Re: [PATCH v13 35/53] ALSA: usb-audio: Prevent starting of audio stream if in use

[Takashi Iwai <tiwai@xxxxxxx>]

On Sat, 03 Feb 2024 03:36:27 +0100,
Wesley Cheng wrote:
> 
> With USB audio offloading, an audio session is started from the ASoC
> platform sound card and PCM devices.  Likewise, the USB SND path is still
> readily available for use, in case the non-offload path is desired.  In
> order to prevent the two entities from attempting to use the USB bus,
> introduce a flag that determines when either paths are in use.
> 
> If a PCM device is already in use, the check will return an error to
> userspace notifying that the stream is currently busy.  This ensures that
> only one path is using the USB substream.
> 
> Signed-off-by: Wesley Cheng <quic_wcheng@xxxxxxxxxxx>

Hm, I'm not sure whether it's safe to hold chip->mutex there for the
long code path.  It even kicks off the auto-resume, which may call
various functions at resuming, and some of them may re-hold
chip->mutex.

If it's only about the open flag, protect only the flag access with
the mutex, not covering the all open function.  At least the re-entry
can be avoided by that.


thanks,

Takashi

> ---
>  sound/usb/card.h |  1 +
>  sound/usb/pcm.c  | 19 +++++++++++++++++--
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/usb/card.h b/sound/usb/card.h
> index ed4a664e24e5..6d59995440c3 100644
> --- a/sound/usb/card.h
> +++ b/sound/usb/card.h
> @@ -165,6 +165,7 @@ struct snd_usb_substream {
>  	unsigned int pkt_offset_adj;	/* Bytes to drop from beginning of packets (for non-compliant devices) */
>  	unsigned int stream_offset_adj;	/* Bytes to drop from beginning of stream (for non-compliant devices) */
>  
> +	unsigned int opened:1;		/* pcm device opened */
>  	unsigned int running: 1;	/* running status */
>  	unsigned int period_elapsed_pending;	/* delay period handling */
>  
> diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
> index 3adb09ce1702..c2cb52cd5d23 100644
> --- a/sound/usb/pcm.c
> +++ b/sound/usb/pcm.c
> @@ -1241,8 +1241,15 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream)
>  	struct snd_usb_stream *as = snd_pcm_substream_chip(substream);
>  	struct snd_pcm_runtime *runtime = substream->runtime;
>  	struct snd_usb_substream *subs = &as->substream[direction];
> +	struct snd_usb_audio *chip = subs->stream->chip;
>  	int ret;
>  
> +	mutex_lock(&chip->mutex);
> +	if (subs->opened) {
> +		mutex_unlock(&chip->mutex);
> +		return -EBUSY;
> +	}
> +
>  	runtime->hw = snd_usb_hardware;
>  	/* need an explicit sync to catch applptr update in low-latency mode */
>  	if (direction == SNDRV_PCM_STREAM_PLAYBACK &&
> @@ -1259,13 +1266,17 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream)
>  
>  	ret = setup_hw_info(runtime, subs);
>  	if (ret < 0)
> -		return ret;
> +		goto out;
>  	ret = snd_usb_autoresume(subs->stream->chip);
>  	if (ret < 0)
> -		return ret;
> +		goto out;
>  	ret = snd_media_stream_init(subs, as->pcm, direction);
>  	if (ret < 0)
>  		snd_usb_autosuspend(subs->stream->chip);
> +	subs->opened = 1;
> +out:
> +	mutex_unlock(&chip->mutex);
> +
>  	return ret;
>  }
>  
> @@ -1274,6 +1285,7 @@ static int snd_usb_pcm_close(struct snd_pcm_substream *substream)
>  	int direction = substream->stream;
>  	struct snd_usb_stream *as = snd_pcm_substream_chip(substream);
>  	struct snd_usb_substream *subs = &as->substream[direction];
> +	struct snd_usb_audio *chip = subs->stream->chip;
>  	int ret;
>  
>  	snd_media_stop_pipeline(subs);
> @@ -1287,6 +1299,9 @@ static int snd_usb_pcm_close(struct snd_pcm_substream *substream)
>  
>  	subs->pcm_substream = NULL;
>  	snd_usb_autosuspend(subs->stream->chip);
> +	mutex_lock(&chip->mutex);
> +	subs->opened = 0;
> +	mutex_unlock(&chip->mutex);
>  
>  	return 0;
>  }