在[mysqld]后面增加一行

log=/tmp/mysql.log

是用于给mysql写日志，重载配置之后，再重启mysql,却报错：
MySQL server PID file could not be found![失败]
Starting MySQL.The server quit without updating PID file (/var/lib/mysql/52jiagou.com.pid).[失败]

查看日志文件：
2014-06-26 17:22:59 0 [ERROR] /usr/sbin/mysqld: ambiguous option '--log=/var/lib/mysql/mysql.log' (log-bin, log_slave_updates)
2014-06-26 17:22:59 0 [ERROR] Aborting

查了google，解决方案是目前log参数弃用，改用

general_log=ON
general_log_file=/tmp/mysql.log