if gp.lockedm == 0 {
		return nil, plainError("goroutine not locked to thread")
	}

	tid := int(gp.lockedm.ptr().procid)
	if tid == 0 {
		return nil, plainError("missing tid")
	}

	f := efaceOf(&fn)
	if f._type == nil || f._type.Kind_&abi.KindMask != abi.Func {
		return nil, plainError("fn must be a function")
	}
	fv := (*funcval)(f.data)

	a := efaceOf(&stackArgs)