Re: [PATCH V3] tick/broadcast: Make movement of broadcast hrtimer robust against hotplug

Preeti U Murthy
2015-01-28T20:34:15Z

On 01/27/2015 09:01 AM, Preeti U Murthy wrote:
> On 01/22/2015 04:45 PM, Thomas Gleixner wrote:
>> 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.

Thomas ping. Would you be posting this patch?
>
> I noticed that tick_handover_do_timer() function also suffers from the
> issue that the patch I posted for moving the broadcast duty had, in that
> it relies on all cpus participating in stop_machine(). In a design where
> all cpus do not participate in stop_machine(), if the freshly nominated
> do_timer cpu is idle, there is no update of jiffies till that cpu gets
> back to being busy. So we must do an explicit take over of *both* the
> broadcast and do_timer duty just before the CPU_DEAD phase.