMPLETE + * flag isn't set yet, so there may be race with timeout hanlder, + * but given rq->deadline is just set in .queue_rq() under + * this sitation, the race won't be possible in reality because + * rq->timeout should be set as big enough to cover the window + * between blk_mq_start_request() called from .queue_rq() and + * clearing REQ_ATOM_STARTED here. + */ static void __blk_mq_requeue_request(struct request *rq) { struct request_queue *q =3D rq->q;@@ -700,6 +709,19 @@ static void blk_mq_check_expired(struct blk_mq_hw_ct=
x *hctx,
if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
return;
=20
+ /*
+ * The rq being checked may have been freed and reallocated
+ * out already here, we avoid this race by checking rq->deadline
+ * and REQ_ATOM_COMPLETE flag together:
+ *
+ * - if rq->deadline is observed as new value because of
+ * reusing, the rq won't be timed out because of timing.
+ * - if rq->deadline is observed as previous value,
+ * REQ_ATOM_COMPLETE flag won't be cleared in reuse path
+ * because we put a barrier between setting rq->deadline
+ * and clearing the flag in blk_mq_start_request(), so
+ * this rq won't be timed out too.
+ */
if (time_after_eq(jiffies, rq->deadline)) {
if (!blk_mark_rq_complete(rq))
blk_mq_rq_timed_out(rq, reserved);Since this explanation applies to the same race addressed by patch 1/3, please consider squashing this patch into patch 1/3. Thanks, Bart.=