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

From: Ming Lei <[email-masked]>
Date: Wed, 3 Nov 2021 23:16:15 +0800

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.

Thanks,
Ming