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

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Date:	Wed, 12 Aug 2009 09:41:35 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>, hpa@...or.com,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCHv2 2/2] vhost_net: a kernel-level virtio server

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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