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

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

[Takashi Iwai]

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
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
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.

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(-)

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

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

[Jaroslav Kysela]

On 20. 03. 23 15:28, 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
> 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
> 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.
> 
> 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>

I think that this is the best variant of the proposed solutions.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>

					Jaroslav

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

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

[Takashi Sakamoto]

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>

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

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

[Takashi Sakamoto]

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

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

[Takashi Iwai]

On Tue, 21 Mar 2023 05:02:58 +0100,
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/'

Corrected at applying.

> > 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.

I find rather that's an extra bonus :)  It allows the existing code to
give a more proper error handling.  For example, the indirect PCM
helper returned -EINVAL, but this can be switched to -EPIPE for
stopping the stream.

I'm going to submit the patch together with the documentation
updates.


thanks,

Takashi

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

[Takashi Sakamoto]

On Thu, Mar 23, 2023 at 07:19:45AM +0100, Takashi Iwai wrote:
> On Tue, 21 Mar 2023 05:02:58 +0100,
> 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/'
> 
> Corrected at applying.
> 
> > > 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.
> 
> I find rather that's an extra bonus :)  It allows the existing code to
> give a more proper error handling.  For example, the indirect PCM
> helper returned -EINVAL, but this can be switched to -EPIPE for
> stopping the stream.
> 
> I'm going to submit the patch together with the documentation
> updates.

In my opinion, extra care is required for the simple replacement of
-EINVAL with -EPIPE, since it may not come from any operation relevant
to hardware. In my understanding, it comes from just careless
implementation of user space PCM application, thus it is still reasonable
to return -EINVAL.

... However, you are the best person who has enough knowledge about the
layer. I would not mind the replacement if you did.


Regards

Takashi Sakamoto

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

[Takashi Iwai]

On Thu, 23 Mar 2023 14:29:16 +0100,
Takashi Sakamoto wrote:
> 
> On Thu, Mar 23, 2023 at 07:19:45AM +0100, Takashi Iwai wrote:
> > On Tue, 21 Mar 2023 05:02:58 +0100,
> > 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/'
> > 
> > Corrected at applying.
> > 
> > > > 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.
> > 
> > I find rather that's an extra bonus :)  It allows the existing code to
> > give a more proper error handling.  For example, the indirect PCM
> > helper returned -EINVAL, but this can be switched to -EPIPE for
> > stopping the stream.
> > 
> > I'm going to submit the patch together with the documentation
> > updates.
> 
> In my opinion, extra care is required for the simple replacement of
> -EINVAL with -EPIPE, since it may not come from any operation relevant
> to hardware. In my understanding, it comes from just careless
> implementation of user space PCM application, thus it is still reasonable
> to return -EINVAL.

Yeah, we can't always replace such condition blindly.  But in the case
of indirect PCM helpers, it's the place where the bogus values have
been already set and processed, so we can't return -EINVAL, hence an
XRUN notification would be appreciate.


Takashi