linux-kernel - Re: [PATCH] cgroup: Fix seqcount lockdep assertion in cgroup freezer

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Message-ID: <nbtofen2pwqmp7r5odbyc4en6vv54rpznyaanxlb6tbx5yyg25@jx2re5hdmt5e>
Date: Fri, 3 Oct 2025 16:00:22 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Nirbhay Sharma <nirbhay.lkd@...il.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Tiffany Yang <ynaffit@...gle.com>, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, 
	syzbot+27a2519eb4dad86d0156@...kaller.appspotmail.com, skhan@...uxfoundation.org, david.hunter.linux@...il.com, 
	linux-kernel-mentees@...ts.linuxfoundation.org, Kuniyuki Iwashima <kuniyu@...gle.com>
Subject: Re: [PATCH] cgroup: Fix seqcount lockdep assertion in cgroup freezer

On Fri, Oct 03, 2025 at 05:15:55PM +0530, Nirbhay Sharma <nirbhay.lkd@...il.com> wrote:
> The commit afa3701c0e45 ("cgroup: cgroup.stat.local time accounting")
> introduced a seqcount to track freeze timing but initialized it as a
> plain seqcount_t using seqcount_init().
> 
> However, the write-side critical section in cgroup_do_freeze() holds
> the css_set_lock spinlock while calling write_seqcount_begin(). On
> PREEMPT_RT kernels, spinlocks do not disable preemption, causing the
> lockdep assertion for a plain seqcount_t, which checks for preemption
> being disabled, to fail.
> 
> This triggers the following warning:
>   WARNING: CPU: 0 PID: 9692 at include/linux/seqlock.h:221
> 
> Fix this by changing the type to seqcount_spinlock_t and initializing
> it with seqcount_spinlock_init() to associate css_set_lock with the
> seqcount. This allows lockdep to correctly validate that the spinlock
> is held during write operations, resolving the assertion failure on all
> kernel configurations.
> 
> Reported-by: syzbot+27a2519eb4dad86d0156@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=27a2519eb4dad86d0156
> Fixes: afa3701c0e45 ("cgroup: cgroup.stat.local time accounting")
> Signed-off-by: Nirbhay Sharma <nirbhay.lkd@...il.com>

Link: https://lore.kernel.org/r/20251002165510.KtY3IT--@linutronix.de/

Yes, this is what was discussed yesterday. Thanks.

Acked-by: Michal Koutný <mkoutny@...e.com>

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