Stack.sh stopped at umount ceph image unexpected

[Allen Zhao]

I followed the install procedure of devstack and ceph plugin, but it was stopped unexpected with following error:

2017-05-07 14:43:51.503 | ++/opt/stack/devstack-plugin-ceph/devstack/lib/ceph:configure_ceph:366 initial_configure_ceph
2017-05-07 14:43:51.510 | ++/opt/stack/devstack-plugin-ceph/devstack/lib/ceph:initial_configure_ceph:352 create_disk /var/lib/ceph/drives/images/ceph.img /var/lib/ceph 10250M
2017-05-07 14:43:51.517 | ++functions:create_disk:654 local node_number
2017-05-07 14:43:51.524 | ++functions:create_disk:655 local disk_image=/var/lib/ceph/drives/images/ceph.img
2017-05-07 14:43:51.530 | ++functions:create_disk:656 local storage_data_dir=/var/lib/ceph
2017-05-07 14:43:51.537 | ++functions:create_disk:657 local loopback_disk_size=10250M
2017-05-07 14:43:51.543 | ++functions:create_disk:660 [[ -e /var/lib/ceph/drives/images/ceph.img ]]
2017-05-07 14:43:51.550 | ++functions:create_disk:661 egrep -q /var/lib/ceph /proc/mounts
2017-05-07 14:43:51.559 | ++functions:create_disk:662 sudo umount /var/lib/ceph/drives/sdb1
2017-05-07 14:43:51.566 | umount: /var/lib/ceph/drives/sdb1: mountpoint not found
2017-05-07 14:43:51.574 | +functions:create_disk:1 exit_trap
2017-05-07 14:43:51.579 | +./stack.sh:exit_trap:492 local r=32

the system is Ubuntu server 16.04, and the system information:
stack@devstack:~/devstack$ uname -a
Linux devstack 4.4.0-78-generic #99-Ubuntu SMP Thu Apr 27 15:29:09 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

stack@devstack:~/devstack$ df -k
Filesystem 1K-blocks Used Available Use% Mounted on
udev 4058580 0 4058580 0% /dev
tmpfs 815752 9200 806552 2% /run
/dev/mapper/devstack--vg-root 32368528 3664324 27036928 12% /
tmpfs 4078740 0 4078740 0% /dev/shm
tmpfs 5120 0 5120 0% /run/lock
tmpfs 4078740 0 4078740 0% /sys/fs/cgroup
/dev/sda1 482922 107827 350161 24% /boot
tmpfs 815752 0 815752 0% /run/user/1000
/dev/loop0 10485760 43860 10441900 1% /var/lib/ceph

stack@devstack:~/devstack$ ps axw |grep ceph
 16444 ? Ssl 0:00 /usr/bin/ceph-mon -f --cluster ceph --id devstack --setuserceph --setgroup ceph
 60005 pts/0 S+ 0:00 grep --color=auto ceph