Re: 6.15/regression/bisected - lockdep warning: circular locking dependency detected when plugging USB stick after ffa1e7ada456

[Hillf Danton <hdanton@sina.com>]

On Thu, 24 Jul 2025 01:36:37 +0500 Mikhail Gavrilov wrote:
> On Wed, Jul 23, 2025 at 6:03 AM Hillf Danton <hdanton@sina.com> wrote:
> >
> > In order to cure the deadlock, queue is thawed before switching elevator,
> > so lets see what comes out with that warning ignored.
> >
> > --- x/block/elevator.c
> > +++ y/block/elevator.c
> > @@ -575,7 +575,6 @@ static int elevator_switch(struct reques
> >         struct elevator_type *new_e = NULL;
> >         int ret = 0;
> >
> > -       WARN_ON_ONCE(q->mq_freeze_depth == 0);
> >         lockdep_assert_held(&q->elevator_lock);
> >
> >         if (strncmp(ctx->name, "none", 4)) {
> > @@ -661,6 +660,7 @@ static int elevator_change(struct reques
> >         unsigned int memflags;
> >         int ret = 0;
> >
> > +       /* updaters should be serialized */
> >         lockdep_assert_held(&q->tag_set->update_nr_hwq_lock);
> >
> >         memflags = blk_mq_freeze_queue(q);
> > @@ -674,11 +674,11 @@ static int elevator_change(struct reques
> >          * Disk isn't added yet, so verifying queue lock only manually.
> >          */
> >         blk_mq_cancel_work_sync(q);
> > +       blk_mq_unfreeze_queue(q, memflags);
> >         mutex_lock(&q->elevator_lock);
> >         if (!(q->elevator && elevator_match(q->elevator->type, ctx->name)))
> >                 ret = elevator_switch(q, ctx);
> >         mutex_unlock(&q->elevator_lock);
> > -       blk_mq_unfreeze_queue(q, memflags);
> >         if (!ret)
> >                 ret = elevator_change_done(q, ctx);
> >
> > --
> 
> Hi Hillf,
> 
> Thanks for the patch.
> 
> With this patch applied, I haven't seen either the lockdep warning or
> a soft lockup within 13 hours of runtime. Not sure if that's
> sufficient yet for a final verdict, but it's definitely promising.

Thank you for testing it.

It works for you so far, but given the "correct" locking order enforced
in ffa1e7ada456, I know the chance for reversing that order is not zero
yet either in Jens or upstream tree. Nor simple to fix every single case.

Hillf Danton