linux-kernel - Re: on CONFIG_MM_OWNER=y, kernel panic is possible.

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Message-ID: <481FF115.8030503@linux.vnet.ibm.com>
Date:	Tue, 06 May 2008 11:18:05 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: on CONFIG_MM_OWNER=y, kernel panic is possible.

KOSAKI Motohiro wrote:
> on CONFIG_MM_OWNER=y (that is automatically turned on by mem-cgroup),
> kernel panic is possible by following scenario in mm_update_next_owner().
> 
> 1. mm_update_next_owner() is called.
> 2. found caller task in do_each_thread() loop.
> 3. thus, BUG_ON(c == p) is true, it become kernel panic.
> 
> end up, We should left out current task.
> 
> 

That is not possible. If you look at where mm_update_next_owner() is called
from, we call it from

exit_mm() and exec_mmap()

In both cases, we ensure that the task's mm has changed (to NULL and the new mm
respectively), before we call mm_update_next_owner(), hence c->mm can never be
equal to p->mm.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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