Re: [PATCH] ALSA: Fix deadlocks with kctl removals at disconnection

[Jaroslav Kysela <perex@xxxxxxxx>]

On 10. 05. 24 12:14, Takashi Iwai wrote:
> In snd_card_disconnect(), we set card->shutdown flag at the beginning,
> call callbacks and do sync for card->power_ref_sleep waiters at the
> end.  The callback may delete a kctl element, and this can lead to a
> deadlock when the device was in the suspended state.  Namely:
>
> * A process waits for the power up at snd_power_ref_and_wait() in
>    snd_ctl_info() or read/write() inside card->controls_rwsem.
>
> * The system gets disconnected meanwhile, and the driver tries to
>    delete a kctl via snd_ctl_remove*(); it tries to take
>    card->controls_rwsem again, but this is already locked by the
>    above.  Since the sleeper isn't woken up, this deadlocks.
>
> An easy fix is to wake up sleepers before processing the driver
> disconnect callbacks but right after setting the card->shutdown flag.
> Then all sleepers will abort immediately, and the code flows again.
>
> So, basically this patch moves the wait_event() call at the right
> timing.  While we're at it, just to be sure, call wait_event_all()
> instead of wait_event(), although we don't use exclusive events on
> this queue for now.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218816
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>

LGTM

Reviewed-by: Jaroslav Kysela <perex@xxxxxxxx>

--
Jaroslav Kysela <perex@xxxxxxxx>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.