Re: [Xenomai-help] Question about getting system time

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

Steve Deiters wrote:
>> Yes, that's expected: If gettimeofday runs syscall-less, it 
>> tries to read the time offset from a page which the Linux 
>> kernel updates regularly. When user-space detects an ongoing 
>> update, it spins until that has completed. But as Xenomai 
>> runs the task with higher priority than the Linux update 
>> handler, you just ran into a live-lock.
>>
>> Switching on the Xenomai watchdog will confirm this: It will 
>> shoot that task, and the system will recover.
>>
>> Jan
>>
>> --
>> Siemens AG, Corporate Technology, CT T DE IT 1 Corporate 
>> Competence Center Embedded Linux
>>
> 
> I'm not all too familiar with the time handling in the kernel.  If you
> have any refernces that help me better follow your explanation that
> would be appreciated.
> 
> If you linked against the POSIX skin it would then seem there is no safe
> way of calling gettimeofday.  Even starting from main in this case is a
> real time task.  In this case I would think that gettimeofday should be
> either wrapped to force a mode change before calling
> __real_gettimeofday, or should somehow be expressly forbidden.


The reason why I did not wrap gettimeofday is that we already have a
wrapped clock_gettime, which is the newer service supposed to be used by
real-time applications. If you are not porting a non real-time
application, there is no reason to use gettimeofday, and if you are,
this guide:
http://www.xenomai.org/index.php/Porting_POSIX_applications_to_Xenomai#Timing_services.

Should explain you what you need to do.

-- 
					    Gilles.