mm: memcg/slab: fix return of child memcg objcg for root memcg

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/[email protected]
Fixes: bf4f059 ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Muchun Song <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Yafang Shao <[email protected]>
Cc: Chris Down <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Eugene Syromiatnikov <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Adrian Reber <[email protected]>
Cc: Marco Elver <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Muchun Song

mm/memcontrol.c

@@ -2975,6 +2975,7 @@ __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();