linux-kernel - Re: [PATCH] af_packet: Don't use skb after dev_queue

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Date:	Thu, 7 Jan 2010 18:56:10 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Michael Breuer <mbreuer@...jas.com>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	flyboy@...il.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: Don't use skb after dev_queue_xmit()

On Thu, Jan 07, 2010 at 10:03:51AM -0500, Michael Breuer wrote:
> On 1/7/2010 3:21 AM, Jarek Poplawski wrote:
> >On Thu, Jan 07, 2010 at 02:55:20AM -0500, Michael Breuer wrote:
> >>Unless I misread the code, I think that in some cases e skb is actually
> >>freed if the cfq (among others perhaps) scheduler returns an error on
> >>enqueue (flow control perhaps).  Thus with alternative 1, it is possible
> >>that the skb is acted upon after being freed - this would be consistent
> >>with the DMAR errors I saw.
> >I can't see your point: could you give some scenario?
> >
> >Jarek P.
> With NET_CLS_ACT set, net_dev_enqueue can return an error after
> freeing the skb. Alternative 1 disregards the error and assumes the
> skb is still valid. The original code and alternative 2 exit the
> loop assuming the skb has been freed.

Not exactly: alternative 1 disregards the error, and tries to send
next skbs if the message was longer. After consuming all the message
it returns without err code (at least wrt. dev_queue_xmit). This is
quite often practice to skip dev_queue_xmit() return (try to grep in
net\). It should never touch any part of an earlier sent skb.

Jarek P.
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