Re: [PATCH] block: mq-deadline: check if elevator is attached to queue in dd_finish_request

[Elijah Wright]

I see. would it be possible to detach the elevator from the queue in 
elevator_exit instead?

On 6/17/2025 3:25 PM, Bart Van Assche wrote:
> On 6/17/25 1:56 PM, Elijah Wright wrote:
> > in dd_finish_request(), per_prio points to a rq->elv.priv[0], which 
> > could be
> > free memory if an in-flight requests completes after its associated 
> > scheduler
> > has been freed
> >
> > Signed-off-by: Elijah Wright <git@elijahs.space>
> > ---
> >   block/mq-deadline.c | 16 +++++++++-------
> >   1 file changed, 9 insertions(+), 7 deletions(-)
> >
> > diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> > index 2edf1cac06d5..4d7b21b144d3 100644
> > --- a/block/mq-deadline.c
> > +++ b/block/mq-deadline.c
> > @@ -751,13 +751,15 @@ static void dd_finish_request(struct request *rq)
> >   {
> >       struct dd_per_prio *per_prio = rq->elv.priv[0];
> > -    /*
> > -     * The block layer core may call dd_finish_request() without having
> > -     * called dd_insert_requests(). Skip requests that bypassed I/O
> > -     * scheduling. See also blk_mq_request_bypass_insert().
> > -     */
> > -    if (per_prio)
> > -        atomic_inc(&per_prio->stats.completed);
> > +    if (rq->q->elevator) {
> > +        /*
> > +        * The block layer core may call dd_finish_request() without 
> > having
> > +        * called dd_insert_requests(). Skip requests that bypassed I/O
> > +        * scheduling. See also blk_mq_request_bypass_insert().
> > +        */
> > +        if (per_prio)
> > +            atomic_inc(&per_prio->stats.completed);
> > +    }
> >   }
>
> The warnings in dd_exit_sched() will be triggered if dd_finish_request()
> is ever called with rq->q->elevator == NULL.
>
> If this can happen, it should be fixed in the block layer core instead
> of in the mq-deadline scheduler.
>
> Thanks,
>
> Bart.