Subject: Re: [bug report] WARNING: CPU: 1 PID: 1386 at block/blk-mq-sched.c:432 blk_mq_sched_insert_request+0x54/0x178

From: Jens Axboe <[email-masked]>
Date: Wed, 3 Nov 2021 09:49:20 -0600

On 11/3/21 9:41 AM, Jens Axboe wrote:
> On 11/3/21 9:16 AM, Ming Lei wrote:
>> On Wed, Nov 03, 2021 at 09:10:17AM -0600, Jens Axboe wrote:
>>> On 11/3/21 9:03 AM, Jens Axboe wrote:
>>>> On 11/3/21 8:57 AM, Ming Lei wrote:
>>>>> On Wed, Nov 03, 2021 at 09:59:02PM +0800, Yi Zhang wrote:
>>>>>> On Wed, Nov 3, 2021 at 7:59 PM Jens Axboe <[email-masked]> wrote:
>>>>>>>
>>>>>>> On 11/2/21 9:54 PM, Jens Axboe wrote:
>>>>>>>> On Nov 2, 2021, at 9:52 PM, Ming Lei <[email-masked]> wrote:
>>>>>>>>>
>>>>>>>>> On Tue, Nov 02, 2021 at 09:21:10PM -0600, Jens Axboe wrote:
>>>>>>>>>>> On 11/2/21 8:21 PM, Yi Zhang wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Can either one of you try with this patch? Won't fix anything, but it'll
>>>>>>>>>>>>> hopefully shine a bit of light on the issue.
>>>>>>>>>>>>>
>>>>>>>>>>> Hi Jens
>>>>>>>>>>>
>>>>>>>>>>> Here is the full log:
>>>>>>>>>>
>>>>>>>>>> Thanks! I think I see what it could be - can you try this one as well,
>>>>>>>>>> would like to confirm that the condition I think is triggering is what
>>>>>>>>>> is triggering.
>>>>>>>>>>
>>>>>>>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>>>>>>>> index 07eb1412760b..81dede885231 100644
>>>>>>>>>> --- a/block/blk-mq.c
>>>>>>>>>> +++ b/block/blk-mq.c
>>>>>>>>>> @@ -2515,6 +2515,8 @@ void blk_mq_submit_bio(struct bio *bio)
>>>>>>>>>>    if (plug && plug->cached_rq) {
>>>>>>>>>>        rq = rq_list_pop(&plug->cached_rq);
>>>>>>>>>>        INIT_LIST_HEAD(&rq->queuelist);
>>>>>>>>>> +        WARN_ON_ONCE(q->elevator && !(rq->rq_flags & RQF_ELV));
>>>>>>>>>> +        WARN_ON_ONCE(!q->elevator && (rq->rq_flags & RQF_ELV));
>>>>>>>>>>    } else {
>>>>>>>>>>        struct blk_mq_alloc_data data = {
>>>>>>>>>>            .q        = q,
>>>>>>>>>> @@ -2535,6 +2537,8 @@ void blk_mq_submit_bio(struct bio *bio)
>>>>>>>>>>                bio_wouldblock_error(bio);
>>>>>>>>>>            goto queue_exit;
>>>>>>>>>>        }
>>>>>>>>>> +        WARN_ON_ONCE(q->elevator && !(rq->rq_flags & RQF_ELV));
>>>>>>>>>> +        WARN_ON_ONCE(!q->elevator && (rq->rq_flags & RQF_ELV));
>>>>>>>>>
>>>>>>>>> Hello Jens,
>>>>>>>>>
>>>>>>>>> I guess the issue could be the following code run without grabbing
>>>>>>>>> ->q_usage_counter from blk_mq_alloc_request() and blk_mq_alloc_request_hctx().
>>>>>>>>>
>>>>>>>>> .rq_flags       = q->elevator ? RQF_ELV : 0,
>>>>>>>>>
>>>>>>>>> then elevator is switched to real one from none, and check on q->elevator
>>>>>>>>> becomes not consistent.
>>>>>>>>
>>>>>>>> Indeed, that’s where I was going with this. I have a patch, testing it
>>>>>>>> locally but it’s getting late. Will send it out tomorrow. The nice
>>>>>>>> benefit is that it allows dropping the weird ref get on plug flush,
>>>>>>>> and batches getting the refs as well.
>>>>>>>
>>>>>>> Yi/Steffen, can you try pulling this into your test kernel:
>>>>>>>
>>>>>>> git://git.kernel.dk/linux-block for-next
>>>>>>>
>>>>>>> and see if it fixes the issue for you. Thanks!
>>>>>>
>>>>>> It still can be reproduced with the latest linux-block/for-next, here is the log
>>>>>>
>>>>>> fab2914e46eb (HEAD, new/for-next) Merge branch 'for-5.16/drivers' into for-next
>>>>>
>>>>> Hi Yi,
>>>>>
>>>>> Please try the following change:
>>>>>
>>>>>
>>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>>> index e1e64964a31b..eb634a9c61ff 100644
>>>>> --- a/block/blk-mq.c
>>>>> +++ b/block/blk-mq.c
>>>>> @@ -494,7 +494,6 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
