On Fri, Jul 29, 2022 at 02:52:43PM +0800, Zhang Wensheng wrote:

From: Zhang Wensheng <redacted>

In the use of q_usage_counter of request_queue, blk_cleanup_queue using
"wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter))"
to wait q_usage_counter becoming zero. however, if the q_usage_counter
becoming zero quickly, and percpu_ref_exit will execute and ref->data
will be freed, maybe another process will cause a null-defef problem
like below:

	CPU0                             CPU1
blk_cleanup_queue
 blk_freeze_queue
  blk_mq_freeze_queue_wait
				scsi_end_request
				 percpu_ref_get
				 ...
				 percpu_ref_put
				  atomic_long_sub_and_test
  percpu_ref_exit
   ref->data -> NULL
   				   ref->data->release(ref) -> null-deref

Fix it by setting flag(QUEUE_FLAG_USAGE_COUNT_SYNC) to add synchronization
mechanism, when ref->data->release is called, the flag will be setted,
and the "wait_event" in blk_mq_freeze_queue_wait must wait flag becoming
true as well, which will limit percpu_ref_exit to execute ahead of time.

Signed-off-by: Zhang Wensheng <redacted>
---
 block/blk-core.c       | 4 +++-
 block/blk-mq.c         | 7 +++++++
 include/linux/blk-mq.h | 1 +
 include/linux/blkdev.h | 2 ++
 4 files changed, 13 insertions(+), 1 deletion(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>