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