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

[Mikhail Gavrilov]

On Wed, Jul 23, 2025 at 6:03 AM Hillf Danton <hdanton@xxxxxxxx> 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.

The full dmesg trace is attached below.

-- 
Best Regards,
Mike Gavrilov.

Attachment: dmesg-6.16.0-rc7-with-fix-2.zip
Description: Zip archive