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

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Message-ID: <20110920153542.GA17731@redhat.com>
Date:	Tue, 20 Sep 2011 17:35:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Serge Hallyn <serge.hallyn@...onical.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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, Serge Hallyn wrote:
>
> Quoting Oleg Nesterov (oleg@...hat.com):
> > 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>
>
> So to check whether I've got this straight, the original __task_cred()
> was allowing rcu read lock to not be held for a non-running task, but
> required rcu read lock if task was running?  With these two patches,
> rcu read lock will not be needed if task == current?

Yes.

> If so, then that sounds good to me, and an unconditional rcu_read_lock()
> at wait_task_zombie() seems better than at send_signal()...

Agreed.

Although sometimes I think it would be better to use rcu_dereference_raw()
with a comment in some places.

Oleg.

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