//
//go:nosplit
func lockextra(nilokay bool) *m {
	const locked = 1

	incr := false
	for {
		old := extraM.Load()
		if old == locked {
			osyield_no_g()
			continue
		}
		if old == 0 && !nilokay {
			if !incr {
				// Add 1 to the number of threads
				// waiting for an M.
				// This is cleared by newextram.
				extraMWaiters.Add(1)