Call sync with the lock held before allowing a new party to come along and start writing to config.xml.

etc/inc/config.inc

@@ -1040,11 +1040,11 @@ function write_config($desc="Unknown", $backup = true) {
 		fclose($fd);
 	}
 
-	config_unlock();
-
 	/* tell kernel to sync fs data */
 	mwexec("/bin/sync");
 
+	config_unlock();
+
 	return $config;
 }