Commit 93a4b36e authored by Nirbhay Sharma's avatar Nirbhay Sharma Committed by Tejun Heo
Browse files

cgroup: Fix seqcount lockdep assertion in cgroup freezer 



The commit afa3701c ("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: default avatar <syzbot+27a2519eb4dad86d0156@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=27a2519eb4dad86d0156


Fixes: afa3701c ("cgroup: cgroup.stat.local time accounting")
Signed-off-by: default avatarNirbhay Sharma <nirbhay.lkd@gmail.com>
Link: https://lore.kernel.org/r/20251002165510.KtY3IT--@linutronix.de/


Acked-by: default avatarMichal Koutný <mkoutny@suse.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent e406d57b

include/linux/cgroup-defs.h

+1 −1
Original line number Diff line number Diff line
@@ -452,7 +452,7 @@ 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.

kernel/cgroup/cgroup.c

+1 −1
Original line number Diff line number Diff line
@@ -5892,7 +5892,7 @@ 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