It seems like it would be possible to implement this in userspace using shared memory to store the data structures and using just one eventfd per thread to park/unpark (or a futex if not waiting for anything else), which should be fully correct and have similar or faster performance, at the cost of not being secure or robust against process crashes (which isn't a big problem for more Wine usage).

It seems that neither esync or fsync do this though - why?

Claude thinks that "nobody was motivated enough to write and debug the complex shared-memory waiter-list logic when simpler (if less correct) approaches worked for 95% of games, and when correctness finally mattered enough, the kernel was the more natural place to put it". Is that true?