From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon, 03 Jan 2011 07:06:41 +0000
Subject: Re: [PATCH] sched, cgroup: Use exit hook to avoid use-after-free
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To: Paul Menage <menage@google.com>
Cc: Mike Galbraith <efault@gmx.de>, Miklos Vajna <vmiklos@frugalware.org>, shenghui <crosslonelyover@gmail.com>, kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu, Greg KH <greg@kroah.com>, Paul Turner <pjt@google.com>, Yong Zhang <yong.zhang0@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>, Balbir Singh <balbir@linux.vnet.ibm.com>, Srivatsa Vaddagiri <vatsa@in.ibm.com>

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.