[PATCH 4.4 108/163] cgroup: make sure a parent css isnt freed before its children

From: Greg Kroah-Hartman
Date: Mon May 02 2016 - 21:35:48 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Tejun Heo <tj@xxxxxxxxxx>

commit 8bb5ef79bc0f4016ecf79e8dce6096a3c63603e4 upstream.

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

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

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/cgroup.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4692,14 +4692,15 @@ static void css_free_work_fn(struct work

if (ss) {
/* css free path */
+ struct cgroup_subsys_state *parent = css->parent;
int id = css->id;

- if (css->parent)
- css_put(css->parent);
cgroup_idr_remove(&ss->css_idr, id);
+ if (parent)
+ css_put(parent);
} else {
/* cgroup free path */