[PATCH v2 2/4] block: add a read barrier in blk_queue_enter()

[Ming Lei <tom.leiming@gmail.com>]

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.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 block/blk-core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index ad388d5e309a..44eed17319c0 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -669,6 +669,14 @@ 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 DEAD flag of .q_usage_counter
+		 * and reading .mq_freeze_depth, otherwise the following
+		 * wait may never return if the two read are reordered.
+		 */
+		smp_rmb();
+
 		ret = wait_event_interruptible(q->mq_freeze_wq,
 				!atomic_read(&q->mq_freeze_depth) ||
 				blk_queue_dying(q));
-- 
2.9.3