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@syzkaller.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@gmail.com>
Link: https://lore.kernel.org/r/20251002165510.KtY3IT--@linutronix.de/
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 93318fce31f3a869bf95ef2c98b974351462c456..b760a3c470a5680d89e278e6bfdcdc9ca8f37710 100644 (file)
--- a/include/linux/cgroup-defs.h
+++ b/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.

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6ae5f48cf64e34e263afb45647848c53ec97a12f..fdee387f0d6be40b0b79987d23cdc9981afb4226 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/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