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

Michael S. Tsirkin
2009-08-12T13:25:40Z

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.

--
MST