Subject: Re: next-20130627 breaks i.MX6 sabre sd UART console

From: Stephen Boyd <[email-masked]>
Date: Mon, 01 Jul 2013 15:14:24 -0700

On 07/01/13 14:24, Thomas Gleixner wrote:
> On Mon, 1 Jul 2013, Stephen Boyd wrote:
>> On 07/01/13 13:14, Thomas Gleixner wrote:
>>> The issue is very subtle. What happens is:
>>>
>>> CPU0						CPU1
>>>
>>> Switch to oneshot mode
>>>
>>>  Copy the bits from tick_broadcast_mask to
>>>  tick_broadcast_oneshot_mask. We need to do
>>>  that so the other cpus reach the timer irq
>>>  and the softirq which switches them to
>>>  oneshot.
>>>
>>>  Kick the broadcast device into oneshot.
>>>
>>> 						Timer interrupt fires
>>> 						
>>> 						irq_enter sees the cpu in
>>> 						tick_broadcast_oneshot_mask and
>>> 						sets the device to oneshot mode
>>> 						
>>> 						handle_periodic:
>>> 						 Sees oneshot mode and adds
>>> 						 period to
>>> 						 dev->next_event(KTIME_MAX)
>>> 			
>> Yep. It is also racing with the timer interrupt so having more than two
>> CPUs must help widen the window (which is why we see it on the higher
>> numbered CPUs).
> The race above is about the timer interrupt. You mean the broadcast
> one which is still enabled due to the dummy -> functional transition
> issue, right? That helps a lot to make this more visible, because we
> double the number of events.

I was thinking that tick_check_oneshot_broadcast() is racing with
tick_switch_to_oneshot() and because we have more CPUs we're more likely
to have a CPU fix up the handler in tick_switch_to_oneshot() after
tick_check_oneshot_broadcast() forces that CPU to oneshot mode and the
periodic handler runs. I wonder if I can reproduce it locally by making
tick_switch_to_oneshot() spin for a jiffy or two on CPU1.

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