我应该在嵌套结构上还是仅在父结构上使用sync.Mutex？

I'm wondering where I should put the mutex in the example? Or should both structs have a mutex?

I have setters/getters for manipulating a *Device and I have a function for adding Devices to my State struct.

type State struct {
    Devices map[string]*Device
    //Should the sync.Mutex be here?
}

func (s *State) AddDevice(id [4]byte, name string, features []string, state string) {
    d := NewDevice(id, name, state, "", features)

    s.Devices[d.Id()] = d
}

func NewState() *State {
    return &State{make(map[string]*Device)}
}
type Device struct {
    //Or Should the sync.Mutex be here?
    SenderId  string
    Name      string
    State     string
    Type      string
    Features  []string
    EEPs      []string
    Power     int64
    PowerUnit string
}

func (d *Device) Power() int64 {
    return d.Power
}
func (d *Device) SetPower(p int64) {
    d.Power = p
}

func NewDevice(id [4]byte, name, state, dtype string, features []string) *Device {
    d := &Device{Name: name, State: state, Type: dtype}
    d.SetId(id)
    return d
}