AZL-58798

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

blk-mq: fix IO hang from sbitmap wakeup race

In blkmqmarktagwait(), __addwaitqueue() may be re-ordered with the following blkmqgetdrivertag() in case of getting driver tag failure.

Then in __sbitmapqueuewakeup(), waitqueueactive() may not observe the added waiter in blkmqmarktagwait() and wake up nothing, meantime blkmqmarktagwait() 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 blkmqmarktagwait(), which is just fine in case of running out of tag.