[PATCH] blk-mq: fix IO hang from sbitmap wakeup race

[Ming Lei <ming.lei@redhat.com>]

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.

Apply the same pattern in blk_mq_get_tag() which should have same risk.

Reported-by: Changhui Zhong <czhong@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
BTW, Changhui is planning to upstream the test case to blktests.

 block/blk-mq-tag.c | 19 +++++++++++++++++++
 block/blk-mq.c     | 16 ++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index cc57e2dd9a0b..29f77cae8eb2 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -179,6 +179,25 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 
 		sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
 
+		/*
+		 * Add one explicit barrier since __blk_mq_get_tag() may not
+		 * imply barrier in case of failure.
+		 *
+		 * Order adding us to wait queue and the following allocating
+		 * tag in  __blk_mq_get_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 tag
+		 * may cause __sbitmap_queue_wake_up() to wake up nothing
+		 * because the waitqueue_active() may not observe us in wait
+		 * queue.
+		 */
+		smp_mb();
+
 		tag = __blk_mq_get_tag(data, bt);
 		if (tag != BLK_MQ_NO_TAG)
 			break;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index fb29ff5cc281..54545a4792bf 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1847,6 +1847,22 @@ 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
-- 
2.42.0