netdev - Re: pskb_copy() in tcp_transmit

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Date:	Tue, 13 Sep 2011 15:41:08 +0300
From:	Christoph Paasch <christoph.paasch@...ouvain.be>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Gaofeng <gaofeng@...fujitsu.com>, netdev@...r.kernel.org
Subject: Re: pskb_copy() in tcp_transmit_skb()

Eric,

On Tuesday 13 September 2011 wrote Eric Dumazet:
> I suggest you read dev_queue_xmit_nit() : Every xmit packet can be
> cloned right here.
> 
> By the time tcp_retransmit_skb() is called, cloned skb might still be
> in a AF_PACKET queue (or even a device TX queue)

Thanks for the clear answer.

So, basically every call to tcp_retransmit_skb on an skb that is also still 
hold in an AF_PACKET queue will end up in pskb_copy, because:
1. it has been cloned previously in tcp_transmit_skb()
2. The data_ref > 1 because it's clone is still in an AF_PACKET queue.

Please correct me, if I'm wrong.

Christoph


--
Christoph Paasch
PhD Student

IP Networking Lab --- http://inl.info.ucl.ac.be
MultiPath TCP in the Linux Kernel --- http://inl.info.ucl.ac.be/mptcp
Université Catholique de Louvain

www.rollerbulls.be
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