[PATCH v3 2/4] block: add a read barrier in blk_queue_enter()

From: Ming Lei <tom.leiming@gmail.com>
To: Jens Axboe <axboe@fb.com>,
	linux-block@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>,
	Hannes Reinecke <hare@suse.de>, Ming Lei <tom.leiming@gmail.com>
Subject: [PATCH v3 2/4] block: add a read barrier in blk_queue_enter()
Date: Mon, 27 Mar 2017 20:06:56 +0800	[thread overview]
Message-ID: <20170327120658.29864-3-tom.leiming@gmail.com> (raw)
In-Reply-To: <20170327120658.29864-1-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.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 block/blk-core.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index ad388d5e309a..5e8963bc98d9 100644
--- 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));
-- 
2.9.3
