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Date: Thu, 22 Jan 2015 12:15:36 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Subject: Re: [PATCH V3] tick/broadcast: Make movement of broadcast hrtimer
 robust against hotplug
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On Thu, 22 Jan 2015, Preeti U Murthy wrote:
> On 01/21/2015 05:16 PM, Thomas Gleixner wrote:
> How about when the cpu that is going offline receives a timer interrupt
> just before setting its state to CPU_DEAD ? That is still possible right
> given that its clock devices may not have been shutdown and it is
> capable of receiving interrupts for a short duration. Even with the
> above patch, is the following scenario possible ?
> 
>                 CPU0                                  CPU1
> t0         Receives timer interrupt
> 
> t1         Sees that there are hrtimers
>            to be serviced (hrtimers are not yet migrated)
> 
> t2         calls hrtimer_interrupt()
> 
> t3         tick_program_event()                   CPU_DEAD notifiers
>                                                 CPU0's td->evtdev = NULL
> 
> t4         clockevent_program_event()
>            references NULL tick device pointer
> 
> So my concern is that since the CLOCK_EVT_NOTIFY_CPU_DEAD callback
> handles shutting down of devices besides moving tick related duties.
> it's functions may race with the hotplug cpu still handling tick events.

  __cpu_disable() is supposed to block interrupts on the dying cpu.

But I agree, we should make it more robust. So we want an explicit
call for disabling the cpu local stuff and an explicit takeover of the
broadcast duty. I'm anyway distangling the clockevents_notify() stuff,
so it should be simple to do so.

Thanks,

	tglx