// cannot happen concurrently with this. allgs grows
	// monotonically and existing entries never change, so we can
	// simply return a copy of the slice header. For added safety,
	// we trim everything past len because that can still change.
	return allgs[:len(allgs):len(allgs)]
}

// atomicAllG returns &allgs[0] and len(allgs) for use with atomicAllGIndex.
func atomicAllG() (**g, uintptr) {