linux-kernel - Re: [PATCH] BUG: using smp_processor_id() in preemptible [00000000] code: caller is __qdisc

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Message-ID: <alpine.DEB.1.10.0808112121370.32393@gandalf.stny.rr.com>
Date:	Mon, 11 Aug 2008 21:22:58 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Miller <davem@...emloft.net>
cc:	a.p.zijlstra@...llo.nl, jkacur@...il.com,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu, tglx@...utronix.de
Subject: Re: [PATCH] BUG: using smp_processor_id() in preemptible [00000000]
 code: caller is __qdisc_run


On Mon, 11 Aug 2008, David Miller wrote:

> From: Peter Zijlstra <a.p.zijlstra@...llo.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@...il.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.
> 

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.

-- Steve

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