On 06/17/2011 01:22 PM, Jan Kiszka wrote:
> On 2011-06-17 13:06, Gilles Chanteperdrix wrote:
>> On 06/17/2011 01:03 PM, Jan Kiszka wrote:
>>> On 2011-06-17 12:55, Gilles Chanteperdrix wrote:
>>>> On 06/17/2011 11:26 AM, Jan Kiszka wrote:
>>>>> Our current interrupt handlers assume that they leave over the same task
>>>>> and CPU they entered. But CONFIG_XENO_HW_UNLOCKED_SWITCH and commit
>>>>> f6af9b831c broke this assumption: xnpod_schedule invoked from the
>>>>> handler tail can now actually trigger a domain migration
>>>>
>>>> What unlocked context swith introduce from my point of view is simply
>>>> sections where interrupt happen which do not reschedule.
>>>>
>>>> f6af9b831c introduce a rescheduling point, but does not change what
>>>> happens during the interrupt handler either.
>>>>
>>>> So, I do not really understand this commit message. Either we can assume
>>>> that interrupt handlers migrate tasks or not, but this does not seem to
>>>> have anything to do with unlocked context switches or commit f6af9b831c.
>>>
>>> It has: Task is about to relax, re-enables interrupts in
>>> xnpod_resume_thread, IRQ hits, handler is entered over the relaxing RT
>>> task, xnpod_schedule in its tail performs the switch to root, which then
>>> continues to relaxed task, IRQ tail resumes over a different task, on
>>> SMP potentially also on a different CPU. I can send you a the trace if
>>> you want to have a closer look.
>>
>> Ok. Got it. But what has this to do with unlocked context switches ?
> 
> Also before commit f6af9b831c, there was a window with enabled IRQs in
> the relaxation path. So the above scenario should have been possible
> even earlier.

Unlocked context swith enables irqs, but sets the XNSWLOCK bit, so, no
other rescheduling can take place.

-- 
                                                                Gilles.

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