Why do you need a CAS? Isn't an atomic write sufficient? Because we're just changing the function to use our code we don't really care what it was pointing to b

Why do you need a CAS? Isn't an atomic write sufficient? Because we're just changing the function to use our code we don't really care what it was pointing to before, and we don't care if someone else changed the pointer before we did: it's just last write wins. In fact, the function pointer method will be able to have multiple threads try to hotpatch at once; idk about the OP code. My guess is the OP's method can probably be extended to allow for more advanced hooking, like returning to the original function definition after running the new code or something. And, it means the caller doesn't have to know that the function is hot patchable, which I'm guessing is the most likely reason.