linux-kernel - Re: [PATCH 1/2] cgroup: make sure a parent css isn't offlined before its children

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Message-ID: <20160121212812.GJ5157@mtj.duckdns.org>
Date:	Thu, 21 Jan 2016 16:28:12 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-kernel@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"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: Re: [PATCH 1/2] cgroup: make sure a parent css isn't offlined before
 its children

On Thu, Jan 21, 2016 at 10:24:16PM +0100, Peter Zijlstra wrote:
> On Thu, Jan 21, 2016 at 03:31:11PM -0500, Tejun Heo wrote:
> > There are three subsystem callbacks in css shutdown path -
> > css_offline(), css_released() and css_free().  Except for
> > css_released(), cgroup core didn't use to 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
> > use-after-free in cpu controller.
> > 
> > This patch updates offline path so that a parent css is never offlined
> > before its children.  Each css keeps online_cnt which reaches zero iff
> > itself and all its children are offline and offline_css() is invoked
> > only after online_cnt reaches zero.
> > 
> > This fixes the reported cpu controller malfunction.  The next patch
> > will update css_free() handling.
> 
> No, I need to fix the cpu controller too, because the offending code
> sits off of css_free() (the next patch), but also does a call_rcu() in
> between, which also doesn't guarantee order.

Ah, I see.  Christian, can you please apply all three patches and see
whether the problem gets fixed?  Once verified, I'll update the patch
description and repost.

Thanks.

-- 
tejun