linux-kernel - [PATCH v2 2/2] cgroup: make sure a parent css isn't freed before its children

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Date:	Fri, 22 Jan 2016 10:45:52 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
	kernel-team@...com
Subject: [PATCH v2 2/2] cgroup: make sure a parent css isn't freed before its
 children

>From 8bb5ef79bc0f4016ecf79e8dce6096a3c63603e4 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@...nel.org>
Date: Thu, 21 Jan 2016 15:32:15 -0500

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@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
Hello,

Applied to cgroup/for-4.5-fixes w/ description updated.  Will push out
to Linus early next week.

Thanks.

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

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d015877..d27904c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4657,14 +4657,15 @@ static void css_free_work_fn(struct work_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);
-
 		ss->css_free(css);
 		cgroup_idr_remove(&ss->css_idr, id);
 		cgroup_put(cgrp);
+
+		if (parent)
+			css_put(parent);
 	} else {
 		/* cgroup free path */
 		atomic_dec(&cgrp->root->nr_cgrps);
-- 
2.5.0