Re: [PATCH] ALSA: pcm: Disable bottom softirqs as part of spin_lock_irq() on PREEMPT_RT

Takashi Iwai
2025-09-16T09:43:56Z

On Mon, 15 Sep 2025 17:28:51 +0200,
Sebastian Andrzej Siewior wrote:
>
> snd_pcm_group_lock_irq() acquires a spinlock_t and disables interrupts
> via spin_lock_irq(). This also implicitly disables the handling of
> softirqs such as TIMER_SOFTIRQ.
> On PREEMPT_RT softirqs are preemptible and spin_lock_irq() does not
> disable them. That means a timer can be invoked during spin_lock_irq()
> on the same CPU. Due to synchronisations reasons local_bh_disable() has
> a per-CPU lock named softirq_ctrl.lock which synchronizes individual
> softirq against each other.
> syz-bot managed to trigger a lockdep report where softirq_ctrl.lock is
> acquired in hrtimer_cancel() in addition to hrtimer_run_softirq(). This
> is a possible deadlock.
>
> The softirq_ctrl.lock can not be made part of spin_lock_irq() as this
> would lead to too much synchronisation against individual threads on the
> system. To avoid the possible deadlock, softirqs must be manually
> disabled before the lock is acquired.
>
> Disable softirqs before the lock is acquired on PREEMPT_RT.
>
> Reported-by: syzbot+10b4363fb0f46527f3f3@syzkaller.appspotmail.com
> Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>
> I don't see a way around this given the report. I don't see how to
> address this within the softirq. Taking this lock as part of every
> spin_lock_irq() would be a mess and while testing I didn't even manage
> to boot the machine. So I probably missed a detail (but then I would
> only know how mad it really is).
>
> This can be an intermediate solution until
> https://lore.kernel.org/all/20250901163811.963326-4-bigeasy@linutronix.de/
>
> gets merged and the !PREEMPT_RT_NEEDS_BH_LOCK case the default (i.e. not
> a config option anymore).

I applied now to for-next branch for 6.18.
It's already at a late stage for 6.17 release, and the issue doesn't
seem like an urgent regression to be fixed.

Thanks!

Takashi