diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 878809d50..ad2753c1b 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -26,7 +26,7 @@ from synapse.types import (
     UserID, RoomAlias, RoomStreamToken, StreamToken, get_domain_from_id
 )
 from synapse.util import unwrapFirstError
-from synapse.util.async import concurrently_execute, run_on_reactor
+from synapse.util.async import concurrently_execute, run_on_reactor, ReadWriteLock
 from synapse.util.caches.snapshot_cache import SnapshotCache
 from synapse.util.logcontext import preserve_fn
 from synapse.visibility import filter_events_for_client
@@ -50,6 +50,8 @@ class MessageHandler(BaseHandler):
         self.validator = EventValidator()
         self.snapshot_cache = SnapshotCache()
 
+        self.pagination_lock = ReadWriteLock()
+
     @defer.inlineCallbacks
     def purge_history(self, room_id, event_id):
         event = yield self.store.get_event(event_id)
@@ -59,9 +61,8 @@ class MessageHandler(BaseHandler):
 
         depth = event.depth
 
-        # TODO: Lock.
-
-        yield self.store.delete_old_state(room_id, depth)
+        with (yield self.pagination_lock.write(room_id)):
+            yield self.store.delete_old_state(room_id, depth)
 
     @defer.inlineCallbacks
     def get_messages(self, requester, room_id=None, pagin_config=None,
@@ -98,42 +99,43 @@ class MessageHandler(BaseHandler):
 
         source_config = pagin_config.get_source_config("room")
 
-        membership, member_event_id = yield self._check_in_room_or_world_readable(
-            room_id, user_id
-        )
-
-        if source_config.direction == 'b':
-            # if we're going backwards, we might need to backfill. This
-            # requires that we have a topo token.
-            if room_token.topological:
-                max_topo = room_token.topological
-            else:
-                max_topo = yield self.store.get_max_topological_token_for_stream_and_room(
-                    room_id, room_token.stream
-                )
-
-            if membership == Membership.LEAVE:
-                # If they have left the room then clamp the token to be before
-                # they left the room, to save the effort of loading from the
-                # database.
-                leave_token = yield self.store.get_topological_token_for_event(
-                    member_event_id
-                )
-                leave_token = RoomStreamToken.parse(leave_token)
-                if leave_token.topological < max_topo:
-                    source_config.from_key = str(leave_token)
-
-            yield self.hs.get_handlers().federation_handler.maybe_backfill(
-                room_id, max_topo
+        with (yield self.pagination_lock.read(room_id)):
+            membership, member_event_id = yield self._check_in_room_or_world_readable(
+                room_id, user_id
             )
 
-        events, next_key = yield data_source.get_pagination_rows(
-            requester.user, source_config, room_id
-        )
+            if source_config.direction == 'b':
+                # if we're going backwards, we might need to backfill. This
+                # requires that we have a topo token.
+                if room_token.topological:
+                    max_topo = room_token.topological
+                else:
+                    max_topo = yield self.store.get_max_topological_token(
+                        room_id, room_token.stream
+                    )
 
-        next_token = pagin_config.from_token.copy_and_replace(
-            "room_key", next_key
-        )
+                if membership == Membership.LEAVE:
+                    # If they have left the room then clamp the token to be before
+                    # they left the room, to save the effort of loading from the
+                    # database.
+                    leave_token = yield self.store.get_topological_token_for_event(
+                        member_event_id
+                    )
+                    leave_token = RoomStreamToken.parse(leave_token)
+                    if leave_token.topological < max_topo:
+                        source_config.from_key = str(leave_token)
+
+                yield self.hs.get_handlers().federation_handler.maybe_backfill(
+                    room_id, max_topo
+                )
+
+            events, next_key = yield data_source.get_pagination_rows(
+                requester.user, source_config, room_id
+            )
+
+            next_token = pagin_config.from_token.copy_and_replace(
+                "room_key", next_key
+            )
 
         if not events:
             defer.returnValue({
diff --git a/synapse/storage/stream.py b/synapse/storage/stream.py
index b9ad965fd..3dda2dab5 100644
--- a/synapse/storage/stream.py
+++ b/synapse/storage/stream.py
@@ -487,13 +487,13 @@ class StreamStore(SQLBaseStore):
             row["topological_ordering"], row["stream_ordering"],)
         )
 
-    def get_max_topological_token_for_stream_and_room(self, room_id, stream_key):
+    def get_max_topological_token(self, room_id, stream_key):
         sql = (
             "SELECT max(topological_ordering) FROM events"
             " WHERE room_id = ? AND stream_ordering < ?"
         )
         return self._execute(
-            "get_max_topological_token_for_stream_and_room", None,
+            "get_max_topological_token", None,
             sql, room_id, stream_key,
         ).addCallback(
             lambda r: r[0][0] if r else 0