Remove db-access semaphore in ML2

With the addition of the deadlock retry decorator in
I635cc49ca69f589f99ab145d4d51e511b24194d2 and the change
to instantaneous deadlock feedback under the pymysql driver,
this semaphore should no longer be necessary. Removing it
will allow us to benefit from processing multiple requests
simultaneously.

Change-Id: If5dc76a2974d13d45f0cc88419bcccb3332576cf

neutron/plugins/ml2/plugin.py

@@ -14,7 +14,6 @@
 #    under the License.
 
 from eventlet import greenthread
-from oslo_concurrency import lockutils
 from oslo_config import cfg
 from oslo_db import api as oslo_db_api
 from oslo_db import exception as os_db_exception
@@ -344,13 +343,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         # After we've attempted to bind the port, we begin a
         # transaction, get the current port state, and decide whether
         # to commit the binding results.
-        #
-        # REVISIT: Serialize this operation with a semaphore to
-        # prevent deadlock waiting to acquire a DB lock held by
-        # another thread in the same process, leading to 'lock wait
-        # timeout' errors.
-        with lockutils.lock('db-access'),\
-                session.begin(subtransactions=True):
+        with session.begin(subtransactions=True):
             # Get the current port state and build a new PortContext
             # reflecting this state as original state for subsequent
             # mechanism driver update_port_*commit() calls.
@@ -729,15 +722,14 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
                 # to 'lock wait timeout' errors.
                 #
                 # Process L3 first, since, depending on the L3 plugin, it may
-                # involve locking the db-access semaphore, sending RPC
-                # notifications, and/or calling delete_port on this plugin.
+                # involve sending RPC notifications, and/or calling delete_port
+                # on this plugin.
                 # Additionally, a rollback may not be enough to undo the
                 # deletion of a floating IP with certain L3 backends.
                 self._process_l3_delete(context, id)
                 # Using query().with_lockmode isn't necessary. Foreign-key
                 # constraints prevent deletion if concurrent creation happens.
-                with lockutils.lock('db-access'),\
-                        session.begin(subtransactions=True):
+                with session.begin(subtransactions=True):
                     # Get ports to auto-delete.
                     ports = (session.query(models_v2.Port).
                              enable_eagerloads(False).
@@ -852,12 +844,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         LOG.debug("Deleting subnet %s", id)
         session = context.session
         while True:
-            # REVISIT: Serialize this operation with a semaphore to
-            # prevent deadlock waiting to acquire a DB lock held by
-            # another thread in the same process, leading to 'lock
-            # wait timeout' errors.
-            with lockutils.lock('db-access'),\
-                    session.begin(subtransactions=True):
+            with session.begin(subtransactions=True):
                 record = self._get_subnet(context, id)
                 subnet = self._make_subnet_dict(record, None)
                 qry_allocated = (session.query(models_v2.IPAllocation).
@@ -1102,12 +1089,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         need_port_update_notify = False
         session = context.session
 
-        # REVISIT: Serialize this operation with a semaphore to
-        # prevent deadlock waiting to acquire a DB lock held by
-        # another thread in the same process, leading to 'lock wait
-        # timeout' errors.
-        with lockutils.lock('db-access'),\
-                session.begin(subtransactions=True):
+        with session.begin(subtransactions=True):
             port_db, binding = db.get_locked_port_and_binding(session, id)
             if not port_db:
                 raise exc.PortNotFound(port_id=id)
@@ -1258,12 +1240,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
             l3plugin, const.L3_DISTRIBUTED_EXT_ALIAS)
 
         session = context.session
-        # REVISIT: Serialize this operation with a semaphore to
-        # prevent deadlock waiting to acquire a DB lock held by
-        # another thread in the same process, leading to 'lock wait
-        # timeout' errors.
-        with lockutils.lock('db-access'),\
-                session.begin(subtransactions=True):
+        with session.begin(subtransactions=True):
             port_db, binding = db.get_locked_port_and_binding(session, id)
             if not port_db:
                 LOG.debug("The port '%s' was deleted", id)
@@ -1392,12 +1369,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         """
         updated = False
         session = context.session
-        # REVISIT: Serialize this operation with a semaphore to
-        # prevent deadlock waiting to acquire a DB lock held by
-        # another thread in the same process, leading to 'lock wait
-        # timeout' errors.
-        with lockutils.lock('db-access'),\
-                session.begin(subtransactions=True):
+        with session.begin(subtransactions=True):
             port = db.get_port(session, port_id)
             if not port:
                 LOG.debug("Port %(port)s update to %(val)s by agent not found",
@@ -1428,8 +1400,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
 
         if (updated and
             port['device_owner'] == const.DEVICE_OWNER_DVR_INTERFACE):
-            with lockutils.lock('db-access'),\
-                    session.begin(subtransactions=True):
+            with session.begin(subtransactions=True):
                 port = db.get_port(session, port_id)
                 if not port:
                     LOG.warning(_LW("Port %s not found during update"),