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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 4A5D8C33CA2 for ; Thu, 9 Jan 2020 03:18:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 28BC620705 for ; Thu, 9 Jan 2020 03:18:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727968AbgAIDSX (ORCPT ); Wed, 8 Jan 2020 22:18:23 -0500 Received: from mga12.intel.com ([192.55.52.136]:33749 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727549AbgAIDSX (ORCPT ); Wed, 8 Jan 2020 22:18:23 -0500 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 08 Jan 2020 19:18:22 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,412,1571727600"; d="scan'208";a="421652315" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by fmsmga005.fm.intel.com with ESMTP; 08 Jan 2020 19:18:20 -0800 Date: Thu, 9 Jan 2020 11:18:21 +0800 From: Wei Yang To: Michal Hocko Cc: Wei Yang , hannes@cmpxchg.org, vdavydov.dev@gmail.com, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, yang.shi@linux.alibaba.com Subject: Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer list Message-ID: <20200109031821.GA5206@richard> Reply-To: Wei Yang References: <20200103143407.1089-1-richardw.yang@linux.intel.com> <20200106102345.GE12699@dhcp22.suse.cz> <20200107012241.GA15341@richard> <20200107083808.GC32178@dhcp22.suse.cz> <20200108003543.GA13943@richard> <20200108094041.GQ32178@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200108094041.GQ32178@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 08, 2020 at 10:40:41AM +0100, Michal Hocko wrote: >On Wed 08-01-20 08:35:43, Wei Yang wrote: >> On Tue, Jan 07, 2020 at 09:38:08AM +0100, Michal Hocko wrote: >> >On Tue 07-01-20 09:22:41, Wei Yang wrote: >> >> On Mon, Jan 06, 2020 at 11:23:45AM +0100, Michal Hocko wrote: >> >> >On Fri 03-01-20 22:34:07, Wei Yang wrote: >> >> >> As all the other places, we grab the lock before manipulate the defer list. >> >> >> Current implementation may face a race condition. >> >> > >> >> >Please always make sure to describe the effect of the change. Why a racy >> >> >list_empty check matters? >> >> > >> >> >> >> Hmm... access the list without proper lock leads to many bad behaviors. >> > >> >My point is that the changelog should describe that bad behavior. >> > >> >> For example, if we grab the lock after checking list_empty, the page may >> >> already be removed from list in split_huge_page_list. And then list_del_init >> >> would trigger bug. >> > >> >And how does list_empty check under the lock guarantee that the page is >> >on the deferred list? >> >> Just one confusion, is this kind of description basic concept of concurrent >> programming? How detail level we need to describe the effect? > >When I write changelogs for patches like this I usually describe, what >is the potential race - e.g. > CPU1 CPU2 > path1 path2 > check lock > operation2 > unlock > lock > # check might not hold anymore > operation1 > unlock > >and what is the effect of the race - e.g. a crash, data corruption, >pointless attempt for operation1 which fails with user visible effect >etc. Hi, Michal, here is my attempt for an example. Hope this one looks good to you. For example, the potential race would be: CPU1 CPU2 mem_cgroup_move_account split_huge_page_to_list !list_empty lock !list_empty list_del unlock lock # !list_empty might not hold anymore list_del_init unlock When this sequence happens, the list_del_init() in mem_cgroup_move_account() would crash since the page is already been removed by list_del in split_huge_page_to_list(). -- Wei Yang Help you, Help me