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

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Message-ID: <dbx87bxbvzel.fsf@ynaffit-andsys.c.googlers.com>
Date: Sat, 04 Oct 2025 01:34:10 -0700
From: Tiffany Yang <ynaffit@...gle.com>
To: Nirbhay Sharma <nirbhay.lkd@...il.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	"Michal Koutný" <mkoutny@...e.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 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
Subject: Re: [PATCH] cgroup: Fix seqcount lockdep assertion in cgroup freezer

Nirbhay Sharma <nirbhay.lkd@...il.com> writes:

> 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>
> ---
>   include/linux/cgroup-defs.h | 2 +-
>   kernel/cgroup/cgroup.c      | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)

> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index 539c64eeef38..933c4487a846 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -435,7 +435,7 @@ struct cgroup_freezer_state {
>   	int nr_frozen_tasks;

>   	/* Freeze time data consistency protection */
> -	seqcount_t freeze_seq;
> +	seqcount_spinlock_t freeze_seq;

>   	/*
>   	 * Most recent time the cgroup was requested to freeze.
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index ab096b884bbc..fe175326b155 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -5789,7 +5789,7 @@ static struct cgroup *cgroup_create(struct cgroup  
> *parent, const char *name,
>   	 * if the parent has to be frozen, the child has too.
>   	 */
>   	cgrp->freezer.e_freeze = parent->freezer.e_freeze;
> -	seqcount_init(&cgrp->freezer.freeze_seq);
> +	seqcount_spinlock_init(&cgrp->freezer.freeze_seq, &css_set_lock);
>   	if (cgrp->freezer.e_freeze) {
>   		/*
>   		 * Set the CGRP_FREEZE flag, so when a process will be

Thanks for this fix, Nirbhay!

-- 
Tiffany Y. Yang