查看status如下：

    1. [root@localhost ~]# systemctl status firewalld.service -l
    2. [0m firewalld.service - firewalld - dynamic firewall daemon
    3.   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
    4.   Active: failed (Result: timeout) since Wed 2017-07-26 10:12:43 CST; 54min ago
    5.     Docs: man:firewalld(1)
    6.  Process: 152761 ExecStart=/usr/sbin/firewalld --nofork --nopid $FIREWALLD_ARGS (code=exited, status=0/SUCCESS)
    7.  Main PID: 152761 (code=exited, status=0/SUCCESS)
    8. Jul 26 10:11:12 localhost.localdomain systemd[1]: Starting firewalld - dynamic firewall daemon...
    9. Jul 26 10:12:42 localhost.localdomain systemd[1]: firewalld.service start operation timed out. Terminating.
    10. Jul 26 10:12:43 localhost.localdomain systemd[1]: Failed to start firewalld - dynamic firewall daemon.
    11. Jul 26 10:12:43 localhost.localdomain systemd[1]: Unit firewalld.service entered failed state.
    12. Jul 26 10:12:43 localhost.localdomain systemd[1]: firewalld.service failed.

    执行以下命令后，恢复running：

    1. systemctl stop firewalld
    2. pkill -f firewalld
    3. systemctl start firewalld

    该方法参考来源：

    http://centosfaq.org/centos/centos-7-firewalldservice-operation-time-out-systemctl-firewalld-issues/

    来自官方的faq,给了一些解释： systemd didn’t know about the process that it didn’t start in the first place of course