[PATCH v2 1/4] ALSA: core: Add async signal helpers

[Jaroslav Kysela]

On 28. 07. 22 14:59, Takashi Iwai wrote:
> Currently the call of kill_fasync() from an interrupt handler might
> lead to potential spin deadlocks, as spotted by syzkaller.
> Unfortunately, it's not so trivial to fix this lock chain as it's
> involved with the tasklist_lock that is touched in allover places.
> 
> As a temporary workaround, this patch provides the way to defer the
> async signal notification in a work.  The new helper functions,
> snd_fasync_helper() and snd_kill_faync() are replacements for
> fasync_helper() and kill_fasync(), respectively.  In addition,
> snd_fasync_free() needs to be called at the destructor of the relevant
> file object.
> 
> Signed-off-by: Takashi Iwai <tiwai at suse.de>

...

> +void snd_kill_fasync(struct snd_fasync *fasync, int signal, int poll)
> +{
> +	unsigned long flags;
> +
> +	if (!fasync || !fasync->on)
> +		return;
> +	spin_lock_irqsave(&snd_fasync_lock, flags);
> +	fasync->signal = signal;
> +	fasync->poll = poll;
> +	list_move(&fasync->list, &snd_fasync_list);
> +	schedule_work(&snd_fasync_work);
> +	spin_unlock_irqrestore(&snd_fasync_lock, flags);
> +}

The schedule_work() may be called outside the spinlock - it calls 
queue_work_on() / __queue_work() which has already own protection for the 
concurrent execution.

				Jaroslav

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