如何强制QPluginLoader unload? [复制链接]

如下代码， 如果iUprade->performUpgrade()已经死掉了，如何才能强制unload plugin呢？我试过保存loader为全局变量，然后在一个slot里面loader->unload(),结果程序crash了.
QPluginLoader  *loader;
...
...

loader = new QPluginLoader (dir.absoluteFilePath(fileName));
QObject *plugin = loader->instance();

if (plugin)
{
UpgradeInterface * iUpgrade = qobject_cast<UpgradeInterface *>(plugin);
if (iUpgrade)
{
iUpgrade->performUpgrade(); }

loader->unload();
}
}