Re: [Xenomai-help] pthread cancelation and scheduling magics

[Wolfgang Grandegger <wg@domain.hid>]

Gilles Chanteperdrix wrote:
> Gilles Chanteperdrix wrote:
>> Wolfgang Grandegger wrote:
>>> Hello,
>>>
>>> I have written the attached test program to cancel Xenomai POSIX
>>> threads. The "calc_task" does some busy work, which the higher priority
>>> task "ctrl_task" interrupts and aborts after some time. The program does
>>> not behave like I expect and it also behaves differently on my PowerPC
>>> and ARM test system. The "calc_task" continues after calling
>>> pthread_cancel() in "ctrl_task". On ARM, the behaviour is even more
>>> wired. Is there anything wrong in my test program or anything else I
>>> should care of?
>> First, you should know that PTHREAD_CANCEL_ASYNCHRONOUS is evil, it will
>> almost inevitably leave things in an unknown state when canceling a
>> thread, you would better use PTHREAD_CANCEL_DEFERRED and ensure that
>> your thread has some cancellation point, if it has not, use
>> pthread_testcancel.
>>
>> Second, your program will only work if root thread priority coupling is
>> enabled. Is it enabled in your case?
> 
> Actually, even with priority coupling I am not sure it can not work. The
> problem is the way do_sigwake_event signal threads: it make them switch
> to secondary mode only if they pass through a syscall. Since the
> "calc_task" thread does not issue any syscall, it will never check see
> that it should relax to handle the cancelation signal.

calc_task() calls clock_gettime() permanently, which does a syscall as
long as vDSO is not used, which might be the case on PowerPC. This would
explain why cancelation of "calc_task" works on ARM.

> Now, the question is, do you realistically plan to write an application
> which makes no syscall in its real-time loop?

Unlikely, but it may happen in case of programming errors. Anyhow, the
pthreads will run legacy code and it would be a pain to add
pthread_testcancel where necessary. But maybe there is a more elegant
and simple solution to do a defined exit/abort.

Wolfgang.