netdev - RE: [net-next PATCH] igb: remove skb

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Date:	Thu, 26 Feb 2009 09:03:39 -0800
From:	"Duyck, Alexander H" <alexander.h.duyck@...el.com>
To:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	David Miller <davem@...emloft.net>,
	"Tantilov, Emil S" <emil.s.tantilov@...el.com>,
	"Ohly, Patrick" <patrick.ohly@...el.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"Williams, Mitch A" <mitch.a.williams@...el.com>
Subject: RE: [net-next PATCH] igb: remove skb_orphan calls

Jeff Kirsher wrote:
> On Thu, Feb 26, 2009 at 4:14 AM, David Miller <davem@...emloft.net>
> wrote: 
>> From: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
>> Date: Thu, 26 Feb 2009 02:46:23 -0800
>> 
>>> From: Alexander Duyck <alexander.h.duyck@...el.com>
>>> 
>>> The skb_orphan call in the tx path has been shown to cause issues
>>> as seen with the workarounds required for timestamping.
>>> 
>>> In order to avoid this it is easiest just to remove the skb_orphan
>>> call as the motivation for including it was purely performance
>>> based, and the overall gain from having the call was minimal.
>>> 
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
>>> Acked-by: Mitch Williams <mitch.a.williams@...el.com>
>>> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
>> 
>> While I'm happy to apply this, I don't see it as helping the
>> timestamping situation. 
>> 
>> All someone has to do is enable the new timestamping on loopback to
>> trigger the problem, the loopback MUST orphan SKBs before it pushes
>> them back into the stack for receive.
>> 
>> Also, Herbert and I have talked about orphaning SKBs even earlier
>> than dev_queue_xmit() 
>> 
>> This post-send timestamping scheme is not going to work, is poorly
>> designed, and needs to be completely rearchitected.
>> 
>> If it isn't fixed soon, I'll have no choice but to completely revert
>> all of the timestamping stuff.
>> --
>> 
> 
> Adding Emil and Patrick.
> 
> While no issues were seen in testing, I will verify with Emil that
> loopback testing was done.  Also I will work with Patrick to ensure
> that we resolve any issues with lookback and timestamping.

This patch was meant to be more general than just timestamping.  I was just using timestamping as one example of where things can go wrong due to the skb_orphan call.  The main problem is triggering desctructors and removing the sk can cause multiple issues with any custom use of the skb.  It is easiest to avoid those by not calling skb_orphan in the first place.

Thanks,

Alex