cgroup/dmem: avoid rcu warning when unregister region

A warnning was detected:

 WARNING: suspicious RCU usage
 6.19.0-rc7-next-20260129+ #1101 Tainted: G           O
 kernel/cgroup/dmem.c:456 suspicious rcu_dereference_check() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by insmod/532:
  #0: ffffffff85e78b38 (dmemcg_lock){+.+.}-dmem_cgroup_unregister_region+

 stack backtrace:
 CPU: 2 UID: 0 PID: 532 Comm: insmod Tainted: 6.19.0-rc7-next-
 Tainted: [O]=OOT_MODULE
 Call Trace:
  <TASK>
  dump_stack_lvl+0xb0/0xd0
  lockdep_rcu_suspicious+0x151/0x1c0
  dmem_cgroup_unregister_region+0x1e2/0x380
  ? __pfx_dmem_test_init+0x10/0x10 [dmem_uaf]
  dmem_test_init+0x65/0xff0 [dmem_uaf]
  do_one_initcall+0xbb/0x3a0

The macro list_for_each_rcu() must be used within an RCU read-side critical
section (between rcu_read_lock() and rcu_read_unlock()). Using it outside
that context, as seen in dmem_cgroup_unregister_region(), triggers the
lockdep warning because the RCU protection is not guaranteed.

Replace list_for_each_rcu() with list_for_each_entry_safe(), which is
appropriate for traversal under spinlock protection where nodes may be
deleted.

Fixes: b168ed4 ("kernel/cgroup: Add "dmem" memory accounting cgroup")
Cc: [email protected] # v6.14+
Signed-off-by: Chen Ridong <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Chen Ridong

kernel/cgroup/dmem.c

@@ -423,7 +423,7 @@ static void dmemcg_free_region(struct kref *ref)
  */
 void dmem_cgroup_unregister_region(struct dmem_cgroup_region *region)
 {
-	struct list_head *entry;
+	struct dmem_cgroup_pool_state *pool, *next;
 
 	if (!region)
 		return;
@@ -433,10 +433,7 @@ void dmem_cgroup_unregister_region(struct dmem_cgroup_region *region)
 	/* Remove from global region list */
 	list_del_rcu(&region->region_node);
 
-	list_for_each_rcu(entry, &region->pools) {
-		struct dmem_cgroup_pool_state *pool =
-			container_of(entry, typeof(*pool), region_node);
-
+	list_for_each_entry_safe(pool, next, &region->pools, region_node) {
 		list_del_rcu(&pool->css_node);
 	}