From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.6 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 892EDC433F5 for ; Tue, 7 Sep 2021 03:04:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6107B6109F for ; Tue, 7 Sep 2021 03:04:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229926AbhIGDFZ (ORCPT ); Mon, 6 Sep 2021 23:05:25 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:9005 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229797AbhIGDFY (ORCPT ); Mon, 6 Sep 2021 23:05:24 -0400 Received: from dggemv703-chm.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4H3VTC0QKQzVqHM; Tue, 7 Sep 2021 11:03:27 +0800 (CST) Received: from dggpeml500025.china.huawei.com (7.185.36.35) by dggemv703-chm.china.huawei.com (10.3.19.46) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.8; Tue, 7 Sep 2021 11:04:16 +0800 Received: from [10.174.176.117] (10.174.176.117) by dggpeml500025.china.huawei.com (7.185.36.35) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.8; Tue, 7 Sep 2021 11:04:16 +0800 Subject: Re: [PATCH v2 3/3] nbd: fix race between nbd_alloc_config() and module removal To: Christoph Hellwig CC: Josef Bacik , Jens Axboe , , References: <20210904122519.1963983-1-houtao1@huawei.com> <20210904122519.1963983-4-houtao1@huawei.com> <20210906093051.GC30790@lst.de> <20210906102521.GA3082@lst.de> From: Hou Tao Message-ID: <730dae5e-5af8-3554-18bf-e22ff576e2b1@huawei.com> Date: Tue, 7 Sep 2021 11:04:16 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210906102521.GA3082@lst.de> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Content-Language: en-US X-Originating-IP: [10.174.176.117] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpeml500025.china.huawei.com (7.185.36.35) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi, On 9/6/2021 6:25 PM, Christoph Hellwig wrote: > On Mon, Sep 06, 2021 at 06:08:54PM +0800, Hou Tao wrote: >>>> + if (!try_module_get(THIS_MODULE)) >>>> + return ERR_PTR(-ENODEV); >>> try_module_get(THIS_MODULE) is an indicator for an unsafe pattern. If >>> we don't already have a reference it could never close the race. >>> >>> Looking at the callers: >>> >>> - nbd_open like all block device operations must have a reference >>> already. >> Yes. nbd_open() has already taken a reference in dentry_open(). >>> - for nbd_genl_connect I'm not an expert, but given that struct >>> nbd_genl_family has a module member I suspect the networkinh >>> code already takes a reference. >> That was my original though, but the fact is netlink code doesn't take a module reference >> >> in genl_family_rcv_msg_doit() and netlink uses genl_lock_all() to serialize between module removal >> >> and nbd_connect_genl_ops calling, so I think use try_module_get() is OK here. > How it this going to work? If there was a race you just shortened it, > but it can still happen before you call try_module_get. So I think we > need to look into how the netlink calling conventions are supposed to > look and understand the issues there first. > . Let me explain first. The reason it works is due to genl_lock_all() in netlink code. If the module removal happens before calling try_module_get(), nbd_cleanup() will call genl_unregister_family() first, and then genl_lock_all(). genl_lock_all() will prevent ops in nbd_connect_genl_ops() from being called, because the calling of nbd ops happens in genl_rcv() which needs to acquire cb_lock first. process A                                       process B module removal genl_unregister_family()   genl_lock_all()     down_write(&cb_lock)                                                 receive a new netlink message                                                 genl_rcv()                                                    // will wait for the removal of nbd ops                                                    down_read(&cb_lock) If nbd_alloc_config() happens before the module removal, genl_rcv() must have been acquired cb_lock & genl_mutex, so nbd_cleanup() will block in genl_unregister_family(). When nbd_alloc_config() calls try_module_get(), it will find out the module is dying, so fail nbd_genl_connect(). process A                                     process B a new netlink message genl_rcv()   down_read(&cb_lock)     mutex_lock(&genl_mutex)       nbd_genl_connect()         nbd_alloc_config()                                                module removal                                                genl_unregister_family           // module is dying, so fail           try_module_get()                                                  genl_lock_all()                                                    // wait for the completion of nbd ops                                                    down_write(&cb_lock) I have checked multiple genl_ops, it seems that the reason why these genl_ops don't need try_module_get() is that these ops don't create new object through genl_ops and just control it. However genl_family_rcv_msg_dumpit() will try to call try_module_get(), but according to the history (6dc878a8ca39 "netlink: add reference of module in netlink_dump_start"), it is because inet_diag_handler_cmd() will call __netlink_dump_start(). Regards, Tao