Date: Thu, 9 Feb 2012 19:43:37 +0400
From: Anton Vorontsov <anton.vorontsov@linaro.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Greg KH <gregkh@linuxfoundation.org>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Paul Mundt <lethal@linux-sh.org>,
        Russell King <linux@arm.linux.org.uk>,
        Mike Frysinger <vapier@gentoo.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Richard Weinberger <richard@nod.at>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/8] arm: Fix possible race on task->mm
Message-ID: <20120209154336.GA4769@oksana.dev.rtsoft.ru>
References: <20120207064809.GA29061@oksana.dev.rtsoft.ru>
 <20120207065004.GC1496@oksana.dev.rtsoft.ru>
 <20120208160808.GB17177@redhat.com>
 <20120209153346.GA346@oksana.dev.rtsoft.ru>
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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@gmail.com
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