"all datastructures would have to be thread-safe is pretty common argument used against usage of native threads (as opposed to green threads) or removal of glob

"all datastructures would have to be thread-safe is pretty common argument used against usage of native threads (as opposed to green threads) or removal of global locks in various VMs. While it's true that they don't have to be thread-safe from user's point of view, they should be thread-safe in the sense that they cannot get corrupted by race condition in user code in a way that could crash (or deadlock) the VM.