Re: [PATCH 2/3] Keys: Use RCU to manage session keyring pointer

Paul E. McKenney
2005-04-12T14:57:30Z

On Tue, Apr 12, 2005 at 10:11:50AM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@us.ibm.com> wrote:
>
> > > spin_lock_irqsave(&tsk->sighand->siglock, flags);
> > > - old = tsk->signal->session_keyring;
> > > - tsk->signal->session_keyring = keyring;
> > > + old = rcu_dereference(tsk->signal->session_keyring);
> >
> > I don't understand why rcu_dereference() is needed in this case.
> > Since we are holding the lock, it should not be possible for
> > this to change, right? Or am I missing something? (Quite possible,
> > am not all that familiar with this code.)
>
> Erm... you're right. I stuck the rcu_dereference() in then added the locks
> back in when I realised I still needed them.
>
> > > + synchronize_kernel();
> >
> > This would want to become synchronize_rcu().
>
> I think the deprecation happened since I wrote my patch.

Yes, sorry, I should have made it clear that this was a change that
affected your code rather than an error on your part.

> > > + if (tsk->signal->session_keyring) {
> > > + rcu_read_lock();
> > > + key = keyring_search_aux(
> > > + rcu_dereference(tsk->signal->session_keyring),
> > > + type, description, match);
> > > + rcu_read_unlock();
> > > + }
> > > + else {
> > > + key = keyring_search_aux(tsk->user->session_keyring,
> > > + type, description, match);
> >
> > This one is constant, right? If not, I don't understand the locking design.
>
> Which one? tsk->user->session_keyring is, tsk->signal->session_keyring is not.

Good, that matches the code!

Thanx, Paul

> Thanks for the review.
>
> David
>