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

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Date:	Mon, 11 Aug 2008 22:06:28 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	David Miller <davem@...emloft.net>, 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

Steven Rostedt wrote:
> 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.
>   
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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