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

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

Wolfgang Grandegger wrote:
> 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.

For architectures with a high-resolution counter,
clock_gettime(CLOCK_MONOTONIC) is a xenomai service which uses the
counter and does not issue any syscall, this includes arm unless you
configured xenomai with --enable-arm-arch=generic or with
--disable-arm-tsc, and this probably includes powerpc.

> 
>> 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.

In case of programming error, enable the xenomai watchdog, it will
forcibly kill the problematic thread.

-- 
                                                 Gilles.