原因：
操作系统需要时间来kill进程
On some platforms it takes some time to kill processes--AIX being one of those platforms
There have been previous reports of shutdown taking time on AIX and after all Oracle waits were taken out of the picture it was determined to be due to the way the kill command is implemented on that platform.
Bug 4931101 ERRORS IN ALERT LOG DURING SHUTDOWN
解决方案：
Solution
无视这个错误，进程最终会被关闭，数据库shutdown也可以成功完成。
Ignore the error as all processes will be closed and shutdown will complete successfully.