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 10:36:11 -0600

On 11/3/21 10:09 AM, Ming Lei wrote:
> On Wed, Nov 03, 2021 at 09:49:20AM -0600, Jens Axboe wrote:
>> 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.
> 
> inflight bio just means that bdev is opened, and request queue won't
> go away.
> 
> But a lot things still can happen: elevator switch, update nr_hw_queues,
> clean up request queue, so looks blk_mq_sched_bio_merge() not safe
> without grabbing .q_usage_counter?

Yes good point, we need a consistent sched/queue view at that point.
I'll update it.

-- 
Jens Axboe