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: Thu, 4 Nov 2021 00:09:49 +0800

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?

Thanks,
Ming