Re: [syzbot] [sound?] possible deadlock in __snd_pcm_lib_xfer (2)

Hillf Danton
2025-09-04T01:06:21Z

On Wed, 3 Sep 2025 16:59:05 +0200 Sebastian Andrzej Siewior wrote:
> On 2025-08-30 14:56:37 [+0800], Hillf Danton wrote:
> > > syz.0.46/6843 is trying to acquire lock:
> > > ffff8880b8823d90 ((softirq_ctrl.lock)){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
> > > ffff8880b8823d90 ((softirq_ctrl.lock)){+.+.}-{3:3}, at: __local_bh_disable_ip+0x264/0x400 kernel/softirq.c:168
> > >
> > Given softirq_ctrl is percpu, this report is false positive.
>
> No. This can happen on a single CPU.
>
But the single CPU theory fails to explain the deadlock reported.

> > > Possible unsafe locking scenario:
> > >
> > > CPU0 CPU1
> > > ---- ----
> > > lock(&group->lock#2);
> > > lock((softirq_ctrl.lock));
> > > lock(&group->lock#2);
> > > lock((softirq_ctrl.lock));
> > >
> > > *** DEADLOCK ***