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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 D12D4C433DB for ; Mon, 8 Mar 2021 10:40:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 99D73651FD for ; Mon, 8 Mar 2021 10:40:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230511AbhCHKkL (ORCPT ); Mon, 8 Mar 2021 05:40:11 -0500 Received: from frasgout.his.huawei.com ([185.176.79.56]:2649 "EHLO frasgout.his.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230495AbhCHKjx (ORCPT ); Mon, 8 Mar 2021 05:39:53 -0500 Received: from fraeml714-chm.china.huawei.com (unknown [172.18.147.226]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4DvF571Tjzz67x4Q; Mon, 8 Mar 2021 18:31:55 +0800 (CST) Received: from lhreml724-chm.china.huawei.com (10.201.108.75) by fraeml714-chm.china.huawei.com (10.206.15.33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2106.2; Mon, 8 Mar 2021 11:39:51 +0100 Received: from [10.210.165.214] (10.210.165.214) by lhreml724-chm.china.huawei.com (10.201.108.75) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2106.2; Mon, 8 Mar 2021 10:39:50 +0000 Subject: Re: [RFC PATCH v3 1/3] blk-mq: Clean up references to old requests when freeing rqs To: Bart Van Assche , , , , CC: , , , , References: <1614957294-188540-1-git-send-email-john.garry@huawei.com> <1614957294-188540-2-git-send-email-john.garry@huawei.com> From: John Garry Message-ID: Date: Mon, 8 Mar 2021 10:37:51 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.210.165.214] X-ClientProxiedBy: lhreml739-chm.china.huawei.com (10.201.108.189) To lhreml724-chm.china.huawei.com (10.201.108.75) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 06/03/2021 18:13, Bart Van Assche wrote: > On 3/5/21 7:14 AM, John Garry wrote: >> @@ -2296,10 +2296,14 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags, >> >> for (i = 0; i < tags->nr_tags; i++) { >> struct request *rq = tags->static_rqs[i]; >> + int j; >> >> if (!rq) >> continue; >> set->ops->exit_request(set, rq, hctx_idx); >> + /* clean up any references which occur in @ref_tags */ >> + for (j = 0; ref_tags && j < ref_tags->nr_tags; j++) >> + cmpxchg(&ref_tags->rqs[j], rq, 0); >> tags->static_rqs[i] = NULL; >> } >> } Hi Bart, > What prevents blk_mq_tagset_busy_iter() from reading hctx->tags[...] > before the cmpxcg() call and dereferencing it after blk_mq_free_rqs() > has called __free_pages()? > So there is nothing in this patch to stop that. But it's pretty unlikely, as the window is very narrow generally between reading hctx->tags[...] and actually dereferencing it. However, something like that should be made safe in patch 2/3. Thanks, John