From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1751180Ab1ITQmv (ORCPT <rfc822;w@1wt.eu>);
	Tue, 20 Sep 2011 12:42:51 -0400
Received: from mx1.redhat.com ([209.132.183.28]:48831 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1750797Ab1ITQmu (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Tue, 20 Sep 2011 12:42:50 -0400
Date: Tue, 20 Sep 2011 18:38:05 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: David Howells <dhowells@redhat.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        "Serge E. Hallyn" <serge.hallyn@canonical.com>,
        lkml <linux-kernel@vger.kernel.org>, richard@nod.at,
        "Eric W. Biederman" <ebiederm@xmission.com>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 2/2] creds: __task_cred(current) doesn't need
	rcu_read_lock_held()
Message-ID: <20110920163805.GA20804@redhat.com>
References: <20110920143959.GC15859@redhat.com> <20110919214531.GA18085@sergelap> <20110920122202.GA26504@redhat.com> <20110920124419.GA10759@hallyn.com> <20110920134108.GA30749@redhat.com> <20110920143920.GA15859@redhat.com> <5088.1316535551@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5088.1316535551@redhat.com>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 09/20, David Howells wrote:
>
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Change __task_cred(task) to accept "task == current" without
> > rcu_read_lock_held(). This is what current_cred() currently does,
> > and with this change __task_cred() becomes more flexible/usable.
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
>
> NAK!
>
> If you compare carefully:
>
> > -	rcu_dereference_protected(current->cred, 1)
>
> and:
>
> > -		rcu_dereference_check(__t->real_cred, 0);		\
>
> you'll notice they aren't quite the same in one very fundamental way.

Do you mean that this patch adds the unnecessary ACCESS_ONCE +
smp_read_barrier_depends() to current_cred() or I missed something
else?

And what do you think about (__t == current) in __task_cred ?

Oleg.