diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 874d41447a..6017c2a6e8 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -466,6 +466,9 @@ class SQLBaseStore(object):
         )
 
     def _simple_upsert_txn(self, txn, table, keyvalues, values, insertion_values={}):
+        # We need to lock the table :(
+        self.database_engine.lock_table(txn, table)
+
         # Try to update
         sql = "UPDATE %s SET %s WHERE %s" % (
             table,
diff --git a/synapse/storage/engines/postgres.py b/synapse/storage/engines/postgres.py
index 00dbae7b60..b8cca9b187 100644
--- a/synapse/storage/engines/postgres.py
+++ b/synapse/storage/engines/postgres.py
@@ -42,3 +42,6 @@ class PostgresEngine(object):
 
     def is_connection_closed(self, conn):
         return bool(conn)
+
+    def lock_table(self, txn, table):
+        txn.execute("LOCK TABLE %s in EXCLUSIVE MODE" % (table,))
diff --git a/synapse/storage/engines/sqlite3.py b/synapse/storage/engines/sqlite3.py
index 39828a597c..f62d5d1205 100644
--- a/synapse/storage/engines/sqlite3.py
+++ b/synapse/storage/engines/sqlite3.py
@@ -38,3 +38,6 @@ class Sqlite3Engine(object):
 
     def is_connection_closed(self, conn):
         return False
+
+    def lock_table(self, txn, table):
+        return
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 7dbf7a396a..a3c260ddc4 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -283,50 +283,6 @@ class EventsStore(SQLBaseStore):
         if context.rejected:
             self._store_rejections_txn(txn, event.event_id, context.rejected)
 
-        if event.is_state():
-            vals = {
-                "event_id": event.event_id,
-                "room_id": event.room_id,
-                "type": event.type,
-                "state_key": event.state_key,
-            }
-
-            # TODO: How does this work with backfilling?
-            if hasattr(event, "replaces_state"):
-                vals["prev_state"] = event.replaces_state
-
-            self._simple_insert_txn(
-                txn,
-                "state_events",
-                vals,
-            )
-
-            if is_new_state and not context.rejected:
-                self._simple_upsert_txn(
-                    txn,
-                    "current_state_events",
-                    keyvalues={
-                        "room_id": event.room_id,
-                        "type": event.type,
-                        "state_key": event.state_key,
-                    },
-                    values={
-                        "event_id": event.event_id,
-                    }
-                )
-
-            for e_id, h in event.prev_state:
-                self._simple_insert_txn(
-                    txn,
-                    table="event_edges",
-                    values={
-                        "event_id": event.event_id,
-                        "prev_event_id": e_id,
-                        "room_id": event.room_id,
-                        "is_state": True,
-                    },
-                )
-
         for hash_alg, hash_base64 in event.hashes.items():
             hash_bytes = decode_base64(hash_base64)
             self._store_event_content_hash_txn(
@@ -356,6 +312,50 @@ class EventsStore(SQLBaseStore):
             txn, event.event_id, ref_alg, ref_hash_bytes
         )
 
+        if event.is_state():
+            vals = {
+                "event_id": event.event_id,
+                "room_id": event.room_id,
+                "type": event.type,
+                "state_key": event.state_key,
+            }
+
+            # TODO: How does this work with backfilling?
+            if hasattr(event, "replaces_state"):
+                vals["prev_state"] = event.replaces_state
+
+            self._simple_insert_txn(
+                txn,
+                "state_events",
+                vals,
+            )
+
+            for e_id, h in event.prev_state:
+                self._simple_insert_txn(
+                    txn,
+                    table="event_edges",
+                    values={
+                        "event_id": event.event_id,
+                        "prev_event_id": e_id,
+                        "room_id": event.room_id,
+                        "is_state": True,
+                    },
+                )
+
+            if is_new_state and not context.rejected:
+                self._simple_upsert_txn(
+                    txn,
+                    "current_state_events",
+                    keyvalues={
+                        "room_id": event.room_id,
+                        "type": event.type,
+                        "state_key": event.state_key,
+                    },
+                    values={
+                        "event_id": event.event_id,
+                    }
+                )
+
     def _store_redaction(self, txn, event):
         # invalidate the cache for the redacted event
         self._invalidate_get_event_cache(event.redacts)
diff --git a/synapse/storage/transactions.py b/synapse/storage/transactions.py
index 7e3add5280..89dd7d8947 100644
--- a/synapse/storage/transactions.py
+++ b/synapse/storage/transactions.py
@@ -76,25 +76,16 @@ class TransactionStore(SQLBaseStore):
             response_json (str)
         """
 
-        return self.runInteraction(
-            "set_received_txn_response",
-            self._set_received_txn_response,
-            transaction_id, origin, code, response_dict
-        )
-
-    def _set_received_txn_response(self, txn, transaction_id, origin, code,
-                                   response_json):
-        self._simple_upsert_txn(
-            txn,
+        return self._simple_insert(
             table=ReceivedTransactionsTable.table_name,
-            keyvalues={
+            values={
                 "transaction_id": transaction_id,
                 "origin": origin,
-            },
-            values={
                 "response_code": code,
-                "response_json": response_json,
-            }
+                "response_json": response_dict,
+            },
+            or_ignore=True,
+            desc="set_received_txn_response",
         )
 
     def prep_send_transaction(self, transaction_id, destination,