Re: [alsa-devel] [PATCH] sound: Fix race condition in the pcm_lib "wait for space loop

[Takashi Iwai <tiwai@suse.de>]

Hi,

sorry for the late follow-up, as I've been on vacation until today.

At Mon, 5 Sep 2011 09:49:47 -0700,
Arjan van de Ven wrote:
> 
> >From 2e37f0a4b2289962e1a45d8e02f8a7f7adad619f Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven <arjan@linux.intel.com>
> Date: Mon, 5 Sep 2011 09:40:18 -0700
> Subject: [PATCH] sound: Fix race condition in the pcm_lib "wait for space" loop
> 
> The wait_for_avail() function in pcm_lib.c has a race in it (observed in
> practice by an Intel validation group).
> 
> The function is supposed to return once space in the buffer has become
> available, or if some timeout happens.  The entity that creates space (irq
> handler of sound driver and some such) will do a wake up on a waitqueue that
> this function registers for.
> 
> However there are two races in the existing code
> 1) If space became available between the caller noticing there was no space and
>    this function actually sleeping, the wakeup is missed and the timeout
>    condition will happen instead
> 2) If a wakeup happened but not sufficient space became available, the code will loop
>    again and wait for more space. However, if the second wake comes in prior
>    to hitting the schedule_timeout_interruptible(), it will be missed, and
>    potentially you'll wait out until the timeout happens.
> 
> The fix consists of using more careful setting of the current state (so that
> if a wakeup happens in the main loop window, the schedule_timeout() falls
> through) and by checking for available space prior to going into the
> schedule_timeout() loop, but after being on the waitqueue and having the
> state set to interruptible.
> 
> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
> CC: Jaroslav Kysela <perex@perex.cz>
> CC: Takashi Iwai <tiwai@suse.de>
> CC: alsa-devel@alsa-project.org
> CC: linux-kernel@vger.kernel.org
> ---
>  sound/core/pcm_lib.c |   29 ++++++++++++++++++++++++++---
>  1 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index 86d0caf..8848080 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -1761,6 +1761,10 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
>  	snd_pcm_uframes_t avail = 0;
>  	long wait_time, tout;
>  
> +	init_waitqueue_entry(&wait, current);
> +	add_wait_queue(&runtime->tsleep, &wait);
> +	set_current_state(TASK_INTERRUPTIBLE);
> +
>  	if (runtime->no_period_wakeup)
>  		wait_time = MAX_SCHEDULE_TIMEOUT;
>  	else {
> @@ -1771,16 +1775,34 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
>  		}
>  		wait_time = msecs_to_jiffies(wait_time * 1000);
>  	}
> -	init_waitqueue_entry(&wait, current);
> -	add_wait_queue(&runtime->tsleep, &wait);
> +
> +	/*
> +	 * We need to check if space became available already (and thus the
> +	 * wakeup happened already) prior to going into the sleep loop to
> +	 * close the race of space already having become available.
> +	 * This check must happen after been added to the waitqueue and
> +	 * having current state be INTERRUPTIBLE.
> +	 */
> +
> +	if (is_playback)
> +		avail = snd_pcm_playback_avail(runtime);
> +	else
> +		avail = snd_pcm_capture_avail(runtime);
> +	if (avail >= runtime->twake)
> +		goto _endloop;
> +

Instead of adding this, we can move the check in the for loop at the
beginning of the loop, no?


>  	for (;;) {
>  		if (signal_pending(current)) {
>  			err = -ERESTARTSYS;
>  			break;
>  		}
>  		snd_pcm_stream_unlock_irq(substream);
> -		tout = schedule_timeout_interruptible(wait_time);
> +
> +		tout = schedule_timeout(wait_time);
> +
>  		snd_pcm_stream_lock_irq(substream);
> +		set_current_state(TASK_INTERRUPTIBLE);
>  		switch (runtime->status->state) {
>  		case SNDRV_PCM_STATE_SUSPENDED:
>  			err = -ESTRPIPE;
> @@ -1814,6 +1836,7 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
>  			break;
>  	}
>   _endloop:
> +	set_current_state(TASK_RUNNING);
>  	remove_wait_queue(&runtime->tsleep, &wait);
>  	*availp = avail;
>  	return err;


thanks,

Takashi