Re: Regression caused by f5bbbbe4d635

[Bart Van Assche <bart.vanassche@gmail.com>]

On Mon, 2018-09-24 at 13:13 -0600, Keith Busch wrote:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 85a1c1a59c72..28d128450621 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -848,22 +848,6 @@ static void blk_mq_timeout_work(struct work_struct *work)
> 	struct blk_mq_hw_ctx *hctx;
> 	int i;
>  
> -	/* A deadlock might occur if a request is stuck requiring a
> -	 * timeout at the same time a queue freeze is waiting
> -	 * completion, since the timeout code would not be able to
> -	 * acquire the queue reference here.
> -	 *
> -	 * That's why we don't use blk_queue_enter here; instead, we use
> -	 * percpu_ref_tryget directly, because we need to be able to
> -	 * obtain a reference even in the short window between the queue
> -	 * starting to freeze, by dropping the first reference in
> -	 * blk_freeze_queue_start, and the moment the last request is
> -	 * consumed, marked by the instant q_usage_counter reaches
> -	 * zero.
> -	 */
> -	if (!percpu_ref_tryget(&q->q_usage_counter))
> -		return;
> -
> 	blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
>  
> 	if (next != 0) {
> @@ -881,7 +865,6 @@ static void blk_mq_timeout_work(struct work_struct *work)
> 				blk_mq_tag_idle(hctx);
> 		}
> 	}
> -	blk_queue_exit(q);
>  }

Hi Keith,

The above introduces a behavior change: if the percpu_ref_tryget() call inside
blk_mq_queue_tag_busy_iter() fails then blk_mq_timeout_work() will now call
blk_mq_tag_idle(). I think that's wrong if the percpu_ref_tryget() call fails
due to the queue having been frozen. Please make blk_mq_queue_tag_busy_iter()
return a bool that indicates whether or not it has iterated over the request
queue.

Thanks,

Bart.