changes are always eventually made visible to other threads in all modern languages - it's the timing of thread wakeup and the possibility that the thread may not flush a cache that is uncertain. the GIL doesn't magically solve race conditions, and in Java and other such languages, you usually have a keyword like volatile that can make flushes explicit. In any case, I very much doubt the GIL work would affect Python's cache coherency model.

here's a python code that would break even with GIL:

current_boolean_value = global_cancelled

global_cancelled = new_boolean_value(current_boolean_value)