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

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

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 :-)
> 
> (maybe we discuss this better further on xenomai-core)

Ok. Got it now. My idea with that the nucleus service xnselect could be
used to implement select-like services which would have different
semantics depending on the skins.

So, the select service with posix semantics was reserved to posix skin
threads.

-- 
                                          Gilles