if old&mutexStarving == 0 {
			new |= mutexLocked
		}
		if old&(mutexLocked|mutexStarving) != 0 {
			new += 1 << mutexWaiterShift
		}
		// The current goroutine switches mutex to starvation mode.
		// But if the mutex is currently unlocked, don't do the switch.
		// Unlock expects that starving mutex has waiters, which will not
		// be true in this case.
		if starving && old&mutexLocked != 0 {
			new |= mutexStarving
		}

//	futexwakeup(addr *uint32, cnt uint32)
//		If any procs are sleeping on addr, wake up at most cnt.

const (
	mutex_unlocked = 0
	mutex_locked   = 1
	mutex_sleeping = 2

	active_spin     = 4
	active_spin_cnt = 30
	passive_spin    = 1
)

// Possible lock states are mutex_unlocked, mutex_locked and mutex_sleeping.
// mutex_sleeping means that there is presumably at least one sleeping thread.