2013/4/1 Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>:
> On 03/29/2013 10:24 AM, ning.n.jiang@xxxxxxxxx wrote:
>> From: Ning Jiang <ning.n.jiang@xxxxxxxxx>
>>
>> Currently there are two problems when we try to stop local timer.
>> First, it calls set_mode function directly so mode state is not
>> updated for the clock event device. Second, it makes the device
>> unused instead of shutdown.
>>
>> A subtle error will happen because of it. When a cpu is plugged out
>> it will stop the local timer. It will call tick_nohz_idle_enter()
>> in idle thread afterwards. It will cancel the sched timer and try
>> to reprogram the next event. This is wrong since the local timer
>> is supposed to be stopped.
>>
>> The right way to stop the local timer is to shutdown it by calling
>> clockevents_set_mode(). Thus when we try to reprogram the clock
>> event device, it will return directly without doing anything since
>> the clock mode is CLOCK_EVT_MODE_SHUTDOWN.
>>
>> Signed-off-by: Ning Jiang <ning.n.jiang@xxxxxxxxx>
>> ---
>
>
> Don't you see a kernel BUG with this patch ?
>
> In the clockevents code, there is:
>
> /**
> * clockevents_notify - notification about relevant events
> */
> void clockevents_notify(unsigned long reason, void *arg)
> {
> ...
> case CLOCK_EVT_NOTIFY_CPU_DEAD:
> ...
> cpu = *((int *)arg);
> list_for_each_entry_safe(dev, tmp, &clockevent_devices,
> list) {
> if (cpumask_test_cpu(cpu, dev->cpumask) &&
> cpumask_weight(dev->cpumask) == 1 &&
> !tick_is_broadcast_device(dev)) {
> BUG_ON(dev->mode !=
> CLOCK_EVT_MODE_UNUSED);
>
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> list_del(&dev->list);
> }
> }
> break;
> ...
> }
>
> This is called triggered from hrtimer_cpu_notify with the CPU_DEAD event.
>
The clockevents_do_notify() in clockevents_notify() will call
tick_notify() which will call tick_shutdown() in turn. tick_shutdown()
will set clock event device mode to UNUSED. So no panic afterwards.
void clockevents_notify(unsigned long reason, void *arg)
{
...
clockevents_do_notify(reason, arg);
switch (reason) {
case CLOCK_EVT_NOTIFY_CPU_DEAD:
...
list_for_each_entry_safe(dev, tmp, &clockevent_devices, list) {
if (cpumask_test_cpu(cpu, dev->cpumask) &&
cpumask_weight(dev->cpumask) == 1 &&
!tick_is_broadcast_device(dev)) {
BUG_ON(dev->mode != CLOCK_EVT_MODE_UNUSED);
list_del(&dev->list);
}
}
....
}
raw_spin_unlock_irqrestore(&clockevents_lock, flags);
}
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