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Thu, 8 Feb 2024 08:33:58 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap1.dmz-prg2.suse.org with ESMTPSA id /uiqOPaRxGWSIAAAD6G6ig (envelope-from ); Thu, 08 Feb 2024 08:33:58 +0000 Date: Thu, 08 Feb 2024 09:33:58 +0100 Message-ID: <87zfwb4ao9.wl-tiwai@suse.de> From: Takashi Iwai To: Wesley Cheng Cc: , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH v13 35/53] ALSA: usb-audio: Prevent starting of audio stream if in use In-Reply-To: <0cb39613-ec01-50aa-807f-b537f201dac0@quicinc.com> References: <20240203023645.31105-1-quic_wcheng@quicinc.com> <20240203023645.31105-36-quic_wcheng@quicinc.com> <87y1bxvj0o.wl-tiwai@suse.de> <877cjg7o0k.wl-tiwai@suse.de> <810161b3-4d98-755f-163f-fdfc9fe37063@quicinc.com> <0cb39613-ec01-50aa-807f-b537f201dac0@quicinc.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/27.2 Mule/6.0 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; 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SUSPICIOUS_RECIPS(1.50)[] X-Spam-Flag: NO 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 > >>>> > >>>> 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