Stackless is an example of the communicating sequential process model, not concurrency. You avoid all the issues around locking etc because switching between processes (tasklets) is explicit - it's cooperative multitasking not preemptive. It's a very nice language but doesn't help at all with running on multiple cores. Once you start having multiple processes running simultaneously and shared mutable state you have to start worrying about things like locks. Erlang nicely sidesteps the problem by not having mutable variables.