[PATCH -next 2/3] cgroup/dmem: avoid rcu warning when unregister region

[Chen Ridong]

From: Chen Ridong <chenrid...@huawei.com>

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: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup")
Signed-off-by: Chen Ridong <chenrid...@huawei.com>
---
 kernel/cgroup/dmem.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 30f96627860c..b17f48bdef81 100644
--- a/kernel/cgroup/dmem.c
+++ b/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);
        }
 
-- 
2.34.1