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

[Takashi Iwai <tiwai@xxxxxxx>]

On Thu, 08 Feb 2024 02:12:00 +0100,
Wesley Cheng wrote:
> 
> Hi Takashi,
> 
> On 2/7/2024 4:02 PM, Wesley Cheng wrote:
> > Hi Takashi,
> > 
> > On 2/6/2024 11:05 PM, Takashi Iwai wrote:
> >> On Wed, 07 Feb 2024 01:08:00 +0100,
> >> Wesley Cheng wrote:
> >>> 
> >>> Hi Takashi,
> >>> 
> >>> On 2/6/2024 5:07 AM, Takashi Iwai wrote:
> >>>> 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.
> >>>> 
> >>> 
> >>> That's a good point.
> >>> 
> >>>> 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.
> >>>> 
> >>> 
> >>> Sure, let me re-order the check/assignment and the mutex locking.
> >>> Since this is now checked here in USB PCM and the QC offload driver,
> >>> we want to make sure that if there was some application attempting to
> >>> open both at the same time, we prevent any possible races.
> >>> 
> >>> I think the best way to address this would be something like:
> >>> 
> >>> static int snd_usb_pcm_open(struct snd_pcm_substream *substream)
> >>> {
> >>> ...
> >>>     mutex_lock(&chip->mutex);
> >>>     if (subs->opened) {
> >>>         mutex_unlock(&chip->mutex);
> >>>         return -EBUSY;
> >>>     }
> >>>     subs->opened = 1;
> >>>     mutex_unlock(&chip->mutex);
> >>> 
> >>> //Execute bulk of PCM open routine
> >>> ...
> >>>     return 0;
> >>> 
> >>> // If any errors are seen, unwind
> >>> err_resume:
> >>>     snd_usb_autosuspend(subs->stream->chip);
> >>> err_open:
> >>>     mutex_lock(&chip->mutex);
> >>>     subs->opened = 0;
> >>>     mutex_unlock(&chip->mutex);
> >>> 
> >>>     return ret;
> >>> }
> >>> 
> >>> Set the opened flag first, so that if QC offload checks it, it can
> >>> exit early and vice versa.  Otherwise, if we set the opened flag at
> >>> the same position as the previous patch, we may be calling the other
> >>> routines in parallel to the QC offload enable stream routine.  The
> >>> only thing with this patch is that we'd need some error handling
> >>> unwinding.
> >> 
> >> The above is what I had in mind.
> >> 
> >> But, thinking on this again, you might be able to get the same result
> >> by using the ALSA PCM core substream open_mutex and hw_opened flag.
> >> This is already held and set at snd_pcm_core() (the hw_opened flag is
> >> set after open callback, though).  The offload driver can use those
> >> instead of the own lock and flag, too, although it's not really
> >> well-mannered behavior (hence you need proper comments).
> >> 
> > 
> > I think I had looked into this as well previously, and it was
> > difficult to achieve, because from the USB offloading perspective,
> > we don't ever call: snd_usb_pcm_open()
> > 
> > This is actually where we populate the pcm_substream parameter
> > within struct snd_usb_substream based on when userspace opens the
> > USB SND PCM device (which is not the case for offloading).  So the
> > offload driver doesn't have a way to fetch the struct snd_pcm that
> > is allocated to the PCM device created by the USB SND card.
> > 
> 
> Sorry, took a look at it again, and found a way.  Although not pretty,
> we can access it using:
> subs->stream->pcm->streams[direction].substream->hw_opened

Yes, it's not easy to follow it.  So if we want to this path, worth
for a detailed comment.  That said, I don't mind to introduce the new
local mutex and flag as you did if the above became too messy in the
end.


thanks,

Takashi