diff --git a/neutron/plugins/ml2/plugin.py b/neutron/plugins/ml2/plugin.py
index bc1a1beb81a..887a88a9dbb 100644
--- a/neutron/plugins/ml2/plugin.py
+++ b/neutron/plugins/ml2/plugin.py
@@ -1059,11 +1059,11 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         session = context.session
         deallocated = set()
         while True:
+            # NOTE(kevinbenton): this loop keeps db objects in scope
+            # so we must expire them or risk stale reads.
+            # see bug/1623990
+            session.expire_all()
             with session.begin(subtransactions=True):
-                # NOTE(kevinbenton): this loop keeps db objects in scope
-                # so we must expire them or risk stale reads.
-                # see bug/1623990
-                session.expire_all()
                 record = self._get_subnet(context, id)
                 subnet = self._make_subnet_dict(record, None, context=context)
                 qry_allocated = (session.query(models_v2.IPAllocation).
@@ -1080,7 +1080,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
                     qry_allocated = (
                         qry_allocated.filter(models_v2.Port.device_owner.
                         in_(db_base_plugin_v2.AUTO_DELETE_PORT_OWNERS)))
-                allocated = set(ipal.port_id for ipal in qry_allocated)
+                allocated = set(qry_allocated.all())
                 LOG.debug("Ports to auto-deallocate: %s", allocated)
                 if not is_auto_addr_subnet:
                     user_alloc = self._subnet_get_user_allocation(
@@ -1143,20 +1143,14 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
                     LOG.debug("Committing transaction")
                     break
 
-            for port_id in to_deallocate:
-                try:
-                    port = self.get_port(context, port_id)
-                except exc.PortNotFound:
-                    LOG.debug("Port %s deleted concurrently", port_id)
-                    deallocated.add(port_id)
-                    continue
-                if port:
+            for a in to_deallocate:
+                if a.port:
                     # calling update_port() for each allocation to remove the
                     # IP from the port and call the MechanismDrivers
-                    fixed_ips = [{'subnet_id': ip['subnet_id'],
-                                  'ip_address': ip['ip_address']}
-                                 for ip in port['fixed_ips']
-                                 if ip['subnet_id'] != id]
+                    fixed_ips = [{'subnet_id': ip.subnet_id,
+                                  'ip_address': ip.ip_address}
+                                 for ip in a.port.fixed_ips
+                                 if ip.subnet_id != id]
                     # By default auto-addressed ips are not removed from port
                     # on port update, so mark subnet with 'delete_subnet' flag
                     # to force ip deallocation on port update.
@@ -1165,8 +1159,11 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
                                           'delete_subnet': True})
                     data = {attributes.PORT: {'fixed_ips': fixed_ips}}
                     try:
+                        # NOTE Don't inline port_id; needed for PortNotFound.
+                        port_id = a.port_id
                         self.update_port(context, port_id, data)
                     except exc.PortNotFound:
+                        # NOTE Attempting to access a.port_id here is an error.
                         LOG.debug("Port %s deleted concurrently", port_id)
                     except exc.SubnetNotFound:
                         # NOTE we hit here if another subnet was concurrently
@@ -1179,7 +1176,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
                             utils.attach_exc_details(
                                 e, _LE("Exception deleting fixed_ip from "
                                        "port %s"), port_id)
-                deallocated.add(port_id)
+                deallocated.add(a)
 
         kwargs = {'context': context, 'subnet': subnet}
         registry.notify(resources.SUBNET, events.AFTER_DELETE, self, **kwargs)
diff --git a/neutron/tests/unit/plugins/ml2/test_plugin.py b/neutron/tests/unit/plugins/ml2/test_plugin.py
index c1e6a4c2ba9..402e7c7349f 100644
--- a/neutron/tests/unit/plugins/ml2/test_plugin.py
+++ b/neutron/tests/unit/plugins/ml2/test_plugin.py
@@ -522,32 +522,6 @@ class TestMl2SubnetsV2(test_plugin.TestSubnetsV2,
             kwargs = after_create.mock_calls[0][2]
             self.assertEqual(s['subnet']['id'], kwargs['subnet']['id'])
 
-    def test_subnet_delete_no_DetachedInstanceError_for_port_update(self):
-        mock_check_subnet_not_used = mock.patch.object(
-            base_plugin, '_check_subnet_not_used').start()
-        with self.network() as n:
-            with self.subnet(network=n, cidr='1.1.1.0/24') as s1,\
-                 self.subnet(network=n, cidr='1.1.2.0/24') as s2:
-                fixed_ips = [{'subnet_id': s1['subnet']['id']},
-                             {'subnet_id': s2['subnet']['id']}]
-                with self.port(subnet=s1, fixed_ips=fixed_ips,
-                               device_owner=constants.DEVICE_OWNER_DHCP) as p:
-
-                    def subnet_not_used(ctx, subnet_id):
-                        # Make the session invalid after the query.
-                        ctx.session.expunge_all()
-                        mock_check_subnet_not_used.side_effect = None
-                    mock_check_subnet_not_used.side_effect = subnet_not_used
-                    req = self.new_delete_request('subnets',
-                                                  s1['subnet']['id'])
-                    res = req.get_response(self.api)
-                    self.assertEqual(204, res.status_int)
-                    # ensure port only has 1 IP on s2
-                    port = self._show('ports', p['port']['id'])['port']
-                    self.assertEqual(1, len(port['fixed_ips']))
-                    self.assertEqual(s2['subnet']['id'],
-                                     port['fixed_ips'][0]['subnet_id'])
-
     def test_port_update_subnetnotfound(self):
         with self.network() as n:
             with self.subnet(network=n, cidr='1.1.1.0/24') as s1,\