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

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Message-ID: <4947A53B.9050600@cn.fujitsu.com>
Date:	Tue, 16 Dec 2008 20:55:23 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] sched: fix another race when reading /proc/sched_debug

Paul Menage wrote:
> On Tue, Dec 16, 2008 at 1:41 AM, Paul Menage <menage@...gle.com> wrote:
>> It should be OK to rcu-free cgrp and have it still safe to call
>> cgroup_path() on it - as long as we're confident that the dentry (and
>> all its parents) won't be released until after the RCU section as
>> well, since that's where we get the path elements from. And that does
>> seem to be the case from looking at dcache.c
>>
>> It's certainly nicer than having two calls to synchronize_rcu() in
>> quick succession in cgroup_diput().
> 
> How about something like this?
> 

This solves half of the problem..

This avoids accessing a destroyed cgroup, but still race with cgroup_create.
You sugguested call cgroup_create_dir() before calling subsystems' create()
method.

> Paul
> 
>  include/linux/cgroup.h |    9 +++++++++
>  kernel/cgroup.c        |   49 ++++++++++++++++++++++++++++---------------------
>  2 files changed, 37 insertions(+), 21 deletions(-)
> 
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