diff --git a/swh/provenance/postgresql/archive.py b/swh/provenance/postgresql/archive.py
index 72734d4..e830363 100644
--- a/swh/provenance/postgresql/archive.py
+++ b/swh/provenance/postgresql/archive.py
@@ -1,73 +1,78 @@
 import psycopg2
+import threading
 
 from ..archive import ArchiveInterface
 
 # from functools import lru_cache
 from methodtools import lru_cache
 from typing import List
 
 
 class ArchivePostgreSQL(ArchiveInterface):
     def __init__(self, conn: psycopg2.extensions.connection):
         self.conn = conn
         self.cursor = conn.cursor()
+        self.mutex = threading.Lock()
 
     @lru_cache(maxsize=1000000)
     def directory_ls(self, id: bytes):
+        self.mutex.acquire()
         self.cursor.execute(
             """WITH
             dir  AS (SELECT id AS dir_id, dir_entries, file_entries, rev_entries
                         FROM directory WHERE id=%s),
             ls_d AS (SELECT dir_id, unnest(dir_entries) AS entry_id from dir),
             ls_f AS (SELECT dir_id, unnest(file_entries) AS entry_id from dir),
             ls_r AS (SELECT dir_id, unnest(rev_entries) AS entry_id from dir)
             (SELECT 'dir'::directory_entry_type AS type, e.target, e.name,
                     NULL::sha1_git
                 FROM ls_d
                 LEFT JOIN directory_entry_dir e ON ls_d.entry_id=e.id)
             UNION
             (WITH known_contents AS
                 (SELECT 'file'::directory_entry_type AS type, e.target, e.name,
                         c.sha1_git
                     FROM ls_f
                     LEFT JOIN directory_entry_file e ON ls_f.entry_id=e.id
                     INNER JOIN content c ON e.target=c.sha1_git)
                 SELECT * FROM known_contents
                 UNION
                 (SELECT 'file'::directory_entry_type AS type, e.target, e.name,
                         c.sha1_git
                     FROM ls_f
                     LEFT JOIN directory_entry_file e ON ls_f.entry_id=e.id
                     LEFT JOIN skipped_content c ON e.target=c.sha1_git
                     WHERE NOT EXISTS (
                         SELECT 1 FROM known_contents
                             WHERE known_contents.sha1_git=e.target
                     )
                 )
             )
             ORDER BY name
             """,
             (id,),
         )
-        return [
+        entries = [
             {"type": row[0], "target": row[1], "name": row[2]}
             for row in self.cursor.fetchall()
         ]
+        self.mutex.release()
+        return entries
 
     def iter_origins(self):
         raise NotImplementedError
 
     def iter_origin_visits(self, origin: str):
         raise NotImplementedError
 
     def iter_origin_visit_statuses(self, origin: str, visit: int):
         raise NotImplementedError
 
     def release_get(self, ids: List[bytes]):
         raise NotImplementedError
 
     def revision_get(self, ids: List[bytes]):
         raise NotImplementedError
 
     def snapshot_get_all_branches(self, snapshot: bytes):
         raise NotImplementedError