RFC: termination detection for STM

[Brandon Michael Moore]

On Wed, Feb 14, 2007 at 10:04:32AM +0000, Simon Marlow wrote:
> Perhaps I'm missing something, but doesn't GHC already detect the kind of 
> deadlock you're talking about here?  When a thread is blocked and cannot be 
> woken up, it is sent the BlockedOnDeadMVar exception.  It's more precise 
> than the extension you propose, because the GC is used to check which 
> threads are unreachable and therefore cannot be woken up, so it can detect 
> mutual-deadlock between two threads in a system that contains other running 
> threads.

Perhaps the idea is specifically to detect from the outside when a group of
threads is deadlocked, maybe like something that can be done with computation
spaces in Oz, definitely like the way tree spaces work in Aardappel
( http://wouter.fov120.com/aardappel/ ).

Based on your description, it sounds like it wouldn't work very well to have
a parent thread waiting on a channel, with one of the child threads set up
to catch BlockedOnDeadMVar and send a message, lest the parent thread be
considered deadlocked and sent BlockedOnDeadMVar itself.

What are the semantics of the exception? It seems like it might be tricky
to provide any guarantees, if a thread can catch the exception and make the
MVar live again.

Brandon