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

Ming Lei
2021-11-03T15:17:07Z

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 <axboe@kernel.dk> wrote:
> >>>>
> >>>> On 11/2/21 9:54 PM, Jens Axboe wrote:
> >>>>> On Nov 2, 2021, at 9:52 PM, Ming Lei <ming.lei@redhat.com> 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