cgroup: make sure a parent css isn't freed before its children

cgroup: make sure a parent css isn't freed before its children

Orabug: 25308065

[ Upstream commit 8bb5ef79bc0f4016ecf79e8dce6096a3c63603e4 ]

There are three subsystem callbacks in css shutdown path -
css_offline(), css_released() and css_free().  Except for
css_released(), cgroup core didn't guarantee the order of invocation.
css_offline() or css_free() could be called on a parent css before its
children.  This behavior is unexpected and led to bugs in cpu and
memory controller.

The previous patch updated ordering for css_offline() which fixes the
cpu controller issue.  While there currently isn't a known bug caused
by misordering of css_free() invocations, let's fix it too for
consistency.

css_free() ordering can be trivially fixed by moving putting of the
parent css below css_free() invocation.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit cef12dbf18ab7617cacf7c271da43624afac5520)
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>