From 1f779fb28aa07350d72976d304591d216ca86f0e Mon Sep 17 00:00:00 2001
From: Li Zefan <lizefan@huawei.com>
Date: Wed, 4 Jun 2014 16:48:15 +0800
Subject: [PATCH] cgroup: don't destroy the default root

The default root is allocated and initialized at boot phase, so we
shouldn't destroy the default root when it's umounted, otherwise
it will lead to disaster.

Just try mount and then umount the default root, and the kernel will
crash immediately.

v2:
- No need to check for CSS_NO_REF in cgroup_get/put(). (Tejun)
- Better call cgroup_put() for the default root in kill_sb(). (Tejun)
- Add a comment.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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