linux-kernel - Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg

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Message-ID: <6599ad830804162117w14364b7cg20d3694ffdfeb867@mail.gmail.com>
Date:	Wed, 16 Apr 2008 21:17:34 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	"Li Zefan" <lizf@...fujitsu.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Linux Containers" <containers@...ts.linux-foundation.org>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Paul Jackson" <pj@....com>
Subject: Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list

On Wed, Apr 16, 2008 at 9:11 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
>
>  I don't fully understand the race.  Both paths hold css_set_lock.
>
>  Can you describe it in more detail please?

Task A starts exiting, passes the check for unlinking current->cg_list.

Before it completely exits task B does the very first
cgroup_iter_begin() call (via reading a cgroups tasks file) which
links all tasks in to their css_set objects via tsk->cg_list.

Then task A finishes exiting and is freed, but doesn't unlink from the cg_list.

>
>  afacit the task at *p could set PF_EXITING immediately after this code has
>  tested PF_EXITING and then the task at *p could proceed until we hit the
>  same race (whatever that is).

The important fact there is that the task sets PF_EXITING *before* it
checks whether it needs to unlink from current->cg_list.

Paul
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