https://rein.kr/posts/2011-06-13-%ED%94%84%EB%A1%9C%EA%B7%B8%EB%9E%98%EB%A8%B8%EC%9D%98-%EC%9D%BC%EC%83%81-race-condition-%EA%B8%80%EC%93%B0%EA%B8%B0/