[PATCH 5/6] blk-mq: fix potential reorder of request state and deadline

[chengming . zhou]

From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>

CPU0				CPU1
blk_mq_start_request()		blk_mq_req_expired()
  WRITE_ONCE(rq->deadline)
  WRITE_ONCE(rq->state)
				  if (READ_ONCE(rq->state) != IN_FLIGHT)
				    return
				  deadline = READ_ONCE(rq->deadline)

If CPU1 speculately reorder rq->deadline LOAD before rq->state, the
deadline will be the initial value 0.

CPU0				CPU1
blk_mq_start_request()		blk_mq_req_expired()
				  deadline = READ_ONCE(rq->deadline)
  WRITE_ONCE(rq->deadline)
  WRITE_ONCE(rq->state)
				  if (READ_ONCE(rq->state) != IN_FLIGHT)
				    return

Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
---
 block/blk-mq.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ff1b0f3ab3a8..49cbf826b100 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1258,6 +1258,8 @@ void blk_mq_start_request(struct request *rq)
 	WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_IDLE);
 
 	blk_add_timer(rq);
+	/* Pair with smp_rmb in blk_mq_req_expired(). */
+	smp_wmb();
 	WRITE_ONCE(rq->state, MQ_RQ_IN_FLIGHT);
 	rq->mq_hctx->tags->rqs[rq->tag] = rq;
 
@@ -1568,6 +1570,12 @@ static bool blk_mq_req_expired(struct request *rq, struct blk_expired_data *expi
 	if (rq->rq_flags & RQF_TIMED_OUT)
 		return false;
 
+	/*
+	 * Order LOADs of rq->state and rq->deadline, pair with
+	 * smp_wmb in blk_mq_start_request().
+	 */
+	smp_rmb();
+
 	deadline = READ_ONCE(rq->deadline);
 	if (time_after_eq(expired->timeout_start, deadline))
 		return true;
-- 
2.41.0