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

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Message-ID: <20251003114555.413804-1-nirbhay.lkd@gmail.com>
Date: Fri,  3 Oct 2025 17:15:55 +0530
From: Nirbhay Sharma <nirbhay.lkd@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: 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>,
	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,
	Nirbhay Sharma <nirbhay.lkd@...il.com>
Subject: [PATCH] cgroup: Fix seqcount lockdep assertion in cgroup freezer

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
-- 
2.51.0