threaded rts deadlock

[Wolfgang Thaller]

If a callback into Haskell code finishes very quickly, a deadlock 
occurs. Adding a small delay helps.

This is what I think happens:

rts_evalIO creates a haskell thread
rts_evalIO calls scheduleExtThread to spawn a new OS thread. When the 
callback is finished, schedule() calls broadcastCondition on m->wakeup. 
But because the callback took very little time, this happens _before_ 
rts_evalIO calls waitThread.
waitThread is responsible for initializing m->wakeup and waiting for a 
broadcast, but broadcastCondition might already have been called 
_before_ that. It won't be called again, so nothing happens anymore...
Some locking to prevent this race condition is probably necessary...

Cheers,

Wolfgang