Re: [PATCHv2 2/2] vhost_net: a kernel-level virtio server

Paul E. McKenney
2009-08-12T15:26:39Z

On Wed, Aug 12, 2009 at 05:15:59PM +0300, Michael S. Tsirkin wrote:
> On Wed, Aug 12, 2009 at 07:11:07AM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 12, 2009 at 04:25:40PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, Aug 12, 2009 at 09:01:35AM -0400, Gregory Haskins wrote:
> > > > I think I understand what your comment above meant: You don't need to
> > > > do synchronize_rcu() because you can flush the workqueue instead to
> > > > ensure that all readers have completed.
> > >
> > > Yes.
> > >
> > > > But if thats true, to me, the
> > > > rcu_dereference itself is gratuitous,
> > >
> > > Here's a thesis on what rcu_dereference does (besides documentation):
> > >
> > > reader does this
> > >
> > > A: sock = n->sock
> > > B: use *sock
> > >
> > > Say writer does this:
> > >
> > > C: newsock = allocate socket
> > > D: initialize(newsock)
> > > E: n->sock = newsock
> > > F: flush
> > >
> > >
> > > On Alpha, reads could be reordered. So, on smp, command A could get
> > > data from point F, and command B - from point D (uninitialized, from
> > > cache). IOW, you get fresh pointer but stale data.
> > > So we need to stick a barrier in there.
> > >
> > > > and that pointer is *not* actually
> > > > RCU protected (nor does it need to be).
> > >
> > > Heh, if readers are lockless and writer does init/update/sync,
> > > this to me spells rcu.
> >
> > If you are using call_rcu(), synchronize_rcu(), or one of the
> > similar primitives, then you absolutely need rcu_read_lock() and
> > rcu_read_unlock(), or one of the similar pairs of primitives.
>
> Right. I don't use any of these though.
>
> > If you -don't- use rcu_read_lock(), then you are pretty much restricted
> > to adding data, but never removing it.
> >
> > Make sense? ;-)
>
> Since I only access data from a workqueue, I replaced synchronize_rcu
> with workqueue flush. That's why I don't need rcu_read_lock.

Well, you -do- need -something- that takes on the role of rcu_read_lock(),
and in your case you in fact actually do. Your equivalent of
rcu_read_lock() is the beginning of execution of a workqueue item, and
the equivalent of rcu_read_unlock() is the end of execution of that same
workqueue item. Implicit, but no less real.

If a couple more uses like this show up, I might need to add this to
Documentation/RCU. ;-)

Thanx, Paul