RT patches expose netdev race [was Re: [RFC] [patch 2/2] powerpc 2.6.21-rt1: fix kernel hang and/or panic

[David Miller]

From: linas at austin.ibm.com (Linas Vepstas)
Date: Wed, 16 May 2007 19:18:02 -0500

> Hi,
> 
> On Tue, May 15, 2007 at 08:09:02PM +1000, Benjamin Herrenschmidt wrote:
> > On Tue, 2007-05-15 at 17:47 +0900, Tsutomu OWA wrote:
> > >   I encountered the following error when doing netperf from other machine 
> > > to Celleb running RT kernel.  PREEPT_NONE kernel works just fine as well.
> > 
> > Hrm... sounds a bit weird. I wonder if there's a locking bug in the
> > driver in the first place.
> > 
> > Linas, what's your take ?
> 
> Heh. I almost deleted the entire email thread cause it
> didn't say "spidernet" in the subject line. :-)
> Seriously, I really almost did ....
> 
> Since this is a long email; let me put a summary up front:
> I think the RT/premption patches are exposing some sort
> of race in the ip header handling code. The rest of the 
> note is forensics pointing to this.

skb->head should never ever be NULL.