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SubjectRe: [syzbot] possible deadlock in snd_timer_interrupt (2)
On Mon, 07 Mar 2022 09:05:20 +0100,
Hillf Danton wrote:
>
> Walk around the deadlock by trying to lock tasklist_lock for write on
> timer irq and scheduling workqueue work if any lock owner detected.

Oh no, that's toooo ugly.

And the problem isn't only here; take a look at commits f671a691e299
and 2f488f698fda. There are other users of kill_fasync() with the
hard-IRQ disabled, too.

So, IMO, the handling of tasklist_lock around kill_fasync() looks
broken and the fix should be needed there (or other core part),
instead of messing round each caller's code.


thanks,

Takashi

>
> Only for thoughts now.
>
> Hillf
>
> #syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ 38f80f42147f
>
> --- x/sound/core/timer.c
> +++ y/sound/core/timer.c
> @@ -916,7 +916,14 @@ void snd_timer_interrupt(struct snd_time
> }
>
> /* now process all fast callbacks */
> - snd_timer_process_callbacks(timer, &timer->ack_list_head);
> + if (write_trylock(&tasklist_lock)) {
> + write_unlock(&tasklist_lock);
> + snd_timer_process_callbacks(timer, &timer->ack_list_head);
> + } else {
> + /* go the slow path to avoid deadlock by calling kill_fasync() */
> + list_splice_init(&timer->ack_list_head,
> + &timer->sack_list_head);
> + }
>
> /* do we have any slow callbacks? */
> use_work = !list_empty(&timer->sack_list_head);
> --
>

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