// Returns the STW context. When starting the world, this context must be
// passed to startTheWorldWithSema.
func stopTheWorldWithSema(reason stwReason) worldStop {
	trace := traceAcquire()
	if trace.ok() {
		trace.STWStart(reason)
		traceRelease(trace)
	}
	gp := getg()

	// If we hold a lock, then we won't be able to stop another M
	// that is blocked trying to acquire the lock.
	if gp.m.locks > 0 {