centos 解决：Another app is currently holding the yum lock; waiting for it to exit

centos执行yum时出现错误：

Loaded plugins: fastestmirror, refresh-packagekit, security
Existing lock /var/run/yum.pid: another copy is running as pid 3174.
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  28 M RSS (341 MB VSZ)
    Started: Wed Apr 19 02:38:44 2017 - 00:21 ago
    State  : Sleeping, pid: 3174
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  28 M RSS (341 MB VSZ)
    Started: Wed Apr 19 02:38:44 2017 - 00:23 ago
    State  : Sleeping, pid: 3174
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: PackageKit
    Memory :  28 M RSS (341 MB VSZ)
    Started: Wed Apr 19 02:38:44 2017 - 00:25 ago
    State  : Sleeping, pid: 3174

解决： rm -fr /var/run/yum.pid