linux-kernel - Re: [PATCH 2/2] creds: __task_cred(current) doesn't need rcu_read_lock

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Message-ID: <20110920163805.GA20804@redhat.com>
Date:	Tue, 20 Sep 2011 18:38:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Serge E. Hallyn" <serge.hallyn@...onical.com>,
	lkml <linux-kernel@...r.kernel.org>, richard@....at,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] creds: __task_cred(current) doesn't need
	rcu_read_lock_held()

On 09/20, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.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@...hat.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.

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