Re: [PATCH] ALSA: usb-audio: Fix recursive locking at XRUN during syncing

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

On Tue, Mar 21, 2023 at 01:02:58PM +0900, Takashi Sakamoto wrote:
> Hi,
> 
> On Mon, Mar 20, 2023 at 03:28:38PM +0100, Takashi Iwai wrote:
> > The recent support of low latency playback in USB-audio driver made
> > the snd_usb_queue_pending_output_urbs() function to be called via PCM
> > ack ops.  In the new code path, the function is performed alread in
> 
> 's/alread/already/' or slang.
> 
> > the PCM stream lock.  The problem is that, when an XRUN is detected,
> > the  function calls snd_pcm_xrun() to notify, but snd_pcm_xrun() is
> 
> 's/the  function/the function/'
> 
> > supposed to be called only outside the stream lock.  As a result, it
> > leads to a deadlock of PCM stream locking.
> > 
> > For avoiding such a recursive locking, this patch adds an additional
> > check to the code paths in PCM core that call the ack callback; now it
> > checks the error code from the callback, and if it's -EPIPE, the XRUN
> > is handled in the PCM core side gracefully.  Along with it, the
> > USB-audio driver code is changed to follow that, i.e. -EPIPE is
> > returned instead of the explicit snd_pcm_xrun() call when the function
> > is performed already in the stream lock.
>  
> Practically, the implementation of 'pcm_hw' in alsa-lib never evaluates
> the return value (see 'snd_pcm_hw_mmap_commit()' and the others). I guess
> that it is inconvenient for the low-latency mode of USB Audio device class
> driver for the case of failure.
> 
> My additional concern is PCM indirect layer, since typically the layer
> is used by drivers with SNDRV_PCM_INFO_SYNC_APPLPTR. But as long as I
> read, the change does not matter to them.
> 
> > Fixes: d5f871f89e21 ("ALSA: usb-audio: Improved lowlatency playback support")
> > Reported-and-tested-by: John Keeping <john@metanate.com>
> > Link: https://lore.kernel.org/r/20230317195128.3911155-1-john@metanate.com
> > Cc: Takashi Sakamoto <o-takashi@sakamocchi.jp>
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > ---
> >  sound/core/pcm_lib.c |  2 ++
> >  sound/usb/endpoint.c | 22 ++++++++++++++--------
> >  sound/usb/endpoint.h |  4 ++--
> >  sound/usb/pcm.c      |  2 +-
> >  4 files changed, 19 insertions(+), 11 deletions(-)
>  
> Anyway, John Keeping reports the change works well to solve the issue. I
> have no objection to it.
> 
> Reviewed-by; Takashi Sakamoto <o-takashi@sakamocchi.jp>
 
I forgot to mention that it is preferable to update 'ack callback' clause
of API Documentation (i.e. writing-an-alsa-driver.rst) as well.

> It is also convenient to IEC 61883-1/6 packet streaming engine for audio
> and music unit in IEEE 1394 bus. I will post patches enough later for it.
> 
> > diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> > index 8b6aeb8a78f7..02fd65993e7e 100644
> > --- a/sound/core/pcm_lib.c
> > +++ b/sound/core/pcm_lib.c
> > @@ -2155,6 +2155,8 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
> >  		ret = substream->ops->ack(substream);
> >  		if (ret < 0) {
> >  			runtime->control->appl_ptr = old_appl_ptr;
> > +			if (ret == -EPIPE)
> > +				__snd_pcm_xrun(substream);
> >  			return ret;
> >  		}
> >  	}
> > diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
> > index 419302e2057e..647fa054d8b1 100644
> > --- a/sound/usb/endpoint.c
> > +++ b/sound/usb/endpoint.c
> > @@ -455,8 +455,8 @@ static void push_back_to_ready_list(struct snd_usb_endpoint *ep,
> >   * This function is used both for implicit feedback endpoints and in low-
> >   * latency playback mode.
> >   */
> > -void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> > -				       bool in_stream_lock)
> > +int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> > +				      bool in_stream_lock)
> >  {
> >  	bool implicit_fb = snd_usb_endpoint_implicit_feedback_sink(ep);
> >  
> > @@ -480,7 +480,7 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> >  		spin_unlock_irqrestore(&ep->lock, flags);
> >  
> >  		if (ctx == NULL)
> > -			return;
> > +			break;
> >  
> >  		/* copy over the length information */
> >  		if (implicit_fb) {
> > @@ -495,11 +495,14 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> >  			break;
> >  		if (err < 0) {
> >  			/* push back to ready list again for -EAGAIN */
> > -			if (err == -EAGAIN)
> > +			if (err == -EAGAIN) {
> >  				push_back_to_ready_list(ep, ctx);
> > -			else
> > +				break;
> > +			}
> > +
> > +			if (!in_stream_lock)
> >  				notify_xrun(ep);
> > -			return;
> > +			return -EPIPE;
> >  		}
> >  
> >  		err = usb_submit_urb(ctx->urb, GFP_ATOMIC);
> > @@ -507,13 +510,16 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> >  			usb_audio_err(ep->chip,
> >  				      "Unable to submit urb #%d: %d at %s\n",
> >  				      ctx->index, err, __func__);
> > -			notify_xrun(ep);
> > -			return;
> > +			if (!in_stream_lock)
> > +				notify_xrun(ep);
> > +			return -EPIPE;
> >  		}
> >  
> >  		set_bit(ctx->index, &ep->active_mask);
> >  		atomic_inc(&ep->submitted_urbs);
> >  	}
> > +
> > +	return 0;
> >  }
> >  
> >  /*
> > diff --git a/sound/usb/endpoint.h b/sound/usb/endpoint.h
> > index 924f4351588c..c09f68ce08b1 100644
> > --- a/sound/usb/endpoint.h
> > +++ b/sound/usb/endpoint.h
> > @@ -52,7 +52,7 @@ int snd_usb_endpoint_implicit_feedback_sink(struct snd_usb_endpoint *ep);
> >  int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep,
> >  				      struct snd_urb_ctx *ctx, int idx,
> >  				      unsigned int avail);
> > -void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> > -				       bool in_stream_lock);
> > +int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
> > +				      bool in_stream_lock);
> >  
> >  #endif /* __USBAUDIO_ENDPOINT_H */
> > diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
> > index d959da7a1afb..eec5232f9fb2 100644
> > --- a/sound/usb/pcm.c
> > +++ b/sound/usb/pcm.c
> > @@ -1639,7 +1639,7 @@ static int snd_usb_pcm_playback_ack(struct snd_pcm_substream *substream)
> >  	 * outputs here
> >  	 */
> >  	if (!ep->active_mask)
> > -		snd_usb_queue_pending_output_urbs(ep, true);
> > +		return snd_usb_queue_pending_output_urbs(ep, true);
> >  	return 0;
> >  }
> >  
> > -- 
> > 2.35.3
> 
> 
> Thanks
> 
> Takashi Sakamoto