Re: [PATCH V2] clockevents: Fix cpu down race for hrtimer based broadcasting

[Ingo Molnar]

* Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> wrote:

> It was found when doing a hotplug stress test on POWER, that the machine
> either hit softlockups or rcu_sched stall warnings.  The issue was
> traced to commit 7cba160ad789a powernv/cpuidle: Redesign idle states
> management, which exposed the cpu down race with hrtimer based broadcast
> mode(Commit 5d1638acb9f6(tick: Introduce hrtimer based broadcast). This
> is explained below.
> 
> Assume CPU1 is the CPU which holds the hrtimer broadcasting duty before
> it is taken down.
> 
> CPU0					CPU1
> 
> cpu_down()				take_cpu_down()
> 					disable_interrupts()
> 
> cpu_die()
> 
>  while(CPU1 != CPU_DEAD) {
>   msleep(100);
>    switch_to_idle();
>     stop_cpu_timer();
>      schedule_broadcast();
>  }
> 
> tick_cleanup_cpu_dead()
> 	take_over_broadcast()
> 
> So after CPU1 disabled interrupts it cannot handle the broadcast hrtimer
> anymore, so CPU0 will be stuck forever.
> 
> Fix this by explicitly taking over broadcast duty before cpu_die().
> This is a temporary workaround. What we really want is a callback in the
> clockevent device which allows us to do that from the dying CPU by
> pushing the hrtimer onto a different cpu. That might involve an IPI and
> is definitely more complex than this immediate fix.

So why not use a suitable CPU_DOWN* notifier for this, instead of open 
coding it all into a random place in the hotplug machinery?

Also, I improved the changelog (attached below), but decided against 
applying it until these questions are cleared - please use that for 
future versions of this patch.

Thanks,

	Ingo

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