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Date:	Sat, 6 Apr 2013 10:31:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
cc:	Dave Hansen <dave@...1.net>, LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, dhillf@...il.com,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: kernel BUG at kernel/smpboot.c:134!

On Sat, 6 Apr 2013, Srivatsa S. Bhat wrote:

> Hi Dave,
> 
> On 04/06/2013 03:13 AM, Dave Hansen wrote:
> > Hey Thomas,
> > 
> > I seem to be running in to smpboot_thread_fn()'s
> > 
> > 	BUG_ON(td->cpu != smp_processor_id());

That should be WARN_ON of course. Stupid me.
 
> > pretty regularly, both at boot and if I boot with maxcpus=x and then
> > online the CPUs from sysfs after boot.  It's a 160-logical-cpu system,
> > so it's quite a beast.  I _seem_ to be hitting it more often at higher
> > cpu counts, but it doesn't trigger on bringing up a particular CPU as
> > far as I can tell.
> > 
> > This is on a pull of mainline from today, e0a77f263.  Any ideas?
> >
> 
> Dave Jones had reported a similar problem some time back and Hillf had
> proposed a fix. I guess it slipped through the cracks and never went
> upstream.
> 
> Here is the link: https://lkml.org/lkml/2013/1/19/1

This is Hillfs proposed patch:

> --- a/kernel/kthread.c	Sat Jan 19 13:03:52 2013
> +++ b/kernel/kthread.c	Sat Jan 19 13:17:54 2013
> @@ -306,6 +306,7 @@ struct task_struct *kthread_create_on_cp
>   	  return p;
> 	  set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
> 	  to_kthread(p)->cpu = cpu;
> +	  __kthread_bind(p, cpu);
> 	  /* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
> 	  kthread_park(p);
> 	  return p;
 
That's bogus. Simply because when we create the thread then the thread
status is HP_THREAD_NONE and the path with the BUG_ON is only entered
with status == HP_THREAD_ACTIVE:

	if (ht->park && td->status == HP_THREAD_ACTIVE) {

So in Dave's case the thread was already created and has entered
active state.

> >> [  790.226909] Pid: 3909, comm: migration/135 Tainted: G        W    3.9.0-rc5-00184-gb6a9b7f-dirty #118 FUJITSU-SV PRIMEQUEST 1800E2/SB

Hmm, it's the migration thread which trips over this. Oh joy!

Dave, does the issue reproduce with function tracing enabled? For a
first shot it's probably enough to filter on smpboot_* functions plus
sched_switch and sched_wakeup events.

Thanks,

	tglx
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