Re: [Xenomai-core] [Xenomai-git] Jan Kiszka : nucleus: Fix interrupt handler tails

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 06/18/2011 12:21 PM, Jan Kiszka wrote:
> On 2011-06-17 20:55, Gilles Chanteperdrix wrote:
>> On 06/17/2011 07:03 PM, Jan Kiszka wrote:
>>> On 2011-06-17 18:53, Gilles Chanteperdrix wrote:
>>>> On 06/17/2011 04:38 PM, GIT version control wrote:
>>>>> Module: xenomai-jki
>>>>> Branch: for-upstream
>>>>> Commit: 7203b1a66ca0825d5bcda1c3abab9ca048177914
>>>>> URL:    http://git.xenomai.org/?p=xenomai-jki.git;a=commit;h=7203b1a66ca0825d5bcda1c3abab9ca048177914
>>>>>
>>>>> Author: Jan Kiszka <jan.kiszka@domain.hid>
>>>>> Date:   Fri Jun 17 09:46:19 2011 +0200
>>>>>
>>>>> nucleus: Fix interrupt handler tails
>>>>>
>>>>> Our current interrupt handlers assume that they leave over the same task
>>>>> and CPU they entered. But commit f6af9b831c broke this assumption:
>>>>> xnpod_schedule invoked from the handler tail can now actually trigger a
>>>>> domain migration, and that can also include a CPU migration. This causes
>>>>> subtle corruptions as invalid xnstat_exectime_t objects may be restored
>>>>> and - even worse - we may improperly flush XNHTICK of the old CPU,
>>>>> leaving Linux timer-wise dead there (as happened to us).
>>>>>
>>>>> Fix this by moving XNHTICK replay and exectime accounting before the
>>>>> scheduling point. Note that this introduces a tiny imprecision in the
>>>>> accounting.
>>>>
>>>> I am not sure I understand why moving the XNHTICK replay is needed: if
>>>> we switch to secondary mode, the HTICK is handled by xnpod_schedule
>>>> anyway, or am I missing something?
>>>
>>> The replay can work on an invalid sched (after CPU migration in
>>> secondary mode). We could reload the sched, but just moving the replay
>>> is simpler.
>>
>> But does it not remove the purpose of this delayed replay?
> 
> Hmm, yes, in the corner case of coalesced timed RT task wakeup and host
> tick over a root thread. Well, then we actually have to reload sched and
> keep the ordering to catch that as well.
> 
>>
>> Note that if you want to reload the sched, you also have to shut
>> interrupts off, because upon return from xnpod_schedule after migration,
>> interrupts are on.
> 
> That would be another severe bug if we left an interrupt handler with
> hard IRQs enabled - the interrupt tail code of ipipe would break.
> 
> Fortunately, only xnpod_suspend_thread re-enables IRQs and returns.
> xnpod_schedule also re-enables but then terminates the context (in
> xnshadow_exit). So we are safe.

I do not think we are, at least on platforms where context switches
happen with irqs on.


-- 
                                                                Gilles.