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 09:03:02 -0600

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.

-- 
Jens Axboe