Rethinking the futex API

You would just do things differently. A model that is relatively common is the "parking lot" where each thread has a single binary semaphore attached to it. The primitives in this model are "parking" yourself (consuming the token in the semaphore, or waiting for one and consuming it immediately if it's not there) and "unparking" another thread (placing a token on its semaphore). The parking lot itself is an associative array from addresses to waitqueues, similar to what the kernel does for futexes, so that you can "park" yourself on a list and "unpark" the head of a list.

By moving waitqueue handling to userspace, you don't need a system call anymore for either requeueing or multiple-futex wait. On the other hand it makes the implementation of pthread_barrier_t less efficient because every slow-path wakeup needs a separate system call.