Date: Thu, 17 May 2007 12:38:46 -0500
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Tsutomu OWA <tsutomu.owa@toshiba.co.jp>, linuxppc-dev@ozlabs.org,
       mingo@elte.hu, tglx@linutronix.de, cbe-oss-dev@ozlabs.org,
       netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Resending: RT patches expose netdev race [was Re: [RFC] [patch 2/2] powerpc 2.6.21-rt1: fix kernel hang and/or panic
Message-ID: <20070517173846.GF4325@austin.ibm.com>
References: <20070517002751.GC4325@austin.ibm.com> <1179362985.32247.252.camel@localhost.localdomain>
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From: linas@austin.ibm.com (Linas Vepstas)
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On Thu, May 17, 2007 at 10:49:45AM +1000, Benjamin Herrenschmidt wrote:
> 
> > I do not know why sk_buff->head would be null, or
> > would be set in a racy kind of way, or why the rt patches
> > would cause this. But the evidence implicates that.
> 
> Would it be possible that a locking bug in spidernet would cause it
> under some circumstances to get a stale skb pointer ?

The skb pointer should be brand-spanking new/fresh. 
It is passed to spidernet by the netdev->hard_start_xmit
callback:

    netdev->hard_start_xmit = &spider_net_xmit;

I'd expect that anything that hard_start_xmit() passed to 
a device driver should have a fully valid skb.  Locking
problems in spidernet could cause it to work with the wrong 
skb; however, in this case, the skb pointer is passed 
unmodified, directly to the spot where it fails.

Maybe there is some "make ip header fresh and clean on skb" call
that should have been made; if so, I don't know what it is. 

--linas
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