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From: Sasha Levin <sashal@kernel.org>
To: linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Cc: Ming Lei <ming.lei@redhat.com>,
	Jan Kara <jack@suse.cz>,
	Kemeng Shi <shikemeng@huaweicloud.com>,
	Changhui Zhong <czhong@redhat.com>,
	Jens Axboe <axboe@kernel.dk>,
	Sasha Levin <sashal@kernel.org>,
	linux-block@vger.kernel.org
Subject: [PATCH AUTOSEL 6.1 19/27] blk-mq: fix IO hang from sbitmap wakeup race
Date: Sun, 28 Jan 2024 11:14:04 -0500
Message-ID: <20240128161424.203600-19-sashal@kernel.org>
In-Reply-To: <20240128161424.203600-1-sashal@kernel.org>
References: <20240128161424.203600-1-sashal@kernel.org>
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From: Ming Lei <ming.lei@redhat.com>

[ Upstream commit 5266caaf5660529e3da53004b8b7174cab6374ed ]

In blk_mq_mark_tag_wait(), __add_wait_queue() may be re-ordered
with the following blk_mq_get_driver_tag() in case of getting driver
tag failure.

Then in __sbitmap_queue_wake_up(), waitqueue_active() may not observe
the added waiter in blk_mq_mark_tag_wait() and wake up nothing, meantime
blk_mq_mark_tag_wait() can't get driver tag successfully.

This issue can be reproduced by running the following test in loop, and
fio hang can be observed in < 30min when running it on my test VM
in laptop.

	modprobe -r scsi_debug
	modprobe scsi_debug delay=0 dev_size_mb=4096 max_queue=1 host_max_queue=1 submit_queues=4
	dev=`ls -d /sys/bus/pseudo/drivers/scsi_debug/adapter*/host*/target*/*/block/* | head -1 | xargs basename`
	fio --filename=/dev/"$dev" --direct=1 --rw=randrw --bs=4k --iodepth=1 \
       		--runtime=100 --numjobs=40 --time_based --name=test \
        	--ioengine=libaio

Fix the issue by adding one explicit barrier in blk_mq_mark_tag_wait(), which
is just fine in case of running out of tag.

Cc: Jan Kara <jack@suse.cz>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Reported-by: Changhui Zhong <czhong@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20240112122626.4181044-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 block/blk-mq.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 368f1947c895..ebfefbcb3604 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1859,6 +1859,22 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx,
 	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
 	__add_wait_queue(wq, wait);
 
+	/*
+	 * Add one explicit barrier since blk_mq_get_driver_tag() may
+	 * not imply barrier in case of failure.
+	 *
+	 * Order adding us to wait queue and allocating driver tag.
+	 *
+	 * The pair is the one implied in sbitmap_queue_wake_up() which
+	 * orders clearing sbitmap tag bits and waitqueue_active() in
+	 * __sbitmap_queue_wake_up(), since waitqueue_active() is lockless
+	 *
+	 * Otherwise, re-order of adding wait queue and getting driver tag
+	 * may cause __sbitmap_queue_wake_up() to wake up nothing because
+	 * the waitqueue_active() may not observe us in wait queue.
+	 */
+	smp_mb();
+
 	/*
 	 * It's possible that a tag was freed in the window between the
 	 * allocation failure and adding the hardware queue to the wait
-- 
2.43.0