linux-kernel - Re: [PATCH 3/8] arm: Fix possible race on task->mm

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Message-ID: <20120209154336.GA4769@oksana.dev.rtsoft.ru>
Date:	Thu, 9 Feb 2012 19:43:37 +0400
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	Mike Frysinger <vapier@...too.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Weinberger <richard@....at>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] arm: Fix possible race on task->mm

On Thu, Feb 09, 2012 at 07:33:46PM +0400, Anton Vorontsov wrote:
> On Wed, Feb 08, 2012 at 05:08:08PM +0100, Oleg Nesterov wrote:
> > On 02/07, Anton Vorontsov wrote:
> > >
> > > Checking for task->mm is dangerous as ->mm might disappear (exit_mm()
> > > assigns NULL under task_lock(), so tasklist lock is not enough).
> > >
> > > We can't use get_task_mm()/mmput() pair as mmput() might sleep,
> > 
> > Yes, but
> > 
> > > so let's take the task lock while we care about its mm.
> > 
> > it seems that this needs find_lock_task_mm() too ?
> > 
> > the same for the rest patches in this series.
> 
> Yep, I think you're right, will add this change.

Thinking about it more... making the code use find_lock_task_mm
would be a behaviour change. Sure, in trivial cases like ARM this
looks like a 100% safe thing to do, but in e.g. UML case, I
wouldn't bet much money on that 'mm->context.id.u.pid' would be
still meaningful.

So, I'd rather do it in a separate change, so this can be easily
reverted.

-- 
Anton Vorontsov
Email: cbouatmailru@...il.com
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