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 3DBDCC83F11 for ; Mon, 28 Aug 2023 02:43:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230009AbjH1Ch0 (ORCPT ); Sun, 27 Aug 2023 22:37:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33990 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230023AbjH1ChC (ORCPT ); Sun, 27 Aug 2023 22:37:02 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 96BC1F9 for ; Sun, 27 Aug 2023 19:36:59 -0700 (PDT) Received: from kwepemm600017.china.huawei.com (unknown [172.30.72.57]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4RYvkg6PkyzJrlg; Mon, 28 Aug 2023 10:33:47 +0800 (CST) Received: from [10.174.179.234] (10.174.179.234) by kwepemm600017.china.huawei.com (7.193.23.234) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.31; Mon, 28 Aug 2023 10:36:57 +0800 Message-ID: Date: Mon, 28 Aug 2023 10:36:56 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.8.0 Subject: Re: [PATCH v2] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs() To: Matthew Wilcox CC: Naoya Horiguchi , "Paul E. McKenney" , Andrew Morton , Naoya Horiguchi , Miaohe Lin , , , References: <20230821091312.2034844-1-tongtiangen@huawei.com> <0bbbb7d8-699b-30ac-9657-840112c41a78@huawei.com> <20230825060221.GA3948311@ik1-406-35019.vs.sakura.ne.jp> <9e205429-1f27-4d18-faca-8a4fe9d429e3@huawei.com> From: Tong Tiangen In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.179.234] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To kwepemm600017.china.huawei.com (7.193.23.234) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2023/8/27 4:28, Matthew Wilcox 写道: > On Sat, Aug 26, 2023 at 09:46:53AM +0800, Tong Tiangen wrote: >> " the ``task_struct`` object is freed only after one or more >> grace periods elapse, with the help of call_rcu(), which is invoked via >> put_task_struct_rcu_user(). " >> >> Combined with the code,when the task exits： >> >> release_task() >> __exit_signal() >> __unhash_process() >> list_del_rcu(&p->tasks) >> >> put_task_struct_rcu_user() >> call_rcu(&task->rcu, delayed_put_task_struct); >> >> delayed_put_task_struct() >> put_task_struct() >> if (refcount_sub_and_test(nr, &t->usage)) >> __put_task_struct() >> free_task() >> >> The code is consistent with the description in the document. >> >> According to this understanding, i think for_each_process() under the >> protection of rcu locl is safe, that is, task_struct in the list will not be >> destroyed, and get_task_struct() is also safe. > > Aha! This is different from the usual pattern. What I'm used to seeing > is: > > if (refcount_sub_and_test()) { > list_del_rcu(); > rcu_free(); > } > > and then on the read side you need a refcount_inc_not_zero(), which we > didn't have here. Given this new information you've found, I withdraw > my objection. It'd be nice to include some of this analysis in an > updated changelog (and maybe improved documentation for tasklist?). OK, commit message and changelog have been updated, and a new patch version v3 has been sent. Thanks, Tong. > > .