netdev - Re: defxx: skb_push() failing?

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Date:	Tue, 26 Mar 2013 08:15:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Oostdyk <daveo@...mit.edu>
Cc:	netdev@...r.kernel.org, macro@...ux-mips.org
Subject: Re: defxx: skb_push() failing?

On Tue, 2013-03-26 at 10:29 -0400, David Oostdyk wrote:
> Hello,
> 
> In dfx_xmt_queue_pkt() in defxx.c, there is a skb_push(3) call which 
> makes room for 3 packet request header bytes.  There is some discussion 
> in the driver explaining why those three bytes will be available.  I 
> have an old FDDI card that I'm trying to bring up:
> 
> 05:05.0 FDDI network controller: Digital Equipment Corporation 
> PCI-to-PDQ Interface Chip [PFI] (rev 02)
> 
> Most skbuffs that come through dfx_xmit_queue_pkt() have 11 bytes 
> between skb->head and skb->data.  On the other hand, at almost exactly 
> 60-second intervals, an skb arrives that has zero bytes between 
> skb->head and skb->data.  This normally causes a kernel panic, and for 
> the time I just skip over such skb's.
> 
> Does anyone have advice on where I should start digging to find the 
> cause of this?
> 

Have you read comments in defxx.c file around line 151 ?

If one skb arrives with not enough headroom, you could add a

WARN_ON_ONCE(skb_headroom(skb) < 3);

and report stack trace so that we can identify and fix the caller.



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