Futex
The lock that is free when nobody is fighting over it — fast userspace mutex, the building block under Linux thread synchronisation.
Classical locks always went through the kernel, even when the lock was free, costing a system call every time. Futex moves the common case into user space: an atomic operation on a shared integer is enough to take an uncontended lock, with the kernel never involved.
Only when someone actually has to wait is the futex() call made, putting the thread to sleep until woken. Because lock contention is rare in well-written code, the fast path is the one taken nearly always. Every mutex and condition variable in glibc is built on this, and most programmers never meet the call directly.