Re: [uml-devel] UML hangs with hrtimer test module

[Richard W. <ri...@no...>]

Am Donnerstag, 29. März 2018, 00:19:39 CEST schrieb Joel Fernandes:
> Thanks for the quick reply.
> 
> On Wed, Mar 28, 2018 at 6:19 AM, Richard Weinberger <ri...@no...> wrote:
> > Am Mittwoch, 28. März 2018, 15:11:29 CEST schrieb Geert Uytterhoeven:
> >> On Wed, Mar 28, 2018 at 12:28 PM, Joel Fernandes <agn...@gm...>
> > wrote:
> >> >         while(release_now == 0);
> >>
> >> while (release_now == 0)
> >>         cpu_relax();
> >
> > Not sure whether a cpu_relax() fixes the problem.
> > I guess the root of the problem is that UML is UP and non-preemptive.
> > Therefore the loop is never interrupted.
> > To verify I asked for the full source.
> >
> 
> cpu_relax actually worked!

Interesting.
 
> Any thoughts on why it helps? Even if its non-preemptive, I did
> receive the timer interrupt, so I expected the variable to be set.

Timers trigger also with preempt off, I forgot...
I think the cpu_relax() issues internally a barrier such that the
release_now variable is read again.
Can you try barrier() instead of cpu_relax()? I bet it works too.
Same if you mark release_now as volatile.

Thanks,
//richard