diff --git a/includes/menu.inc b/includes/menu.inc
index fa5a71e..2dc3d37 100644
--- a/includes/menu.inc
+++ b/includes/menu.inc
@@ -2707,11 +2707,29 @@ function menu_reset_static_cache() {
  *   in parallel and the current thread just waited for completion.
  */
 function menu_rebuild() {
+  global $conf;
   if (!lock_acquire('menu_rebuild')) {
     // Wait for another request that is already doing this work.
     // We choose to block here since otherwise the router item may not
     // be available in menu_execute_active_handler() resulting in a 404.
     lock_wait('menu_rebuild');
+
+    // Refresh all variables written while waiting for the lock. This will
+    // ensure that $conf['menu_masks'] is set correctly.
+    variable_initialize($conf);
+    // Ensure that no further rebuild is triggered by this process.
+    if (isset($conf['menu_rebuild_needed'])) {
+      unset($conf['menu_rebuild_needed']);
+    }
+    return FALSE;
+  }
+
+  // Account for a race condition where the menu needed rebuilding at the
+  // beginning of this request, but was subsequently rebuilt by another process
+  // in the meantime. In this case, avoid another rebuild.
+  variable_initialize($conf);
+  if (!variable_get('menu_rebuild_needed')) {
+    lock_release('menu_rebuild');
     return FALSE;
   }
 
@@ -2729,13 +2747,22 @@ function menu_rebuild() {
       variable_set('menu_rebuild_needed', TRUE);
     }
     else {
-      variable_del('menu_rebuild_needed');
+      // Explicitly set the value to false so that variable_initialize() calls
+      // elsewhere in this function will overwrite $conf to FALSE when we hit
+      // race conditions.
+      variable_set('menu_rebuild_needed', FALSE);
     }
   }
   catch (Exception $e) {
     $transaction->rollback();
     watchdog_exception('menu', $e);
   }
+  // Explicitly commit the transaction now, this ensures that the database
+  // operations during the menu rebuild are committed before the lock is made
+  // available again, since locks may not always reside in the same database
+  // connection. The lock is acquired outside of the transaction so should also
+  // be released outside of it.
+  unset($transaction);
 
   lock_release('menu_rebuild');
   return TRUE;