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

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Date:	Wed, 12 Aug 2009 16:25:40 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <gregory.haskins@...il.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

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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