Concurrent read and write to a map in Go

What happens if you perform read and write operations on a map concurrently from different goroutines? Will errors occur, and are there any restrictions?

var data = map[string]int{"a": 1}

func main() {
    go reader()
    time.Sleep(1 * time.Second)
    go writer()
    time.Sleep(1 * time.Second)
}

func reader() {
    for {
        fmt.Println(data["a"])
    }
}

func writer() {
    for {
        data["a"] = 2
    }
}