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

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Message-ID: <20110920171341.GA22317@redhat.com>
Date:	Tue, 20 Sep 2011 19:13:41 +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:
>
> > > > -	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:

Argh!!! I am soooo stupid.

Thanks a lot for the quick NACK!


So. This patch shouldn't touch current_cred().

But this also means, in theory it is not good to assume that
send_signal() can use __task_cred(current) instead of current_cred(),
although I doubt very much that someone can do override_creds() +
kill(SEND_SIG_NOINFO). So the change in __task_cred() is probably not
really good too.

Thanks.

Oleg.

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