Re: [PATCH] blk-mq: Allow blocking queue tag iter callbacks

[Bart Van Assche <bvanassche@acm.org>]

On 9/24/18 7:11 PM, jianchao.wang wrote:
> Hi Keith
> 
> On 09/25/2018 05:09 AM, Keith Busch wrote:
>> -	/* 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))
>> +	if (!blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next))
>>   		return;
> 
> We cannot discard the percpu_ref_tryget here.
> 
> There following code in blk_mq_timeout_work still need it:
> 
> 	if (next != 0) {
> 		mod_timer(&q->timeout, next);
> 	} else {
> 		queue_for_each_hw_ctx(q, hctx, i) {
> 			/* the hctx may be unmapped, so check it here */
> 			if (blk_mq_hw_queue_mapped(hctx))
> 				blk_mq_tag_idle(hctx);
> 		}
> 	}

Hi Jianchao,

Had you noticed that the percpu_ref_tryget() call has been moved into
blk_mq_queue_tag_busy_iter()?

Bart.