In the Linux kernel, the following vulnerability has been resolved:

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.

kernel.org https://git.kernel.org/stable/c/1d9c777d3e70bdc57dddf7a14a80059d65919e56 [No types assigned]

kernel.org https://git.kernel.org/stable/c/5266caaf5660529e3da53004b8b7174cab6374ed [No types assigned]

kernel.org https://git.kernel.org/stable/c/6d8b01624a2540336a32be91f25187a433af53a0 [No types assigned]

kernel.org https://git.kernel.org/stable/c/7610ba1319253225a9ba8a9d28d472fc883b4e2f [No types assigned]

kernel.org https://git.kernel.org/stable/c/89e0e66682e1538aeeaa3109503473663cd24c8b [No types assigned]

kernel.org https://git.kernel.org/stable/c/9525b38180e2753f0daa1a522b7767a2aa969676 [No types assigned]

kernel.org https://git.kernel.org/stable/c/ecd7744a1446eb02ccc63e493e2eb6ede4ef1e10 [No types assigned]

kernel.org https://git.kernel.org/stable/c/f1bc0d8163f8ee84a8d5affdf624cfad657df1d2 [No types assigned]