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 B5CE7CCA480 for ; Wed, 20 Jul 2022 09:49:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238726AbiGTJtD (ORCPT ); Wed, 20 Jul 2022 05:49:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48034 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231826AbiGTJtA (ORCPT ); Wed, 20 Jul 2022 05:49:00 -0400 Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com [115.124.30.133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D834965D7C; Wed, 20 Jul 2022 02:48:58 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R461e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046050;MF=joseph.qi@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0VJwWVRl_1658310535; Received: from 30.227.66.165(mailfrom:joseph.qi@linux.alibaba.com fp:SMTPD_---0VJwWVRl_1658310535) by smtp.aliyun-inc.com; Wed, 20 Jul 2022 17:48:55 +0800 Message-ID: <65e6bbcb-2c33-2e43-1826-a62387572310@linux.alibaba.com> Date: Wed, 20 Jul 2022 17:48:55 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH 2/3] ocfs2: Remove a useless spinlock Content-Language: en-US To: Marion & Christophe JAILLET Cc: David.Laight@ACULAB.COM, jlbec@evilplan.org, kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org, mark@fasheh.com, ocfs2-devel@oss.oracle.com References: <8ba7004d330cbe5f626539a8a3bff696d0c4285e.1658224839.git.christophe.jaillet@wanadoo.fr> <7b644e5d32d74d3d90dfc5b1786ae5b9@AcuMS.aculab.com> <29c3fbdd-7695-46c5-bb75-fe358c574ab3@wanadoo.fr> <07c924de-78bf-c993-ce73-635af71f4edd@linux.alibaba.com> From: Joseph Qi In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/20/22 4:26 PM, Marion & Christophe JAILLET wrote: > > Le 20/07/2022 à 03:59, Joseph Qi a écrit : >> >> On 7/19/22 9:25 PM, Christophe JAILLET wrote: >>> Le 19/07/2022 à 12:24, David Laight a écrit : >>>> From: Christophe JAILLET >>>>> Sent: 19 July 2022 11:02 >>>>> >>>>> 'node_map_lock' is a spinlock only used to protect calls to set_bit(), >>>>> clear_bit() and test_bit(). >>>>> >>>>> {set|clear}_bit() are already atomic and don't need this extra spinlock. >>>>> test_bit() only reads the bitmap for a given bit. >>>>> >>>>> Remove this useless spinlock. >>>> It looks to me like the calling code is racy >>>> unless there is another lock in the callers. >>> The call chains are: >>>    ocfs2_recover_orphans() >>>      ocfs2_mark_recovering_orphan_dir() >>>        spin_lock(&osb->osb_lock);        <-- osb_lock spinlock >>>        ocfs2_node_map_set_bit()            <-- uses node_map_lock >>>        ... >>>        spin_unlock(&osb->osb_lock); >>>      ... >>>      ocfs2_clear_recovering_orphan_dir() >>>        ocfs2_node_map_clear_bit()        <-- uses node_map_lock >>>                              osb_lock is NOT taken >>> >>> >>>    ocfs2_check_orphan_recovery_state() >>>      spin_lock(&osb->osb_lock);            <-- osb_lock spinlock >>>      ... >>>      ocfs2_node_map_test_bit()            <-- uses node_map_lock >>>      ... >>>      spin_unlock(&osb->osb_lock); >>> >>> >>> So the code looks already protected by the 'osb_lock' spinlock, but I don't know this code and ocfs2_mark_recovering_orphan_dir() looks tricky to me. (so some other eyes are much welcome) >>   osb_lock is to protect osb filed such as 'osb_orphan_wipes', while >> node_map_lock is to protect the node map 'osb_recovering_orphan_dirs' >> specifically. > > Thanks for this explanation. > > But does "node_map_lock" really protects anything? > It is just around some atomic function calls which shouldn't need any, right? > > test_bit() is not documented as atomic, but {clear|set}_bit() could be executed just before or just after it with the current locking mechanism, so I don't really see how it would make a difference. > > I don't understand the logic of this lock here. > > Can you elaborate? These code are introduced long time ago... Refer to commit b4df6ed8db0c "[PATCH] ocfs2: fix orphan recovery deadlock", I guess it plays a role 'barrier' and make sure test node map is executed prior than signal orphan recovery thread. In other words, to serialize evict inode and orphan recovery. Thanks, Joseph