Symfony删除实体从不刷新

    $form = $this->createForm('AppBundle\Form\UserType', $user);
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        $em = $this->getDoctrine()->getManager();
        $user->setRoles($this->setUserRoles($user));

        $changed = $this->checkChanged($em->getUnitOfWork()->getOriginalEntityData($user), $request->request->get('app_user_profile'));
        if ($changed) {
//            $sql = "DELETE FROM sessions WHERE sess_id = '".$user->getSessId()."'";
//            $em->getConnection()->exec($sql);
//            $em->getConnection()->commit();

            $entity = $em->getRepository('AppBundle:Sessions')->find($user->getSessId());
            if ($entity) {
                $em->remove($entity);
//                $em->flush($entity);
            }
            $user->setSessId(null);
        }
        $em->persist($user);
        $em->flush();
        return $this->redirectToRoute('users_list');
    }

SQLSTATE[HY000]: General error: 1205 Lock wait timeout exceeded; try restarting transaction

我用同样的结果注释了我尝试过的代码。MySql日志文件显示删除命令后暂停10秒,我不知道它在等待什么,甚至不知道为什么 $em->getConnection()->commit();

2017-08-09T10:02:36.334195Z   217 Connect   user@localhost on mydb using TCP/IP
2017-08-09T10:02:36.336556Z   217 Query SELECT t0.username AS username_1, <...> FROM fos_user t0 WHERE t0.id = 1 LIMIT 1
2017-08-09T10:02:36.373640Z   217 Query DELETE FROM sessions WHERE sess_id = '1gjo6har0vttn7ru63pjrptti4'
2017-08-09T10:03:27.489764Z   216 Query INSERT INTO sessions (sess_id, sess_data <...>