Another app is currently holding the yum lock; waiting for it to exit Error Log [CentOS Linux]

Sometime, I can not use yum. When I type yum command, I get following response.

Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  23 M RSS (332 MB VSZ)
    Started: Thu Feb 11 15:10:08 2016 - 00:22 ago
    State  : Sleeping, pid: 31981

Here is the troubleshooting log.
First, grep yum and kill yum PID.

[root@root ~]# ps -ef | grep yum
root     30594 28617  0 14:54 pts/1    00:00:00 /usr/bin/python /usr/bin/yum check-update
root     30637 28801  0 14:55 pts/3    00:00:00 grep yum
[root@root ~]# kill -9 30594

Second, delete /var/run/yum.pid

[root@root ~]# cd /var/run/
[root@root run]# rm yum.pid
rm: remove regular file `yum.pid'? yes

This is because the yum is already running, but You trying to run the yum again.

I recommend waiting for the normal process ends.