Re: [Xenomai-core] select: native tasks with posix skin mqueues

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

Peter Soetens wrote:
> On Thu, Nov 5, 2009 at 02:46, Gilles Chanteperdrix
> <gilles.chanteperdrix@xenomai.org> wrote:
>> Peter Soetens wrote:
>>> Hi,
>>>
>>> I'm creating my RT threads using the native API and I'm creating
>>> mqueues, wrapped to the pthread_rt library.
>>> I can read and write the mqueue (and it goes through Xenomai), but
>>> when I select() on a receiving mqd_t, the select() calls returns that
>>> there is data available on the mq (it fills in the FD_SET), but keeps
>>> doing so even when it's empty (the select() is in a loop). Also, it's
>>> modeswitching like nuts.
>>>
>>> I found out that the __wrap_select is correctly called, but returns
>>> -EPERM. Kernel sources indicate that this is caused by
>>> pse51_current_thread() alias thread2pthread() returning null. Since
>>> EPERM is returned to userspace, the __real_select is called from user
>>> space, causing the mode switches and bad behaviour. This is almost
>>> certainly the thing that native + RTDM + select() is seeing too.
>>>
>>> My mqueues-only work probably because mq.c only uses
>>> pse51_current_thread() in the mq_notify function. I'm guessing that
>>> mq_notify would also not work in combination with native skin.
>>>
>>> I had two options in fixing this: add a xnselector to the native task
>>> struct or to the nucleus xnthread_t. I choose the latter, such that
>>> every skin kan use select() + RTDM and migrate gradualy to the RTDM
>>> and/or Posix skin.
>>> I needed to free the xnselector structure in xnpod_delete_thread() , I
>>> chose a spot, but it causes a segfault in my native thread (which did
>>> the select) during program cleanup. Any advice ? Also, maybe we should
>>> separate select() from the posix skin and put it in a separate place
>>> (in RTDM as rtdm_select() ?), such that we can start building around
>>> it (posix just forwards to rtdm_select() then).
>>>
>>> A second patch was necessary to return the timeout case properly to
>>> userspace (independent of first patch).
>>>
>>> Tested with native + posix loaded and mq. If you never quit your
>>> application, this works :-)
>> Hi,
>>
>> I have included a lightly modified version of this patch on head, I do
>> not see any crash.  However, I have some doubts about the current
>> implementation: calling xnselector_destroy() opens opportunities for a
>> rescheduling, which I am not sure is really what we want in the middle
>> of xnpod_delete_thread(). Philippe, what do you think?
> 
> I'll test this patch this week too. It seems you forgot to apply the
> second patch, which can go straight into the 2.4 head, since it fixes
> a bug in the select() wrapping code when timeouts are used. See
> parent's 0002-posix-Fix-__wrap_select-when-timeout-happens.patch

Yes, I forgot it, will apply soon, thanks.

-- 
                                          Gilles