Ruby threads working with native threads

I recently wrote a network-event extension for Ruby ("eventmachine" in
Rubyforge) and had considerable difficulty working with Ruby threads. Of
course you can guess the problem: select(2) can't be called by an extension
if more than one Ruby thread is running. I had to use a rather ugly hack to
get it to work, since there are no shared synchronization objects between
Ruby and native code.

Is it possible to call the Ruby thread scheduler directly from extension
code? Does that approach even make sense?
Does it make sense to enable Ruby's sync primitives to interoperate with
native ones?
Thank you,
-francis