Donn,
a thread interlock that serializes access from different threads to the runtime, The thread that holds the lock runs until an I/O operation, then releases the lock,
Provided we are talking about a single sequential processor, then concurrent thread access to the heap should be largely OK I guess, because heap values are immutable. The only thing the RTS needs to be careful about is if one thread accesses a heap cell and finds it is blackholed (i.e. currently under evaluation by a different thread), then it should block until the cell is updated. I believe Yhc already does this. However, if there are multiple processors accessing the same shared memory containing the heap, then I suppose the RTS would additionally need to place a lock around an individual cell during the heap-update operation. The GC would also need to stop all threads whilst collecting. Regards, Malcolm