extraMInUse atomic.Uint32
)

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
//
//go:nosplit
func lockextra(nilokay bool) *m {
	const locked = 1