// goroutines.
//
// Returns the STW context. When starting the world, this context must be
// passed to startTheWorld.
func stopTheWorld(reason stwReason) worldStop {
	semacquire(&worldsema)
	gp := getg()
	gp.m.preemptoff = reason.String()
	systemstack(func() {
		// Mark the goroutine which called stopTheWorld preemptible so its
		// stack may be scanned.