diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -191,6 +191,14 @@
             db.copy_to(content_with_data, 'tmp_content',
                        db.content_add_keys, cur)
 
+            # Create a read/write dependency between transactions that would
+            # write the same content, so that we get a SerializationFailure
+            # (read/write conflict) instead of an IntegrityError (write/write
+            # conflict)
+            cur.execute('SELECT 1 FROM content WHERE sha1 IN %s',
+                        (tuple(cont['sha1'] for cont in content_with_data),))
+            list(cur)
+
             # move metadata in place
             try:
                 db.content_add_from_temp(cur)
@@ -264,6 +272,8 @@
                 content:add:bytes: Sum of the contents' length data
                 skipped_content:add: New skipped contents (no data) added
         """
+        cur.execute('SET TRANSACTION ISOLATION LEVEL SERIALIZABLE')
+
         content = [dict(c.items()) for c in content]  # semi-shallow copy
         now = datetime.datetime.now(tz=datetime.timezone.utc)
         for item in content:
@@ -379,6 +389,7 @@
                 content:add: New contents added
                 skipped_content:add: New skipped contents (no data) added
         """
+        cur.execute('SET TRANSACTION ISOLATION LEVEL SERIALIZABLE')
 
         content = [self._normalize_content(c) for c in content]
         for c in content:
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -3238,7 +3238,6 @@
 
 @pytest.mark.db
 class TestStorageRaceConditions:
-    @pytest.mark.xfail
     def test_content_add_race(self, swh_storage):
 
         results = queue.Queue()
@@ -3267,9 +3266,16 @@
 
         with pytest.raises(queue.Empty):
             results.get(block=False)
-        assert r1[0] != r2[0]
-        assert r1[1] == 'data', 'Got exception %r in Thread%s' % (r1[2], r1[0])
-        assert r2[1] == 'data', 'Got exception %r in Thread%s' % (r2[2], r2[0])
+        assert r1[0] != r2[0]  # Ident is unique
+        assert r1[1] != r2[1]  # Don't have same result
+        if r1[1] == 'data':
+            (r_data, r_error) = (r1, r2)
+        else:
+            (r_data, r_error) = (r2, r1)
+        assert r_data[1] == 'data'
+        assert r_error[1] == 'exc', 'Got no exception %r in Thread%s' % (
+            r_error[2], r_error[0])
+        assert isinstance(r_error[2], psycopg2.errors.SerializationFailure)
 
 
 @pytest.mark.db