If I'm reading the post correctly, the HN title is inaccurate. The in-progress STM was already written in C. It was the garbage collector that was written in RPython, and <i>that</i> has been rewritten in C as part of a combined STM/GC library.<p>edit: fixed now.
The original blog post they did on STM is well worth reading, especially if you're not familiar with the python GIL and STM.<p><a href="http://morepypy.blogspot.com.au/2011/08/we-need-software-transactional-memory.html" rel="nofollow">http://morepypy.blogspot.com.au/2011/08/we-need-software-tra...</a>