Re: INFO: task hung in blk_queue_enter

[Bart Van Assche]

On Wed, 2018-05-16 at 22:05 +0900, Tetsuo Handa wrote:
> One ore more threads are waiting for q->mq_freeze_depth to become 0. But the
> thread who incremented q->mq_freeze_depth at blk_freeze_queue_start(q) from
> blk_freeze_queue() is waiting at blk_mq_freeze_queue_wait(). Therefore,
> atomic_read(&q->mq_freeze_depth) == 0 condition for wait_event() in
> blk_queue_enter() will never be satisfied. But what does that wait_event()
> want to do? Isn't "start freezing" a sort of blk_queue_dying(q) == true?
> Since percpu_ref_tryget_live(&q->q_usage_counter) failed and the queue is
> about to be frozen, shouldn't we treat atomic_read(&q->mq_freeze_depth) != 0
> as if blk_queue_dying(q) == true? That is, something like below:
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 85909b4..59e2496 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -951,10 +951,10 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
>  		smp_rmb();
>  
>  		wait_event(q->mq_freeze_wq,
> -			   (atomic_read(&q->mq_freeze_depth) == 0 &&
> -			    (preempt || !blk_queue_preempt_only(q))) ||
> +			   atomic_read(&q->mq_freeze_depth) ||
> +			   (preempt || !blk_queue_preempt_only(q)) ||
>  			   blk_queue_dying(q));
> -		if (blk_queue_dying(q))
> +		if (atomic_read(&q->mq_freeze_depth) || blk_queue_dying(q))
>  			return -ENODEV;
>  	}
>  }

That change looks wrong to me. Additionally, I think that you are looking in
the wrong direction. Since blk_mq_freeze_queue_wait() and blk_queue_enter()
work fine for all block drivers except the loop driver I think that you should
have a closer look at how the loop driver uses this block layer functionality.

Thanks,

Bart.