lebrush

I observed it while testing the callback of the `xtimer_mutex_lock_timeout`. Removing the `thread_yield_higher` call from the `_mutex_timeout` callback, blocks indefinitely. I thought it might be because no yielding is done,...

So far I'm using `tests/posix_semaphore`, keeping/removing the `thread_yield_higher` in the callback of the timer to trigger this. I can't promise I have time to look further into this for this...

As just mentioned in #6441 I also realized that in `native` when one xtimer is set and fired I can't any future xtimer does not fire... **EDIT**: had a quick...

@LudwigKnuepfer try the following "snippet" to see what I mean. It's not that the thread doesn't yield but that the timer stops "working" if fired to fast. Main will print...

Thanks for testing! Description changed :-)