Commit 8bb5ef79 authored Jan 21, 2016 by Tejun Heo

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



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>

parent aa226ff4

kernel/cgroup.c

+4 −3

Original line number	Original line	Diff line number	Diff line
	@@ -4657,14 +4657,15 @@ static void css_free_work_fn(struct work_struct *work)

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

	if (css->parent)
	css_put(css->parent);

	ss->css_free(css);		ss->css_free(css);
	cgroup_idr_remove(&ss->css_idr, id);		cgroup_idr_remove(&ss->css_idr, id);
	cgroup_put(cgrp);		cgroup_put(cgrp);

			if (parent)
			css_put(parent);
	} else {		} else {
	/* cgroup free path */		/* cgroup free path */
	atomic_dec(&cgrp->root->nr_cgrps);		atomic_dec(&cgrp->root->nr_cgrps);