diff -u b/core/includes/module.inc b/core/includes/module.inc
--- b/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -144,7 +144,7 @@
       }
     }
     else {
-      $lists = _system_list_rebuild();
+      $lists = _system_list_prepare();
     }
     // We only return the module names here since module_list() doesn't need
     // the filename itself.
@@ -156,10 +156,7 @@
       $lists = $cached->data;
     }
     else {
-      if (_system_list_rebuild(TRUE) && $type == 'module_enabled') {
-        return drupal_map_assoc(array_keys(config('system.module')->get()));
-      }
-      $lists = _system_list_rebuild();
+      $lists = _system_list_prepare();
       $lists['bootstrap'] = array_keys($lists['bootstrap']);
     }
     foreach ($lists['filepaths'] as $item) {
@@ -171,15 +168,13 @@
 }
 
 /**
- * Rebuild the system list data structure.
+ * Prepares the system list data structure.
+ *
+ * @return
+ *   An associative array of modules and themes, keyed by list type: bootstrap,
+ *   module_enabled, theme, filepaths.
  */
-function _system_list_rebuild($test_only = FALSE) {
-  static $in_rebuild = FALSE;
-
-  if ($test_only) {
-    return $in_rebuild;
-  }
-  $in_rebuild = TRUE;
+function _system_list_prepare() {
   // The module name (rather than the filename) is used as the fallback
   // weighting in order to guarantee consistent behavior across different
   // Drupal installations, which might have modules installed in different