[PATCH 2/3] cgroup: make the default root invisible when it's umounted

[Li Zefan <lizefan@huawei.com>]

Before this patch (in a fresh system):

   # cat /proc/$$/cgroup
   # mount -t cgroup -o __DEVEL__sane_behavior xxx /cgroup
   # umount /cgroup
   # cat /proc/$$/cgroup
   0:cpuset,cpu,cpuacct,memory,devices,freezer,net_cls,blkio,perf_event,net_prio,hugetlb:/

After this patch (in a fresh system):

   # cat ...
   # mount ...
   # umount ...
   # cat /proc/$$/cgroup
   #

You won't see the default root after it's umounted.

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

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f73fe48..dabc486 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1788,6 +1788,8 @@ static void cgroup_kill_sb(struct super_block *sb)
 	} else {
 		if (root != &cgrp_dfl_root)
 			percpu_ref_kill(&root->cgrp.self.refcnt);
+		else
+			cgrp_dfl_root_visible = false;
 	}
 
 	kernfs_kill_sb(sb);
-- 
1.8.0.2