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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A437C7EE23 for ; Fri, 2 Jun 2023 01:53:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233193AbjFBBxI (ORCPT ); Thu, 1 Jun 2023 21:53:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38730 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233179AbjFBBxH (ORCPT ); Thu, 1 Jun 2023 21:53:07 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EC3D3C0 for ; Thu, 1 Jun 2023 18:53:05 -0700 (PDT) Received: from kwepemm600013.china.huawei.com (unknown [172.30.72.53]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4QXQrR4S8sz18Lnv; Fri, 2 Jun 2023 09:48:23 +0800 (CST) Received: from [10.174.178.46] (10.174.178.46) by kwepemm600013.china.huawei.com (7.193.23.68) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.23; Fri, 2 Jun 2023 09:53:03 +0800 Subject: Re: [PATCH 4/5] jbd2: Fix wrongly judgement for buffer head removing while doing checkpoint To: Jan Kara , Zhang Yi CC: , , , , References: <20230531115100.2779605-1-yi.zhang@huaweicloud.com> <20230531115100.2779605-5-yi.zhang@huaweicloud.com> <20230601094156.m4b7rxntmaxc5zy7@quack3> <20230601163121.jjdo4f2xfpfx6dzi@quack3> From: Zhihao Cheng Message-ID: <9d8555fd-818f-24af-1324-0135a79b086f@huawei.com> Date: Fri, 2 Jun 2023 09:52:51 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20230601163121.jjdo4f2xfpfx6dzi@quack3> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.178.46] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To kwepemm600013.china.huawei.com (7.193.23.68) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org 在 2023/6/2 0:31, Jan Kara 写道: > On Thu 01-06-23 22:20:38, Zhang Yi wrote: >> On 2023/6/1 21:44, Zhihao Cheng wrote: >>> 在 2023/6/1 17:41, Jan Kara 写道: >>> >>> Hi, Jan >>>> On Wed 31-05-23 19:50:59, Zhang Yi wrote: >>>>> From: Zhihao Cheng >>>>> >>>>> Following process, >>>>> >>>>> jbd2_journal_commit_transaction >>>>> // there are several dirty buffer heads in transaction->t_checkpoint_list >>>>>            P1                   wb_workfn >>>>> jbd2_log_do_checkpoint >>>>>   if (buffer_locked(bh)) // false >>>>>                              __block_write_full_page >>>>>                               trylock_buffer(bh) >>>>>                               test_clear_buffer_dirty(bh) >>>>>   if (!buffer_dirty(bh)) >>>>>    __jbd2_journal_remove_checkpoint(jh) >>>>>     if (buffer_write_io_error(bh)) // false >>>>>                               >> bh IO error occurs << >>>>>   jbd2_cleanup_journal_tail >>>>>    __jbd2_update_log_tail >>>>>     jbd2_write_superblock >>>>>     // The bh won't be replayed in next mount. >>>>> , which could corrupt the ext4 image, fetch a reproducer in [Link]. >>>>> >>>>> Since writeback process clears buffer dirty after locking buffer head, >>>>> we can fix it by checking buffer dirty firstly and then checking buffer >>>>> locked, the buffer head can be removed if it is neither dirty nor locked. >>>>> >>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217490 >>>>> Fixes: 470decc613ab ("[PATCH] jbd2: initial copy of files from jbd") >>>>> Signed-off-by: Zhihao Cheng >>>>> Signed-off-by: Zhang Yi >>>> >>>> OK, the analysis is correct but I'm afraid the fix won't be that easy.  The >>>> reordering of tests you did below doesn't really help because CPU or the >>>> compiler are free to order the loads (and stores) in whatever way they >>>> wish. You'd have to use memory barriers when reading and modifying bh flags >>>> (although the modification side is implicitely handled by the bitlock >>>> code) to make this work reliably. But that is IMHO too subtle for this >>>> code. >>>> >>> >> >> I write two litmus-test files in tools/memory-model to check the memory module >> of these two cases as jbd2_log_do_checkpoint() and __cp_buffer_busy() does. > > > >> So it looks like the out-of-order situation cannot happen, am I miss something? > > After thinking about it a bit, indeed CPU cannot reorder the two loads > because they are from the same location in memory. Thanks for correcting me > on this. I'm not sure whether a C compiler still could not reorder the > tests - technically I suspect the C standard does not forbid this although > it would have to be really evil compiler to do this. > > But still I think with the helper function I've suggested things are much > more obviously correct. Thanks for suggestions, we will modify it in next iteration.