From mboxrd@z Thu Jan 1 00:00:00 1970 From: jamal Subject: Re: RFC: on [ab]use of skb->cb by VLAN code Date: Mon, 30 Jul 2007 22:02:04 -0400 Message-ID: <1185847324.5162.31.camel@localhost> References: <1185843556.5162.8.camel@localhost> <20070730.183337.27784025.davem@davemloft.net> Reply-To: hadi@cyberus.ca Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: greearb@candelatech.com, kaber@trash.net, netdev@vger.kernel.org, mcarlson@broadcom.com To: David Miller Return-path: Received: from wx-out-0506.google.com ([66.249.82.233]:63305 "EHLO wx-out-0506.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758515AbXGaCCX (ORCPT ); Mon, 30 Jul 2007 22:02:23 -0400 Received: by wx-out-0506.google.com with SMTP id h31so1516613wxd for ; Mon, 30 Jul 2007 19:02:22 -0700 (PDT) In-Reply-To: <20070730.183337.27784025.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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