linux-kernel - Re: [RFC][PATCH 2/5] pid: Generalize task_active_pid

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Date:	Thu, 27 Nov 2008 22:19:25 +0100
From:	Greg Kurz <gkurz@...ibm.com>
To:	Bastian Blank <bastian@...di.eu.org>
Cc:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, oleg@...hat.com,
	ebiederm@...ssion.com, containers@...ts.osdl.org,
	roland@...hat.com, xemul@...nvz.org
Subject: Re: [RFC][PATCH 2/5] pid: Generalize task_active_pid_ns

On Thu, 2008-11-27 at 02:17 +0100, Bastian Blank wrote:
> On Tue, Nov 25, 2008 at 07:45:28PM -0800, Sukadev Bhattiprolu wrote:
> > Currently task_active_pid_ns is not safe to call after a
> > task becomes a zombie and exit_task_namespaces is called,
> > as nsproxy becomes NULL.
> 
> Why do you need to be able to get the pid namespace from zombie
> processes? Also according to nsproxy.h this access variant is only
> allowed for the current task, anything else needs to take a rcu lock.
> 

That doesn't save ->nsproxy to be NULL... as shown in the very same
example you're talking about.

I agree with Eric and Sukadev that task_active_pid_ns() is unsafe. There
isn't even a /* don't use with zombies */ in pid_namespace.h...

> >                           By reading the pid namespace from
> > the pid of the task we can trivially solve this problem at
> > the cost of one extra memory read in what should be the
> > same cacheline as we read the namespace from.
> 
> The pid namespace may not be destructed yet?
> 

Yes as long as a pid from that namespace is still in use...

-- 
Gregory Kurz                                     gkurz@...ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)534 638 479                           Fax +33 (0)561 400 420

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.

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