netdev - Re: [PATCH 3/4 v2 net-next] net: make GRO aware of skb->head

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Date:	Tue, 01 May 2012 08:39:38 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Michael Chan <mchan@...adcom.com>,
	Matt Carlson <mcarlson@...adcom.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH 3/4 v2 net-next] net: make GRO aware of skb->head_frag

On Mon, 2012-04-30 at 22:33 -0700, Alexander Duyck wrote:

> The question I had was more specific to GRO.  As long as we have
> skb->users == 1 and the skb isn't cloned we should be fine.   It just
> hadn't occurred to me before that napi_gro_receive had the extra
> requirement that the skb couldn't be cloned.
> 

OK

By the way, even if skb was cloned, we would be allowed to steal
skb->head.

When we clone an oskb we :

1) allocate a struct nskb sk_buff (or use the shadow in case of TCP)
2) increment dataref
3) link skb->head

After cloning()  nskb->users == 1, and oskb->users is unchanged

I believe you are referring to skb_get(oskb), that ones increment
oskb->users and skb_shared(oskb) then returns true.



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