This isn't really an issue with threads though, the exact same issue is present in green thread/fiber implementations; it just so happens that in Async Ruby the

This isn't really an issue with threads though, the exact same issue is present in green thread/fiber implementations; it just so happens that in Async Ruby the GIL saves you from this specific problem due to making variable accesses atomic (as I understand it anyway, I'm not super familiar with the Ruby VM). In general, green threads/fibers are vulnerable to the exact same shared memory issues as threads, the only benefit to them is that they are, for a certain class of problems, a more efficient concurrency primitive than threads by avoiding context switches out of userspace, and in many cases provide you the ability to plug in your own scheduler if you so desire allowing you to optimize scheduling for your own workload.