Re: [PATCH] BUG: using smp_processor_id() in preemptible [00000000]code: caller is __qdisc_run

[Gregory Haskins]

Steven Rostedt wrote:
> On Mon, 11 Aug 2008, David Miller wrote:
>
>   
> > From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > 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@xxxxxxxxx>
> > > > 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.
> >
> >     
>
> Yes, we have a softirq thread per CPU. We should have a test in the 
> smp_processor_id for rt to not bug if it is called by known "per_cpu" 
> threads.
>   
something like

#ifdef CONFIG_PREEMPT_RT
   WARN_ON(!in_atomic() && current->rt.nr_cpus_allowed > 1);
#else
   WARN_ON(!in_atomic());
#endif

would probably work and be fairly efficient.  Of course nr_cpus_allowed 
technically could be adjusted at any time, so perhaps not.  Its probably 
good enough for a warning check, however.

> -- Steve
>
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