After running:

  # mount -t cgroup cpu xxx /cgroup && mkdir /cgroup/sub && \
    rmdir /cgroup/sub && umount /cgroup

I found the cgroup root still existed:

  # cat /proc/cgroups
  #subsys_name    hierarchy       num_cgroups     enabled
  cpuset  0       1       1
  cpu     1       1       1
  ...

It turned out css_has_online_children() is broken.

Signed-off-by: Li Zefan <lize...@huawei.com>
---
 kernel/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 05b8ca4..1c65f24 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3327,7 +3327,7 @@ bool css_has_online_children(struct cgroup_subsys_state 
*css)
 
        rcu_read_lock();
        css_for_each_child(child, css) {
-               if (css->flags & CSS_ONLINE) {
+               if (child->flags & CSS_ONLINE) {
                        ret = true;
                        break;
                }
-- 
1.8.0.2
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