From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] BUG: using smp_processor_id() in preemptible
 [00000000] code: caller is __qdisc_run
Date: Mon, 11 Aug 2008 14:23:59 -0700 (PDT)
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From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Mon, 11 Aug 2008 23:09:38 +0200

> On Mon, 2008-08-11 at 14:00 -0700, David Miller wrote:
> > From: "John Kacur" <jkacur@gmail.com>
> > Date: Mon, 11 Aug 2008 15:11:46 +0200
> > 
> > > __qdisc_run() calls qdisc_restart() which calls
> > > handle_dev_cpu_collision(skb, dev, q); and then the problem shows up
> > > here:
> > > __get_cpu_var(netdev_rx_stat).cpu_collision++;
> > > 
> > > The solution is to disable interrupts around the above increment. Here
> > > is an attached patch to do so. (Thank's to Peter Zijlstra for help in
> > > the analysis and dropping the answer in my lap, so if I got it right
> > > it is due to his help, but if I messed it up, then I did that part all
> > > by myself.)
> > 
> > __qdisc_run() always runs in software interrupt context,
> > so I guess this is some problem with the -rt stuff running
> > software interrupts in threads?
> 
> Hmm, good point - and those threads should be cpu affine on -rt if I'm
> not mistaken. Steven, do you happen to remember details?

The key issue is whether those threads run software interrupts
in a compatible environment.  And such a proper environment allows
plain smp_processor_id() without any special preparations.