cgroup: Fix seqcount lockdep assertion in cgroup freezer

The commit afa3701 ("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: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=27a2519eb4dad86d0156
Fixes: afa3701 ("cgroup: cgroup.stat.local time accounting")
Signed-off-by: Nirbhay Sharma <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/
Acked-by: Michal Koutný <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Nirbhay Sharma

include/linux/cgroup-defs.h

@@ -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

@@ -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