linux-kernel - Re: [PATCH] sched: fix another race when reading /proc/sched

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Date:	Mon, 15 Dec 2008 10:11:57 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Paul Menage <menage@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sched: fix another race when reading /proc/sched_debug

KAMEZAWA Hiroyuki wrote:
> On Mon, 15 Dec 2008 09:39:18 +0800
> Li Zefan <lizf@...fujitsu.com> wrote:
> 
>> Paul Menage wrote:
>>> I sent out some patches last week (search for hierarchy_mutex) that would
>>> mean that you'd only need to take a subsystem-local lock to keep a cgroup
>>> alive. People seemed to like them so I'll tweak them based on feedback and
>>> send them on to Andrew.
>>>
>> Unfortunately, AFAICS the proposed hierarchy_mutex can't solve this bug. :(
>>
> Hmm ? how about this way if cgroup->dentry is problem ?
> 

Sounds feasable, though the resulting code might a bit tricky.

> at creation:
>   - cpu_cgroup_populate() should record "tg" that "this cgroup has valid dentry"

This avoids /proc/sched_debug accessing invalid cgrp dentry while a cgroup is
being created.

> at deletion
>   - css_tryget() will be useful to avoid the race.

And this avoids accessing a dead task_group.

Since css_tryget() is a 2.6.29 material, I think we can have this bug fixed for
2.6.28 by holding cgroup_lock, and try to remove the global lock for 2.6.29 ?
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