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=-2.3 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 7E8A4C49ED7 for ; Fri, 20 Sep 2019 14:20:30 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 49A69207E0 for ; Fri, 20 Sep 2019 14:20:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 49A69207E0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=virtuozzo.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id D7E2D6B0003; Fri, 20 Sep 2019 10:20:29 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D2F366B0005; Fri, 20 Sep 2019 10:20:29 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C1D746B000E; Fri, 20 Sep 2019 10:20:29 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0215.hostedemail.com [216.40.44.215]) by kanga.kvack.org (Postfix) with ESMTP id A1EA86B0003 for ; Fri, 20 Sep 2019 10:20:29 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with SMTP id 57C1D180AD805 for ; Fri, 20 Sep 2019 14:20:29 +0000 (UTC) X-FDA: 75955509378.04.north81_21532fe5b800f X-HE-Tag: north81_21532fe5b800f X-Filterd-Recvd-Size: 2066 Received: from relay.sw.ru (relay.sw.ru [185.231.240.75]) by imf28.hostedemail.com (Postfix) with ESMTP for ; Fri, 20 Sep 2019 14:20:28 +0000 (UTC) Received: from [172.16.24.104] by relay.sw.ru with esmtp (Exim 4.92.2) (envelope-from ) id 1iBJlo-0006am-HJ; Fri, 20 Sep 2019 17:20:24 +0300 Subject: Re: [PATCH] mm, memcg: assign shrinker_map before kvfree To: "Kirill A. Shutemov" , Cyrill Gorcunov Cc: LKML , Linux MM , Johannes Weiner , Michal Hocko , Vladimir Davydov References: <20190920122907.GG2507@uranus.lan> <20190920132114.ofzphp53vqqjb3fs@box> From: Kirill Tkhai Message-ID: <55c22ce3-5e42-f7a1-3861-8e0cb73d26de@virtuozzo.com> Date: Fri, 20 Sep 2019 17:20:14 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20190920132114.ofzphp53vqqjb3fs@box> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 20.09.2019 16:21, Kirill A. Shutemov wrote: > On Fri, Sep 20, 2019 at 03:29:07PM +0300, Cyrill Gorcunov wrote: >> Currently there is a small gap between fetching pointer, calling >> kvfree and assign its value to nil. In current callgraph it is >> not a problem (since memcg_free_shrinker_maps is running from >> memcg_alloc_shrinker_maps and mem_cgroup_css_free only) still >> this looks suspicious and we can easily eliminate the gap at all. > > With this logic it will still look suspicious since you don't wait a grace > period before freeing the map. This freeing occurs in the moment, when nobody can dereference shrinker_map in parallel: memcg is either not yet online or its css->refcnt is already dead. This NULLifying is needed just to prevent double freeing of shrinker_map. Please, see the explanation in my email to our namesake. Kirill