Messages in this thread | | | Date | Wed, 12 Aug 2009 16:25:40 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCHv2 2/2] vhost_net: a kernel-level virtio server |
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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
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