blk-mq: fix IO hang from sbitmap wakeup race

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.