>>>>>  		.q		= q,
>>>>>  		.flags		= flags,
>>>>>  		.cmd_flags	= op,
>>>>> -		.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>>>  		.nr_tags	= 1,
>>>>>  	};
>>>>>  	struct request *rq;
>>>>> @@ -504,6 +503,7 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
>>>>>  	if (ret)
>>>>>  		return ERR_PTR(ret);
>>>>>  
>>>>> +	data.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>>>  	rq = __blk_mq_alloc_requests(&data);
>>>>>  	if (!rq)
>>>>>  		goto out_queue_exit;
>>>>> @@ -524,7 +524,6 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>>>  		.q		= q,
>>>>>  		.flags		= flags,
>>>>>  		.cmd_flags	= op,
>>>>> -		.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>>>  		.nr_tags	= 1,
>>>>>  	};
>>>>>  	u64 alloc_time_ns = 0;
>>>>> @@ -551,6 +550,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>>>  	ret = blk_queue_enter(q, flags);
>>>>>  	if (ret)
>>>>>  		return ERR_PTR(ret);
>>>>> +	data.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>>
>>>> Don't think that will compile, but I guess the point is that we can't do
>>>> this assignment before queue enter, in case we're in the midst of
>>>> switching schedulers. Which is indeed a valid concern.
>>>
>>> Something like the below. Maybe? On top of the for-next that was already
>>> pulled in.
>>>
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index b01e05e02277..121f1898d529 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -433,9 +433,11 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
>>>  	if (data->cmd_flags & REQ_NOWAIT)
>>>  		data->flags |= BLK_MQ_REQ_NOWAIT;
>>>  
>>> -	if (data->rq_flags & RQF_ELV) {
>>> +	if (q->elevator) {
>>>  		struct elevator_queue *e = q->elevator;
>>>  
>>> +		data->rq_flags |= RQF_ELV;
>>> +
>>>  		/*
>>>  		 * Flush/passthrough requests are special and go directly to the
>>>  		 * dispatch list. Don't include reserved tags in the
>>> @@ -494,7 +496,6 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
>>>  		.q		= q,
>>>  		.flags		= flags,
>>>  		.cmd_flags	= op,
>>> -		.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>  		.nr_tags	= 1,
>>>  	};
>>>  	struct request *rq;
>>> @@ -524,7 +525,6 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>  		.q		= q,
>>>  		.flags		= flags,
>>>  		.cmd_flags	= op,
>>> -		.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>  		.nr_tags	= 1,
>>>  	};
>>>  	u64 alloc_time_ns = 0;
>>> @@ -565,6 +565,8 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>  
>>>  	if (!q->elevator)
>>>  		blk_mq_tag_busy(data.hctx);
>>> +	else
>>> +		data.rq_flags |= RQF_ELV;
>>>  
>>>  	ret = -EWOULDBLOCK;
>>>  	tag = blk_mq_get_tag(&data);
>>> @@ -2560,7 +2562,6 @@ void blk_mq_submit_bio(struct bio *bio)
>>>  			.q		= q,
>>>  			.nr_tags	= 1,
>>>  			.cmd_flags	= bio->bi_opf,
>>> -			.rq_flags	= q->elevator ? RQF_ELV : 0,
>>>  		};
>>
>> The above patch looks fine.
>>
>> BTW, 9ede85cb670c ("block: move queue enter logic into
>> blk_mq_submit_bio()") moves the queue enter into blk_mq_submit_bio(),
>> which seems dangerous, especially blk_mq_sched_bio_merge() needs
>> hctx/ctx which requires q_usage_counter to be grabbed.
> 
> I think the best solution is to enter just for that as well, and just
> retain that enter state. I'll update the patch, there's some real fixes
> in there too for the batched alloc. Will post them later today.

Is it needed, though? As far as I can tell, it's only needed
persistently for having the IO inflight, otherwise if the premise is
that the queue can just go away, we're in trouble before that too. And I
don't think that's the case.

-- 
Jens Axboe