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

Ming Lei
2021-11-03T03:52:19Z

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.

Thanks,
Ming