block: add a read barrier in blk_queue_enter()

Without the barrier, reading DEAD flag of .q_usage_counter
and reading .mq_freeze_depth may be reordered, then the
following wait_event_interruptible() may never return.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-core.c b/block/blk-core.c
index 4234332aa23c487354fd412c6cd7ba142c623aaf..6373febc77169bfbf0e38635dbb7ec2885c293d7 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -669,6 +669,15 @@ int blk_queue_enter(struct request_queue *q, bool nowait)
                if (nowait)
                        return -EBUSY;
 
+               /*
+                * read pair of barrier in blk_mq_freeze_queue_start(),
+                * we need to order reading __PERCPU_REF_DEAD flag of
+                * .q_usage_counter and reading .mq_freeze_depth,
+                * otherwise the following wait may never return if the
+                * two reads are reordered.
+                */
+               smp_rmb();
+
                ret = wait_event_interruptible(q->mq_freeze_wq,
                                !atomic_read(&q->mq_freeze_depth) ||
                                blk_queue_dying(q));