netdev - Re: [PATCH] net: increase skb->users instead of skb

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Message-ID: <AANLkTikdYi-Cs=4UtjJx-X4bW+LSKTQBTcSv23NS2S+S@mail.gmail.com>
Date:	Thu, 16 Dec 2010 22:05:01 +0800
From:	Junchang Wang <junchangwang@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tom Herbert <therbert@...gle.com>,
	Jiri Pirko <jpirko@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Xinan Tang <xinan.tang@...el.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: increase skb->users instead of skb_clone()

On Thu, Dec 16, 2010 at 1:57 PM, Changli Gao <xiaosuo@...il.com> wrote:
> In dev_queue_xmit_nit(), we have to clone skbs as we need to mangle skbs,
> however, we don't need to clone skbs for all the packet_types.
>
> Except for the first packet_type, we increase skb->users instead of
> skb_clone().

Hi Changli,
Take af_packet for example, I can't see benefit from this patch.

> +static inline int deliver_skb(struct sk_buff *skb,
> +                             struct packet_type *pt_prev,
> +                             struct net_device *orig_dev)
> +{
> +       atomic_inc(&skb->users);
> +       return pt_prev->func(skb, skb->dev, pt_prev, orig_dev);
> +}
The increment call will incur skb_shared() failure in packet_rcv.
In reality, packet_rcv has to clone this packet by itself.


Thanks

-- 
--Junchang
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