I think he means that if the OS knows about locking, it can manage a list of "waiters" to quickly know which thread to wake (resp. let sleep) once (resp. before

I think he means that if the OS knows about locking, it can manage a list of "waiters" to quickly know which thread to wake (resp. let sleep) once (resp. before) the lock is released. A bit like what classic UNIX does with wchan, but between the kernel and userspace this time. Related: https://rdmsr.github.io/writing/turnstiles/ https://rdmsr.github.io/writing/turnstiles/