Patch "cgroup: make sure a parent css isn't freed before its children" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     cgroup-make-sure-a-parent-css-isn-t-freed-before-its-children.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 8bb5ef79bc0f4016ecf79e8dce6096a3c63603e4 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Thu, 21 Jan 2016 15:32:15 -0500
Subject: cgroup: make sure a parent css isn't freed before its children

From: Tejun Heo <tj@kernel.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@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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);


Patches currently in stable-queue which might be from tj@kernel.org are

queue-4.4/cgroup-make-sure-a-parent-css-isn-t-freed-before-its-children.patch
queue-4.4/sched-cgroup-fix-cleanup-cgroup-teardown-init.patch
queue-4.4/workqueue-fix-ghost-pending-flag-while-doing-mq-io.patch
queue-4.4/cgroup-cpuset-replace-cpuset_post_attach_flush-with-cgroup_subsys-post_attach-callback.patch
queue-4.4/memcg-relocate-charge-moving-from-attach-to-post_attach.patch