Having worked on moving a proprietary language from its own green threaded VM to the JVM, and working on TruffleRuby I’m saddened to see this FUD still being trotted out. The GIL and similar mechanisms do not make your code thread safe. It _might_ save you from a small set of concurrency bugs, but they are fewer than you might think, and mostly it will just make intermittent existing issues that little bit more obvious when you move to real threads. Occasionally we would need to fix something in a core library or add a mutex, but those bugs could often be seen in a stress test with green threads or Ruby’s GVL.

My guess is the GIL or smaller mutexes will be needed for C extensions and a few other areas, but it’s also likely that could be moved to an opt in mechanism over time.