FreeBSD's pthread_mutex is shared between the kernel and user land.
So it does a compare and set to take the lock. Uncontested and unheld
locks will mean we've taken the lock and return. Contested locks
are kicked to the kernel to wait. When userland releases the lock
it signals the kernel to wakeup via a system call. The kernel then
does a cas to try to acquire the lock. It either returns with the lock
held, or goes back to sleep. This we have atomics operating both in
the kernel (via standard host atomics) and userland atomics done
via start/end_exclusive.