nvme: hung task in blk_mq_freeze_queue_wait

[Francesco Ruggeri]

We are running into hung tasks in blk_mq_freeze_queue_wait, sometimes
in nvme_reset_work and sometimes in nvme_scan_work.
In some cases we also see a warning in blk_mq_unfreeze_queue
(WARN_ON_ONCE(q->mq_freeze_depth < 0)):
WARNING: CPU: 10 PID: 152 at block/blk-mq.c:199 blk_mq_unfreeze_queue+0x26/0x5a
The hung tasks are preceded by two instances of nvme_timeout racing in
nvme_dev_disable after a device becomes unresponsive.
The first nvme_dev_disable blocks in wait_for_completion_io_timeout, and
a while later a second instance blocks in mutex_lock(&dev->shutdown_lock).
When the timeout expires and the first instance releases the mutex,
the two instances start racing.
We have seen the hung tasks in 4.19, but from a quick look at the
latest 6.1 code the issue is probably there as well.
There seem to be two different scenarios where unbalanced
blk_freeze_queue_start/blk_mq_unfreeze_queue could be the cause.

1) In this case we have an unfreeze without a corresponding freeze
preceding it.

TIMEOUT 1	TIMEOUT 2	RESET WORK 1	RESET WORK 2

state = NVME_CTRL_LIVE
nvme_timeout
nvme_dev_disable
mutex_lock
nvme_start_freeze
blk_freeze_queue_start
nvme_disable_io_queues
wait_for_completion_io_timeout
		nvme_timeout
		nvme_dev_disable
		mutex_lock
mutex_unlock
nvme_reset_ctrl
state = NVME_CTRL_RESETTING
queue_work(nvme_reset_work)
				nvme_reset_work
				state = NVME_CTRL_CONNECTING
		(state != NVME_CTRL_LIVE and
		state != NVME_CTRL_RESETTING)
		skips nvme_start_freeze
		mutex_unlock
		nvme_reset_ctrl
		state = NVME_CTRL_RESETTING
		queue_work(nvme_reset_work)
				nvme_unfreeze (matches
				nvme_start_freeze in
				TIMEOUT 1)
						nvme_reset_work
						nvme_unfreeze (no
						match in TIMEOUT 2)


2) In this case a freeze has no corresponding unfreeze following it.
TIMEOUT 2 cannot schedule nvme_reset_work because TIMEOUT 1's is already
scheduled but not yet running.

TIMEOUT 1	TIMEOUT 2	RESET WORK 1	RESET WORK 2

state = NVME_CTRL_LIVE
nvme_timeout
nvme_dev_disable
mutex_lock
nvme_start_freeze
blk_freeze_queue_start
nvme_disable_io_queues
wait_for_completion_io_timeout
		nvme_timeout
		nvme_dev_disable
		mutex_lock
mutex_unlock
nvme_reset_ctrl
state = NVME_CTRL_RESETTING
queue_work(nvme_reset_work)
		(state == NVME_CTRL_LIVE or
		state == NVME_CTRL_RESETTING)
		nvme_start_freeze
		blk_freeze_queue_start
		mutex_unlock
		nvme_reset_ctrl
		state = NVME_CTRL_RESETTING
		queue_work(nvme_reset_work)
		fails because nvme_reset_work
		is not running yet
				nvme_reset_work
				nvme_unfreeze (matches
				nvme_start_freeze in
				TIMEOUT 1)
						It gets never
						scheduled.


Following is one such backtrace.

[ 4390.119745] INFO: task kworker/u80:2:8279 blocked for more than 300 seconds.
[ 4390.471456]       Tainted: P           O      4.19.142.Ar-29007847.buytenhb7335070 #1
[ 4390.832568] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 4391.193663] kworker/u80:2   D    0  8279      2 0x80000000
[ 4391.526603] Workqueue: nvme-reset-wq nvme_reset_work
[ 4391.853272] Call Trace:
[ 4392.149696]  __schedule+0x75b/0x7c2
[ 4392.458627]  schedule+0x78/0x8c
[ 4392.763399]  blk_mq_freeze_queue_wait+0x8b/0xb6
[ 4393.084849]  ? wait_woken+0x92/0x92
[ 4393.393791]  nvme_wait_freeze+0x39/0x4e
[ 4393.706900]  nvme_reset_work+0x1464/0x1665
[ 4394.023142]  ? dequeue_entity+0x694/0x6a3
[ 4394.338340]  process_one_work+0x1c2/0x30f
[ 4394.511642]  worker_thread+0x1e9/0x2cc
[ 4394.556626]  ? rescuer_thread+0x2b7/0x2b7
[ 4394.604738]  kthread+0x15d/0x165
[ 4394.643460]  ? kthread_park+0x98/0x98
[ 4394.687396]  ret_from_fork+0x1f/0x30