[Xenomai] trying to shadow a pthread fails with -EINVAL error

[Philippe Gerum]

On 04/19/2016 04:08 PM, Andrea wrote:
> Hello,
>
> I'm currently trying to integrate Xenomai 3.0.2 using Cobalt with alchemy
> skin in a C++  project, under kernel 4.1.8  with i-pipe patch.
>
> In my application I'm creating some threads using boost library, and I
> would like to shadow one of them to a rt_task in order to use some xenomai
> function in there.
>
> If I call rt_task_shadow(NULL, NULL, 0,0) from the running thread, -EINVAL
> is returned.
>
> While the API docs says that this can be due to an invalid prio, tracking
> down the source of the error, it appears that in my case it is generated in
> the function syncobj_lock (copperplate/syncobj.c), where a call to
> monitor_enter (which internally calls cobalt_monitor_enter) fails and
> returns -1.
>
> The same happens with threads created both using standard pthread_create or
> via the boost library.
>
> Trying to shadow the main thread (or creating rt_task from there) works
> correctly. instead.
>
> A reproducible example in my system:
>
> #include "pthread.h"
> #include "alchemy/task.h"
> #include <iostream>
> #include <unistd.h>
>
> void* shadow(void*)
> {
>      std::cerr<<"Shadow returned "<< rt_task_shadow(NULL, NULL, 0,0);
> }
>
> int main(int argc, char* argv[])
> {
>      pthread_t test_thread;
>     // new boost::thread(shadow);
>      pthread_create(&test_thread, NULL, shadow, NULL);
>      sleep(10);
>
>      return 0;
> }
>
> with output: Shadow returned -22
>
> Do you have any insight on where the problem could be? Am I missing
> something basic?
>

I suspect some chicken and egg situation in the rt_task_shadow() 
implementation. I'll have a look as soon as I can.

> If possible, I would like to stick to alchemy skin.

Unfortunately, the only way to work around this issue at the moment 
would be to call libcobalt's pthread_setschedparam() for pthread_self(), 
until this issue is fixed in libalchemy. You could do this for a 
particular statement without wrapping the POSIX API by calling:

struct sched_param parm = { .sched_priority = 0 };
__RT(pthread_setschedparam(phread_self(), SCHED_OTHER, &parm);

instead of rt_task_shadow(). libcobalt is underlying all Xenomai 3 
applications, so you would not need to fixup the build rules or 
dependencies. It's already there.

-- 
Philippe.