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Mon, 16 Mar 2026 08:40:01 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id yd5PJeHBt2lsZwAAD6G6ig (envelope-from ); Mon, 16 Mar 2026 08:40:01 +0000 Date: Mon, 16 Mar 2026 09:40:01 +0100 Message-ID: <87fr609nha.wl-tiwai@suse.de> From: Takashi Iwai To: Cen Zhang Cc: perex@perex.cz, tiwai@suse.com, chleroy@kernel.org, linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org, baijiaju1990@gmail.com, r33s3n6@gmail.com, gality369@gmail.com, zhenghaoran154@gmail.com, hanguidong02@gmail.com, ziyuzhang201@gmail.com Subject: Re: [PATCH] ALSA: pcm: oss: annotate data-races around runtime->state In-Reply-To: <20260316030550.2848349-1-zzzccc427@gmail.com> References: <20260316030550.2848349-1-zzzccc427@gmail.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/30.2 Mule/6.0 Precedence: bulk X-Mailing-List: linux-sound@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 X-Spam-Score: -3.30 X-Spam-Level: X-Spamd-Result: default: False [-3.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_TLS_ALL(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCPT_COUNT_TWELVE(0.00)[12]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; FREEMAIL_TO(0.00)[gmail.com]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[perex.cz,suse.com,kernel.org,vger.kernel.org,gmail.com]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:mid] X-Spam-Flag: NO On Mon, 16 Mar 2026 04:05:50 +0100, Cen Zhang wrote: > > __snd_pcm_set_state() writes runtime->state under the PCM stream lock: > > runtime->state = state; > > However, the OSS I/O functions snd_pcm_oss_write3(), snd_pcm_oss_read3(), > snd_pcm_oss_writev3() and snd_pcm_oss_readv3() read runtime->state > without holding the stream lock, only holding oss.params_lock (a > different mutex that does not synchronize with the stream lock): > > if (runtime->state == SNDRV_PCM_STATE_XRUN || ...) > > Since __snd_pcm_set_state() is called from IRQ context (e.g., > snd_pcm_period_elapsed -> snd_pcm_update_state -> __snd_pcm_xrun -> > snd_pcm_stop -> snd_pcm_post_stop) while the OSS read/write paths > run in process context, these are concurrent accesses that constitute > a data race. > > The code handles stale reads gracefully through its retry loop > (re-checking after __snd_pcm_lib_xfer returns -EPIPE), so the race > is not harmful under simple interleaving. However, plain C accesses > are formally undefined under LKMM, and without READ_ONCE the compiler > is free to fuse or cache the loads across loop iterations. > > Add WRITE_ONCE() in __snd_pcm_set_state() for the write side and > READ_ONCE() on all lockless reads of runtime->state in the four OSS > I/O functions. > > Signed-off-by: Cen Zhang Thanks for the patch. I believe it's better not to go with barriers but rather taking the proper spinlock, as it's only for this OSS layer, and other places are already doing so. That said, - Export snd_pcm_set_state() in pcm_native.c - Introduce snd_pcm_get_state() helper just to call like snd_pcm_state_t snd_pcm_get_state(struct snd_pcm_substream *substream) { guard(pcm_stream_lock_irqsave)(substream); return substream->runtime->state; } - Use those for setting the state in pcm_oss.c appropriately; some places are already in the lock, and they don't use the above. Also avoid calling snd_pcm_get_state() repeatedly if not needed. Care to revise the patch and resubmit? thanks, Takashi