yum出现Existing lock /var/run/yum.pid: another copy is running as pid 3358

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Existing lock /var/run/yum.pid: another copy is running as pid 3358.

Another app is currently holding the yum lock; waiting for it to exit...

  The other application is: PackageKit

    Memory :  50 M RSS (365 MB VSZ)

    Started: Fri Jun 27 09:46:17 2014 - 13 day(s) 1:45:42 ago

    State  : Sleeping, pid: 3358

可能是系统自动升级正在运行，yum在锁定状态中。
可以通过强制关掉yum进程：
#rm -f /var/run/yum.pid
然后就可以使用yum了