netdev - Re: RFC: on [ab]use of skb->cb by VLAN code

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Date:	Mon, 30 Jul 2007 22:02:04 -0400
From:	jamal <hadi@...erus.ca>
To:	David Miller <davem@...emloft.net>
Cc:	greearb@...delatech.com, kaber@...sh.net, netdev@...r.kernel.org,
	mcarlson@...adcom.com
Subject: Re: RFC: on [ab]use of skb->cb by VLAN code

On Mon, 2007-30-07 at 18:33 -0700, David Miller wrote:

> I understand the concern, but how much qdisc stuff can possibly
> happen between those two ->hard_start_xmit() calls and do we
> want to support that in any way anyways?

>>From a quick glance only netem seems to use it in the fast path (in a
legit way)
Theoretically, you could have many generations (i.e parents and
children, grandchildren etc) of netdevices stacked on top of each other
each with qdiscs. In a simple example: dont know how well these days
Vlans->bonding->somehardwarenetdevice works. 
Redirect will could also result in a graph of unrelated netdevices (and
it is fair game to trample on cb anywhere along the path)
I came across the issue because i used cb in batching to store transient
state which is used between qdisc dequeueing and hardware enqueueing
(looked and smelled legit to me).
 
> The only alternative I see is to add more things to struct sk_buff
> and that's usually very unpopular :-)

I know ;-> Thats why i asked the question.

cheers,
jamal

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