netdev - Re: [patch net-next-2.6] bonding: remove skb_share_check in handle

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Message-ID: <4D6D52F1.6020407@gmail.com>
Date:	Tue, 01 Mar 2011 21:11:29 +0100
From:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
CC:	Jiri Pirko <jpirko@...hat.com>, netdev@...r.kernel.org,
	davem@...emloft.net, fubar@...ibm.com, eric.dumazet@...il.com,
	andy@...yhouse.net
Subject: Re: [patch net-next-2.6] bonding: remove skb_share_check in handle_frame

Le 01/03/2011 16:12, Changli Gao a écrit :
> On Tue, Mar 1, 2011 at 5:29 PM, Jiri Pirko<jpirko@...hat.com>  wrote:
>> Unapplicable, sorry (wrong branch :(). Here's corrected patch:
>>
>> Subject: [PATCH net-next-2.6 v2] bonding: remove skb_share_check in handle_frame
>>
>> No need to do share check here.
>>
>
> I don't think so. Although you avoid netif_rx(), you can't avoid
> ptype_all handlers. In fact, all the RX handlers should has this
> check(), if they may modify the skb.

Can you please develop your explanation?

In current __netif_receive_skb() (after the recent patch from Jiri), we deliver the skb to ptype_all 
handlers inside a loop, while possibly changing skb->dev inside this loop.

Then, at the end of __netif_receive_skb(), we loop on ptype_base, without changing anything in skb.

Should we consider ptype_*->func() to be called in a pure sequential way? Should we consider that 
when a ptype_*->func() returns, nothing from this handler will use the skb in anyway later, in a 
parallel way?

Or should we, instead, consider that special precautions must be taken, because protocol handlers 
may run in parallel for the same skb? Which kind of precautions?

Thanks.

	Nicolas.
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