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From: David Howells <dhowells@redhat.com>
In-Reply-To: <20110920163805.GA20804@redhat.com>
References: <20110920163805.GA20804@redhat.com> <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>
To: Oleg Nesterov <oleg@redhat.com>
Cc: dhowells@redhat.com, "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()
Date: Tue, 20 Sep 2011 17:50:04 +0100
Message-ID: <5820.1316537404@redhat.com>
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Oleg Nesterov <oleg@redhat.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