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

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Date:	Tue, 20 Sep 2011 17:50:04 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, "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()

Oleg Nesterov <oleg@...hat.com> wrote:

> > > -	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?

Something else.  The current_cred() uses ->cred:

 * current_cred - Access the current task's subjective credentials

and __task_cred() uses ->real_cred:

 * __task_cred - Access a task's objective credentials

Ordinarily both pointers will point to the same set of creds, but this change
will break anything that uses override_creds() + revert_creds() such as
faccessat() and fscache.

David
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