blk-mq: fix IO hang from sbitmap wakeup race

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>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index aa9a05fdd023776d30f98485063b2c291100853d..a4c54c5895a1390f52b67045a494442b20b910c1 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1852,6 +1852,22 @@ 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