[kprotty]

futex APIs are both prevalent and somewhat unfortunate: Since the task enqueue happens in the kernel, you have no control over how it does it. This makes something like "wait if not equal" or requeue either not portable (few non-linux api's support it) or inefficient to adopt (too many syscalls from intermediate locks or eager wakes when using). It also means custom queueing (i.e. readers in one list, writers in another, same addr key) isn't possible.

NetBSD and Windows seemed to have made a more perf-flexible API here: Each thread has an implicit AutoResetEvent which it can wait on while any other thread can set it. All task queueing can then be done in userspace, in a lock-free fashion too for Windows. This avoids that extra syscall at the end for futex based locks/rwlocks and employs natural/efficient requeue. NetBSD has a slight edge in that you can set the AutoResetEvent of multiple threads using a single syscall.