[PATCH 2/6] blk-mq: fix a memory ordering bug in blk_mq_queue_enter()

[Tejun Heo]

blk-mq uses a percpu_counter to keep track of how many usages are in
flight.  The percpu_counter is drained while freezing to ensure that
no usage is left in-flight after freezing is complete.

blk_mq_queue_enter/exit() and blk_mq_[un]freeze_queue() implement this
per-cpu gating mechanism; unfortunately, it contains a subtle bug -
smp_wmb() in blk_mq_queue_enter() doesn't prevent prevent the cpu from
fetching @q->bypass_depth before incrementing @q->mq_usage_counter and
if freezing happens inbetween the caller can slip through and freezing
can be complete while there are active users.

Use smp_mb() instead so that bypass_depth and mq_usage_counter
modifications and tests are properly interlocked.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Nicholas A. Bellinger <nab@xxxxxxxxxxxxxxx>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4787c3d..1c09d57 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -81,7 +81,7 @@ static int blk_mq_queue_enter(struct request_queue *q)
 	int ret;
 
 	__percpu_counter_add(&q->mq_usage_counter, 1, 1000000);
-	smp_wmb();
+	smp_mb();
 
 	/* we have problems freezing the queue if it's initializing */
 	if (!blk_queue_dying(q) &&
-- 
1.9.3

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