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Date:   Mon, 07 Mar 2022 09:31:16 +0100
Message-ID: <s5hfsnup3jf.wl-tiwai@suse.de>
From:   Takashi Iwai <tiwai@suse.de>
To:     Hillf Danton <hdanton@sina.com>
Cc:     syzbot <syzbot+1ee0910eca9c94f71f25@syzkaller.appspotmail.com>,
        alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, tiwai@suse.com
Subject: Re: [syzbot] possible deadlock in snd_timer_interrupt (2)
In-Reply-To: <20220307080520.3199-1-hdanton@sina.com>
References: <00000000000048c71405d96594c7@google.com>
        <20220307080520.3199-1-hdanton@sina.com>
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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);
> --
>