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 23:09:22 +0800

On Wed, Nov 03, 2021 at 09:03:02AM -0600, 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

It can compile.

> this assignment before queue enter, in case we're in the midst of
> switching schedulers. Which is indeed a valid concern.

Yeah, for scsi, real io sched is switched when adding disk, before that, the
passthrough command need to see consistent q->elevator.

Thanks,
Ming