MYSQL dead but pid file exists问题

服务器突然断电，造成MySQL数据库启动不了的问题，现象如下：

[root@localhost html]# service mysql51-mysqld start
kill 31011: No such process
Another MySQL daemon already running with the same unix socket.
Starting mysql51-mysqld:                                   [FAILED]

[root@localhost html]# service mysql51-mysqld status
mysql51-mysqld dead but pid file exists

[root@localhost html]# mv /var/lib/mysql/mysql.sock /var/lib/mysql/mysql.sock.back
[root@localhost html]# service mysql51-mysqld start
kill 31011: No such process
Starting mysql51-mysqld:                                   [  OK  ]