diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 9bb255e41cf2..4e610863cc37 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1420,6 +1420,11 @@ current_cgns_cgroup_from_root(struct cgroup_root *root)
 
 	rcu_read_unlock();
 
+	/*
+	 * The namespace_sem is held by current, so the root cgroup can't
+	 * be umounted. Therefore, we can ensure that the res is non-NULL.
+	 */
+	WARN_ON_ONCE(!res);
 	return res;
 }