linux-kernel - Re: [PATCH] sched, cgroup: Use exit hook to avoid use-after-free crash

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Message-ID: <1294038401.2016.5.camel@laptop>
Date:	Mon, 03 Jan 2011 08:06:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Menage <menage@...gle.com>
Cc:	Mike Galbraith <efault@....de>,
	Miklos Vajna <vmiklos@...galware.org>,
	shenghui <crosslonelyover@...il.com>,
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu, Greg KH <greg@...ah.com>,
	Paul Turner <pjt@...gle.com>,
	Yong Zhang <yong.zhang0@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>
Subject: Re: [PATCH] sched, cgroup: Use exit hook to avoid use-after-free
 crash

On Sat, 2010-12-25 at 20:59 +0000, Paul Menage wrote:

> While this patch is likely fine for solving the problem, it does add
> extra work into the task exit path.
> 
> Could you instead just use the pre_destroy callback to return -EBUSY
> if there are still any tasks on the cfs_rq? That way there'd only be a
> penalty on cgroup destruction, which is a much rarer operation.

No, that's broken! That would mean userspace gets the cgroup-empty
notification thing and then encounters -EBUSY, which then forces it to
go poll the state, rendering the whole notification thing pointless.
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