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 11:51:56 +0800

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