Synchronisation

• Need to ensure that a thunk is evaluated only once, and no two threads evaluates the same thunk simultaneously.
• This is achieved by requiring a thread entering a thunk to overwrite the thunk with a black-hole . A thread entering a black-hole saves its state in its TSO, attaches its TSO to the queue of threads blocked on the black-hole and enters the scheduler.
• A thread completing the evaluation of a thunk overwrites it with its value and moves its TSO to the runnable pool.
• Synchronisation costs only when two threads collide.