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

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Date:	Tue, 16 Dec 2008 17:39:56 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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

Paul Menage wrote:
> On Sun, Dec 14, 2008 at 5:50 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
>> at creation:
>>  - cpu_cgroup_populate() should record "tg" that "this cgroup has valid dentry"
>> at deletion
> 
> Wouldn't moving the call to cgroup_create_dir() to before doing
> subsystem initialization fix this problem more straightforwardly? Then
> by the time the cpu subsytem create() callback is called, dentry will
> already be valid.
> 

Yes, it's true, and actually it was the first idea came into my mind when I
started to fix this bug. But I was not sure whether this is an apropriate way.
Now I think it's ok, and we'd better add a comment to indicate we want to
make cgrp->dentry valid before calling subsystem create() method.

Note in print_cfs_rq(), due to the race, tg->css.cgroup can be NULL.
Fortunately, the code checks this condition, but I think it's just by accident
but not awaring there is a race.
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