[PATCH 5.10 449/717] mm: memcg/slab: fix return of child memcg objcg for root memcg

[Greg Kroah-Hartman]

From: Muchun Song <songmuchun@xxxxxxxxxxxxx>

[ Upstream commit 2f7659a314736b32b66273dbf91c19874a052fde ]

Consider the following memcg hierarchy.

                    root
                   /    \
                  A      B

If we failed to get the reference on objcg of memcg A, the
get_obj_cgroup_from_current can return the wrong objcg for the root
memcg.

Link: https://lkml.kernel.org/r/20201029164429.58703-1-songmuchun@xxxxxxxxxxxxx
Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Yafang Shao <laoar.shao@xxxxxxxxx>
Cc: Chris Down <chris@xxxxxxxxxxxxxx>
Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Eugene Syromiatnikov <esyr@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Adrian Reber <areber@xxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/memcontrol.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 29459a6ce1c7a..74b85077f89ad 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2987,6 +2987,7 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 		objcg = rcu_dereference(memcg->objcg);
 		if (objcg && obj_cgroup_tryget(objcg))
 			break;
+		objcg = NULL;
 	}
 	rcu_read_unlock();
 
-- 
2.27.0