Why:

main_atomic.go

package main

import (
	"sync"
	"time"
	"fmt"
)

type mutexCounter struct {
	mu sync.Mutex
	x int64
}
func (c *mutexCounter) Add(x int64) {
	c.mu.Lock()
	c.x += x
	c.mu.Unlock()
}
func (c *mutexCounter) Value() (x int64) {
	c.mu.Lock()
	x = c.x
	c.mu.Unlock()
	return
}
func main() {
	counter := mutexCounter{}
	for i := 0; i < 100; i++ {
		go func(no int) {
			for i := 0; i < 10000; i++ {
				counter.Add(1)
			}
		}(i)
	}
	time.Sleep(time.Second)
	fmt.Println(counter.Value())
}