Existing lock /var/run/yum.pid: another copy is running as pid XXXX. Another app is currently holding the yum lock; waiting for it to exit..

When you are searching / installing the RPM and if you ever come across the below exception, 

[root@dhcp-ptp2-10-177-154-84 yum.repos.d]# yum search libaio
Loaded plugins: langpacks
Existing lock /var/run/yum.pid: another copy is running as pid 3339.
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  25 M RSS (429 MB VSZ)
    Started: Wed Jul  6 14:52:53 2016 - 02:30 ago
    State  : Sleeping, pid: 3339
Another app is currently holding the yum lock; waiting for it to 

Follow the below steps.

cd /var/run/ and you could see a file "yum.pid". Just delete it and try again. Now you can seach / install the RPM.