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

From	Tejun Heo <tj@kernel.org>
Newsgroups	linux.kernel
Subject	[PATCH v2 2/2] cgroup: make sure a parent css isn't freed before its children
Date	2016-01-22 16:50 +0100
Message-ID	<qTM8j-5YV-23@gated-at.bofh.it> (permalink)
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From 8bb5ef79bc0f4016ecf79e8dce6096a3c63603e4 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.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@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.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

Thread

[PATCH 1/2] cgroup: make sure a parent css isn't offlined before its  children Tejun Heo <tj@kernel.org> - 2016-01-21 21:40 +0100
  [PATCH 2/2] cgroup: make sure a parent css isn't freed before its  children Tejun Heo <tj@kernel.org> - 2016-01-21 21:40 +0100
    [PATCH v2 2/2] cgroup: make sure a parent css isn't freed before its  children Tejun Heo <tj@kernel.org> - 2016-01-22 16:50 +0100
  Re: [PATCH 1/2] cgroup: make sure a parent css isn't offlined before  its children Tejun Heo <tj@kernel.org> - 2016-01-21 22:30 +0100
    Re: [PATCH 1/2] cgroup: make sure a parent css isn't offlined before  its children Christian Borntraeger <borntraeger@de.ibm.com> - 2016-01-22 09:20 +0100
  Re: [PATCH 1/2] cgroup: make sure a parent css isn't offlined before  its children Peter Zijlstra <peterz@infradead.org> - 2016-01-21 22:30 +0100
  [PATCH v2 1/2] cgroup: make sure a parent css isn't offlined before  its children Tejun Heo <tj@kernel.org> - 2016-01-22 16:50 +0100

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