centos7 安装docker后，无法启动。

环境信息：

[root@joinApp2 ~]# uname -r
3.10.0-123.9.3.el7.x86_64
[root@joinApp2 ~]# cat /etc/redhat-release
CentOS Linux release 7.2.1511 (Core）

症状：

[root@joinApp2 ~]# systemctl start docker.service
Job for docker.service failed because the control process exited with error code. See "systemctl status docker.service" and "journalctl -xe" for details.
[root@joinApp2 ~]#
[root@joinApp2 ~]#
[root@joinApp2 ~]#
[root@joinApp2 ~]# systemctl status docker.service
● docker.service - Docker Application Container Engine
Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled; vendor preset: disabled)
Active: failed (Result: start-limit) since Thu 2016-02-25 17:26:11 CST; 16s ago

 Docs: http://docs.docker.com

Process: 16384 ExecStart=/usr/bin/docker daemon $OPTIONS $DOCKER_STORAGE_OPTIONS $DOCKER_NETWORK_OPTIONS $ADD_REGISTRY $BLOCK_REGISTRY $INSECURE_REGISTRY (code=exited, status=1/FAILURE)
Main PID: 16384 (code=exited, status=1/FAILURE)

Feb 25 17:26:10 joinApp2 systemd[1]: Failed to start Docker Application Container Engine.
Feb 25 17:26:10 joinApp2 systemd[1]: Unit docker.service entered failed state.
Feb 25 17:26:10 joinApp2 systemd[1]: docker.service failed.
Feb 25 17:26:11 joinApp2 systemd[1]: docker.service holdoff time over, scheduling restart.
Feb 25 17:26:11 joinApp2 systemd[1]: start request repeated too quickly for docker.service
Feb 25 17:26:11 joinApp2 systemd[1]: Failed to start Docker Application Container Engine.
Feb 25 17:26:11 joinApp2 systemd[1]: Unit docker.service entered failed state.
Feb 25 17:26:11 joinApp2 systemd[1]: docker.service failed.