linux-kernel - Re: [syzbot] [sound?] possible deadlock in __snd_pcm_lib

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Message-ID: <20250904061245.pqGf3VwT@linutronix.de>
Date: Thu, 4 Sep 2025 08:12:45 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Hillf Danton <hdanton@...a.com>
Cc: syzbot <syzbot+10b4363fb0f46527f3f3@...kaller.appspotmail.com>,
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [sound?] possible deadlock in __snd_pcm_lib_xfer (2)

On 2025-09-04 09:05:28 [+0800], Hillf Danton wrote:
> 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
> > > >        ----                    ----
               Thead0                  Thread1
	       ------                  -------c
> > > >   lock(&group->lock#2);
           preempt to ->
> > > >                                lock((softirq_ctrl.lock));
> > > >                                lock(&group->lock#2);
                                       <- preempt to
> > > >   lock((softirq_ctrl.lock));
> > > > 
> > > >  *** DEADLOCK ***
               now nobody makes progress

Sebastian