[root@localhost ~]# yum install php
Loading "installonlyn" plugin
Existing lock /var/run/yum.pid: another copy is running as pid 29802.
Another app is currently holding the yum lock; waiting for it to exit...
Another app is currently holding the yum lock; waiting for it to exit...
Another app is currently holding the yum lock; waiting for it to exit...
Another app is currently holding the yum lock; waiting for it to exit...
Another app is currently holding the yum lock; waiting for it to exit...
Another app is currently holding the yum lock; waiting for it to exit...
Another app is currently holding the yum lock; waiting for it to exit...

となるのですが、なぜエラーになるのでしょうか