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

Gregory Haskins
2009-08-12T13:41:35Z

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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.

Yes, that is understood. Perhaps you should just use a normal barrier,
however. (Or at least a comment that says "I am just using this for its
barrier").

>
>> 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.

More correctly: it "smells like" RCU, but its not. ;) It's rcu-like,
but you are not really using the rcu facilities. I think anyone that
knows RCU and reads your code will likely be scratching their heads as well.

Its probably not a big deal, as I understand your code now. Just a
suggestion to help clarify it.

Regards,
-Greg

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