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Wed, 7 Feb 2024 07:05:16 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap1.dmz-prg2.suse.org with ESMTPSA id TiDJDqwrw2XfLAAAD6G6ig (envelope-from ); Wed, 07 Feb 2024 07:05:16 +0000 Date: Wed, 07 Feb 2024 08:05:15 +0100 Message-ID: <877cjg7o0k.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: References: <20240203023645.31105-1-quic_wcheng@quicinc.com> <20240203023645.31105-36-quic_wcheng@quicinc.com> <87y1bxvj0o.wl-tiwai@suse.de> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/27.2 Mule/6.0 Precedence: bulk X-Mailing-List: linux-doc@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; charset=US-ASCII Authentication-Results: smtp-out1.suse.de; dkim=pass header.d=suse.de header.s=susede2_rsa header.b=BVXP1nkI; dkim=pass header.d=suse.de header.s=susede2_ed25519 header.b=CfugtL1Q X-Spamd-Result: default: False [-4.31 / 50.00]; RCVD_VIA_SMTP_AUTH(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_DN_SOME(0.00)[]; R_RATELIMIT(0.00)[to_ip_from(RLe67txhfobum3fqdb5xx8e3au)]; RCVD_COUNT_THREE(0.00)[3]; DKIM_TRACE(0.00)[suse.de:+]; MX_GOOD(-0.01)[]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; BAYES_HAM(-3.00)[100.00%]; ARC_NA(0.00)[]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; TAGGED_RCPT(0.00)[dt]; MIME_GOOD(-0.10)[text/plain]; DWL_DNSWL_HI(-3.50)[suse.de:dkim]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCPT_COUNT_TWELVE(0.00)[23]; MID_CONTAINS_FROM(1.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[quicinc.com:email,suse.de:dkim]; FUZZY_BLOCKED(0.00)[rspamd.com]; FREEMAIL_CC(0.00)[linaro.org,intel.com,perex.cz,kernel.org,lwn.net,gmail.com,linuxfoundation.org,synopsys.com,quicinc.com,suse.com,vger.kernel.org,alsa-project.org]; RCVD_TLS_ALL(0.00)[]; SUSPICIOUS_RECIPS(1.50)[] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Queue-Id: E21512216B X-Spam-Level: X-Spam-Score: -4.31 X-Spam-Flag: NO 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). thanks, Takashi