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

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Message-Id: <20160503000512.051650879@linuxfoundation.org>
Date:	Mon,  2 May 2016 17:12:16 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 4.4 108/163] cgroup: make sure a parent css isnt freed before its children

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

------------------

From: Tejun Heo <tj@...nel.org>

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
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 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);
-
 		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);