double_check_unsafe_test.go

func Test_double_check_unsafe(t *testing.T) {
	m := make(map[int]int64)
	mu := sync.Mutex{}
	wg := sync.WaitGroup{}
	wg.Add(100 * 10)
	for i := 0; i < 100; i++ {
		for j := 0; j < 10; j++ {
			go func(j int) {
				defer wg.Done()
				v := m[j]
				if v != 0 {
					return
				}
				mu.Lock()
				defer mu.Unlock()
				mm := make(map[int]int64)
				mm[j] = 1
				m = mm
			}(1)
		}
	}
	wg.Wait()
}

WARNING: DATA RACE

and why：
https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
http://erdani.com/publications/DDJ_Jul_Aug_2004_revised.pdf