volatile关键字

转载：https://www.bilibili.com/video/BV1514y1G7wA/?spm_id_from=trigger_reload&vd_source=46d50b5d646b50dcb2a208d3946b1598
https://www.bilibili.com/video/BV1x3411B7GE/?spm_id_from=trigger_reload&vd_source=46d50b5d646b50dcb2a208d3946b1598