netdev - Re: [PATCH 1/4] net: skb_orphan on dev_hard_start

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Date:	Mon, 01 Jun 2009 21:47:22 +0200
From:	Patrick Ohly <patrick.ohly@...el.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Divy Le Ray <divy@...lsio.com>,
	Roland Dreier <rolandd@...co.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Dan Williams <dcbw@...hat.com>,
	libertas-dev@...ts.infradead.org
Subject: Re: [PATCH 1/4] net: skb_orphan on dev_hard_start_xmit

On Fri, 2009-05-29 at 23:44 +0930, Rusty Russell wrote:
> This patch adds skb_orphan to the start of dev_hard_start_xmit(): it
> can be premature in the NETDEV_TX_BUSY case, but that's uncommon.

Would it be possible to make the new skb_orphan() at the start of
dev_hard_start_xmit() conditionally so that it is not executed for
packets that are to be time stamped?

As discussed before
(http://article.gmane.org/gmane.linux.network/121378/), the skb->sk
socket pointer is required for sending back the send time stamp from
inside the device driver. Calling skb_orphan() unconditionally as in
this patch would break the hardware time stamping of outgoing packets.

This should work without much overhead (skb_tx() expands to a lookup of
the skb_shared_info):
if (!skb_tx(skb)->flags)
    skb_orphan(skb);

Instead of looking at the individual skb_shared_tx "hardware" and
"software" bits I'm just checking the whole byte here because it is
shorter and perhaps faster. Semantically the result is the same.

-- 
Best Regards, Patrick Ohly

The content of this message is my personal opinion only and although
I am an employee of Intel, the statements I make here in no way
represent Intel's position on the issue, nor am I authorized to speak
on behalf of Intel on this matter.

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