[Ceph] Failed to delete a volume with its associated snapshot

Description of problem:
I have created a volume and snapshot. I have tried to delete the volume with its snapshot by
using 'cascade' argument. And indeed, both volume and snapshot were deleted.
But when I have repeated on this scenario while there is another volume from
that snapshot, the deletion process failed.
The deletion failure happens only in ceph driver.

Version-Release number of selected component (if applicable):
python-cinder-9.1.1-3.el7ost.noarch
openstack-cinder-9.1.1-3.el7ost.noarch
python-cinderclient-1.9.0-4.el7ost.noarch
puppet-cinder-9.4.1-3.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:

1. Create a volume.

[stack@undercloud-0 ~]$ cinder create 1
+--------------------------------+--------------------------------------+
| Property | Value |
+--------------------------------+--------------------------------------+
| attachments | [] |
| availability_zone | nova |
| bootable | false |
| consistencygroup_id | None |
| created_at | 2017-03-30T08:10:20.000000 |
| description | None |
| encrypted | False |
| id | a0d485d5-f893-43f5-8a40-e1e762482c11 |
| metadata | {} |
| migration_status | None |
| multiattach | False |
| name | None |
| os-vol-host-attr:host | hostgroup@tripleo_ceph#tripleo_ceph |
| os-vol-mig-status-attr:migstat | None |
| os-vol-mig-status-attr:name_id | None |
| os-vol-tenant-attr:tenant_id | 7f554817cae44d38a697954b360ed350 |
| replication_status | disabled |
| size | 1 |
| snapshot_id | None |
| source_volid | None |
| status | available |
| updated_at | 2017-03-30T08:10:20.000000 |
| user_id | 59007ad16aca4714888a74a3b63dde57 |
| volume_type | None |
+--------------------------------+--------------------------------------+

2. Create a snapshot from that volume.

[stack@undercloud-0 ~]$ cinder snapshot-create a0d485d5-f893-43f5-8a40-e1e762482c11
+-------------+--------------------------------------+
| Property | Value |
+-------------+--------------------------------------+
| created_at | 2017-03-30T08:10:31.767236 |
| description | None |
| id | ffe07fae-ba40-48a5-b14c-3c5c51894ff2 |
| metadata | {} |
| name | None |
| size | 1 |
| status | creating |
| updated_at | None |
| volume_id | a0d485d5-f893-43f5-8a40-e1e762482c11 |
+-------------+--------------------------------------+

3. Create a volume from that snapshot.

[stack@undercloud-0 ~]$ cinder create --snapshot-id ffe07fae-ba40-48a5-b14c-3c5c51894ff2
+--------------------------------+--------------------------------------+
| Property | Value |
+--------------------------------+--------------------------------------+
| attachments | [] |
| availability_zone | nova |
| bootable | false |
| consistencygroup_id | None |
| created_at | 2017-03-30T08:10:58.000000 |
| description | None |
| encrypted | False |
| id | 7156d8fd-7434-48d4-a926-b64bef121f27 |
| metadata | {} |
| migration_status | None |
| multiattach | False |
| name | None |
| os-vol-host-attr:host | hostgroup@tripleo_ceph#tripleo_ceph |
| os-vol-mig-status-attr:migstat | None |
| os-vol-mig-status-attr:name_id | None |
| os-vol-tenant-attr:tenant_id | 7f554817cae44d38a697954b360ed350 |
| replication_status | disabled |
| size | 1 |
| snapshot_id | ffe07fae-ba40-48a5-b14c-3c5c51894ff2 |
| source_volid | None |
| status | available |
| updated_at | 2017-03-30T08:10:59.000000 |
| user_id | 59007ad16aca4714888a74a3b63dde57 |
| volume_type | None |
+--------------------------------+--------------------------------------+

[stack@undercloud-0 ~]$ cinder list
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
| ID | Status | Name | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
| 7156d8fd-7434-48d4-a926-b64bef121f27 | available | - | 1 | - | false | |
| a0d485d5-f893-43f5-8a40-e1e762482c11 | available | - | 1 | - | false | |
+--------------------------------------+-----------+------+------+-------------+----------+-------------+

[stack@undercloud-0 ~]$ cinder snapshot-list
+--------------------------------------+--------------------------------------+-----------+------+------+
| ID | Volume ID | Status | Name | Size |
+--------------------------------------+--------------------------------------+-----------+------+------+
| ffe07fae-ba40-48a5-b14c-3c5c51894ff2 | a0d485d5-f893-43f5-8a40-e1e762482c11 | available | - | 1 |
+--------------------------------------+--------------------------------------+-----------+------+------+

4. Try delete the volume with its associated snapshot.

[stack@undercloud-0 ~]$ cinder delete a0d485d5-f893-43f5-8a40-e1e762482c11 --cascade
Request to delete volume a0d485d5-f893-43f5-8a40-e1e762482c11 has been accepted.

[stack@undercloud-0 ~]$ cinder list
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
| ID | Status | Name | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
| 7156d8fd-7434-48d4-a926-b64bef121f27 | available | - | 1 | - | false | |
| a0d485d5-f893-43f5-8a40-e1e762482c11 | available | - | 1 | - | false | |
+--------------------------------------+-----------+------+------+-------------+----------+-------------+

[stack@undercloud-0 ~]$ cinder snapshot-list
+--------------------------------------+--------------------------------------+-----------+------+------+
| ID | Volume ID | Status | Name | Size |
+--------------------------------------+--------------------------------------+-----------+------+------+
| ffe07fae-ba40-48a5-b14c-3c5c51894ff2 | a0d485d5-f893-43f5-8a40-e1e762482c11 | available | - | 1 |
+--------------------------------------+--------------------------------------+-----------+------+------+

Actual results:
Failed to delete a volume with its associated snapshot when there is another volume from that snapshot.

Expected results:
Both volume and snapshot should be deleted when there is another volume from that snapshot.

Additional info:

from /cinder/volume.log:
2017-03-30 08:50:23.178 7901 ERROR cinder.volume.manager [req-4e083fa9-ce06-4881-8f0f-df06f9f3f267 59007ad16aca4714888a74a3b63dde57 7f554817cae44d38a697954b360ed350 - default default] Delete snapshot failed, due to snapshot busy.
2017-03-30 08:50:23.244 7901 ERROR cinder.volume.manager [req-4e083fa9-ce06-4881-8f0f-df06f9f3f267 59007ad16aca4714888a74a3b63dde57 7f554817cae44d38a697954b360ed350 - default default] Unable to delete busy volume.