linux-kernel - Re: [PATCH] clockevent: don't remove broadcast device when cpu is dead

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Date:	Thu, 14 Jan 2010 12:52:38 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Xiaotian Feng <dfeng@...hat.com>
cc:	linux-kernel@...r.kernel.org, Magnus Damm <damm@...l.co.jp>,
	H Hartley Sweeten <hsweeten@...ionengravers.com>
Subject: Re: [PATCH] clockevent: don't remove broadcast device when cpu is
 dead

On Thu, 14 Jan 2010, Xiaotian Feng wrote:
> On 01/14/2010 06:08 AM, Thomas Gleixner wrote:
> > On Wed, 13 Jan 2010, Xiaotian Feng wrote:
> > > On 01/12/2010 09:20 PM, Thomas Gleixner wrote:
> > > > On Thu, 7 Jan 2010, Xiaotian Feng wrote:
> > > > 
> > > > > Marc reported BUG during shutdown, after debugging, kernel is trying
> > > > > to remove a broadcast device which mode is CLOCK_EVT_MODE_ONESHOT.
> > > > > 
> > > > > The root cause for this bug is that in clockevents_notify,
> > > > > "cpumask_weight(dev->cpumask) == 1" is always true even if dev is a
> > > > 
> > > > Why is cpumask_weight(dev->cpumask) == 1 always true when we shutdown
> > > > a non boot cpu ?
> > > > 
> > > > The broadcast device is not a per cpu device and the cpumask should
> > > > not only contain the CPU which is shut down !
> > > 
> > > At least for hpet broadcast dev, it's dev->cpumask is only contain the CPU
> > > which it is initialized from.
> > 
> > Which is fundamentaly wrong and the root cause of the problem. I'll
> > have a look tomorrow morning when my brain is more awake than now.
> 
> hpet_legacy_clockevent_register is trying to register new CE, but replace
> failed,
> then in tick_check_new_device -> tick_check_broadcast_device, the legacy hpet
> CE
> was registered as multicast device, but its dev->cpumask is cpumask of
> smp_processor_id().
> 
> on my system its dev->cpumask is cpumask of 0, but in Marc's, dev->cpumask is
> cpumask of 4.
> So when kernel is trying to offline cpu 4, the broadcast hpet is removed.

I know, but the point is that a broadcast device should not be bound
to the CPU which is registering it. Working on a fix.

Thanks,

	tglx
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