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Mon, 3 Aug 2026 06:56:08 +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 oIlkGIg7cGqCBAAAD6G6ig (envelope-from ); Mon, 03 Aug 2026 06:56:08 +0000 Date: Mon, 03 Aug 2026 08:56:07 +0200 Message-ID: <87ecgfbsl4.wl-tiwai@suse.de> From: Takashi Iwai To: Yu-Hsuan Hsu Cc: Takashi Iwai , linux-kernel@vger.kernel.org, Jaroslav Kysela , Takashi Iwai , =?ISO-8859-1?Q?C?= =?ISO-8859-1?Q?=E1ssio?= Gabriel , linux-sound@vger.kernel.org Subject: Re: [PATCH] ALSA: aloop: Fix spinlock deadlock in loopback_hrtimer_stop() In-Reply-To: References: <20260731074255.1513402-1-yuhsuan@chromium.org> <8733wycn2p.wl-tiwai@suse.de> 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=ISO-2022-JP X-Spamd-Result: default: False [-3.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCPT_COUNT_SEVEN(0.00)[7]; RCVD_VIA_SMTP_AUTH(0.00)[]; TO_DN_SOME(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; ARC_NA(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[suse.de,vger.kernel.org,perex.cz,suse.com,gmail.com]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[chromium.org:email,suse.de:email,suse.de:mid,suse.de:dkim,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo]; URIBL_BLOCKED(0.00)[chromium.org:email,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:email,suse.de:mid,suse.de:dkim]; DKIM_TRACE(0.00)[suse.de:+] X-Spam-Flag: NO X-Spam-Score: -3.51 X-Spam-Level: X-Rspamd-Queue-Id: 9EAC97EF33 X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action On Sun, 02 Aug 2026 15:58:26 +0200, Yu-Hsuan Hsu wrote: > > Hi Takashi, > > Thanks. I actually encountered a CPU deadlock during the aloop stress > test. (Resent as plain text.) Thanks for confirmation. I asked it to judge for 7.2-rc7 fix, but I missed the point that it's the fix for the new change for 7.3 :) Now applied to for-next branch. thanks, Takashi > > Best, > Yu-Hsuan > > Takashi Iwai 於 2026年8月1日週六 下午3:33寫道： > > > > On Fri, 31 Jul 2026 09:39:35 +0200, > > Yu-Hsuan Hsu wrote: > > > > > > In loopback_hrtimer_stop(), calling hrtimer_cancel() while holding > > > cable->lock triggers an AB-BA spinlock deadlock if the hrtimer softirq > > > is executing concurrently on another CPU: > > > > > > 1) CPU A runs loopback_trigger(STOP), acquires spin_lock(&cable->lock), > > > and calls hrtimer_cancel(). Since hrtimer_cancel() is synchronous, > > > it spins waiting for the executing callback to complete before > > > returning. > > > 2) CPU B executes loopback_hrtimer_function(), which immediately tries > > > to acquire spin_lock(&cable->lock). > > > > > > This mutual dependency leads to a CPU hard lockup and NMI watchdog > > > panic when multiple streams start and stop concurrently with small > > > period sizes. > > > > > > Replace hrtimer_cancel() in loopback_hrtimer_stop() with the non-blocking > > > hrtimer_try_to_cancel(), matching the behavior of jiffies timers > > > (timer_delete vs timer_delete_sync). If try_to_cancel returns -1 > > > because the handler is running, CPU A releases cable->lock cleanly. > > > When the running handler subsequently acquires cable->lock, it observes > > > that the stream is no longer in running state (cleared by trigger STOP) > > > and terminates without re-arming the timer. Synchronous hrtimer_cancel() > > > remains preserved in loopback_hrtimer_stop_sync() where cable->lock is > > > not held. > > > > > > Fixes: bf08a5f698dc ("ALSA: aloop: Add 'hrtimer' option to timer_source") > > > Signed-off-by: Yu-Hsuan Hsu > > > > While I find it's fine to change like this, I wonder whether you > > really hit a CPU deadlock. Or it's just hypothetical? > > > > > > thanks, > > > > Takashi