出现的状况如下：
[root@iZwz951sp834mvbed8gdzzZ ~]# yum update kernel
Loaded plugins: fastestmirror
Existing lock /var/run/yum.pid: another copy is running as pid 27773.
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: yum
Memory : 83 M RSS (474 MB VSZ)
Started: Tue Jan 2 11:45:47 2018 - 06:25 ago
State : Sleeping, pid: 27773
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: yum
Memory : 83 M RSS (474 MB VSZ)
Started: Tue Jan 2 11:45:47 2018 - 06:27 ago
State : Sleeping, pid: 27773
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: yum
Memory : 83 M RSS (474 MB VSZ)
Started: Tue Jan 2 11:45:47 2018 - 06:29 ago
说明：这是yum被锁住了
解决方法：
rm -rf /var/run/yum.pid 来强行解除锁定，然后重新进行yum就可以了。