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=-8.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 98EBCCA9EC4 for ; Tue, 29 Oct 2019 18:47:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 69F8821721 for ; Tue, 29 Oct 2019 18:47:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Kn3CZE3x" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731330AbfJ2Sq7 (ORCPT ); Tue, 29 Oct 2019 14:46:59 -0400 Received: from mail-ot1-f66.google.com ([209.85.210.66]:38048 "EHLO mail-ot1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729528AbfJ2Sq6 (ORCPT ); Tue, 29 Oct 2019 14:46:58 -0400 Received: by mail-ot1-f66.google.com with SMTP id r14so5928281otn.5 for ; Tue, 29 Oct 2019 11:46:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=ReWNXiQSJHk1Ohmt4ko0ZYhBstNAojS1cH/ggSlVoAI=; b=Kn3CZE3xBzziKVWaQmk/w5rlpZO2GbLA16umwkgVooj88NX/DmIyNIa6Isn2UqLS3F Fm+cD+GDopHSMsxgxlBeiPULD6tNKXBO67r0TMwCr/VOs30OEn5etwD5cMzd76l9ADJv bEPEN4IhS26FRzf4+PXqg5MERXrdSITqMRw5E2dC1/uboYjloD56iI96ELRsUrcpxLP5 yc1s6tD8CjoBQyxKweBAhpoGiE7mzUtYafmLLspCafldR8G622VHkePlx/x8soweCzFX PCQ7ktRLpjSRRT04ctRSkLGV/puDZijKegMjGo+KDZNPPwCZYRTuI5e1XHQlrNwIo8QJ DbXg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=ReWNXiQSJHk1Ohmt4ko0ZYhBstNAojS1cH/ggSlVoAI=; b=egsW72tRX00sdPnNcbTqvkmgPav5Sdw6z2QdkL+6lnW/oxcd8pE3kBWs9Wh/JeCeLR /FQVFBFSs4xbSfln+A8BKgaaZMJm6Kkc0cTvDaY/wJ35pe6j26zHuM9MiECCSsHLGol1 0Z9XSjUr/OrmlWBpJC3JxX0NHw/GZon4hl5qJvVHZEZU4P9LbJXvqA23vPKNNHJK4uL2 Z57+gKekyQX/yFKogKqeNJfZs7HAHe9RIBHD7Ay+oGtTHLlD1N2A0jqxr3z4R9AJ6raI YXb5/czh3ibQ/I5JqTh6a9jIxgtCYRCtXhhE7V/h2aYEL31Q1Rdwgs0tLtBnNQ3gtQzf 5KqA== X-Gm-Message-State: APjAAAUgd/DuB4ENLuQo/7Af18MZPhWaSVyzdSOvJs9Shsi3NccuYXYs P/5akUX4Htl8jvPnSgae0MSlihv3NJy3VJulolfU4Q== X-Google-Smtp-Source: APXvYqwoXXMB6jQHL01y3ti03FRq/3phCHL6sPdA/P1ffR5g9Hi0PMIpVSujjn/UUi7eJJGETGQD8n0OnIo+4nmy1DA= X-Received: by 2002:a05:6830:1e8c:: with SMTP id n12mr4813345otr.360.1572374817658; Tue, 29 Oct 2019 11:46:57 -0700 (PDT) MIME-Version: 1.0 References: <20191029005405.201986-1-shakeelb@google.com> <20191029090347.GG31513@dhcp22.suse.cz> <20191029182802.GA193152@google.com> In-Reply-To: <20191029182802.GA193152@google.com> From: Shakeel Butt Date: Tue, 29 Oct 2019 11:46:46 -0700 Message-ID: Subject: Re: [PATCH] mm: memcontrol: fix data race in mem_cgroup_select_victim_node To: Marco Elver Cc: Michal Hocko , Roman Gushchin , Johannes Weiner , Andrew Morton , Linux MM , Cgroups , LKML , Eric Dumazet , Greg Thelen , syzbot+13f93c99c06988391efe@syzkaller.appspotmail.com Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 29, 2019 at 11:28 AM Marco Elver wrote: > > > > On Tue, 29 Oct 2019, Shakeel Butt wrote: > > > +Marco > > > > On Tue, Oct 29, 2019 at 2:03 AM Michal Hocko wrote: > > > > > > On Mon 28-10-19 17:54:05, Shakeel Butt wrote: > > > > Syzbot reported the following bug: > > > > > > > > BUG: KCSAN: data-race in mem_cgroup_select_victim_node / mem_cgroup_select_victim_node > > > > > > > > write to 0xffff88809fade9b0 of 4 bytes by task 8603 on cpu 0: > > > > mem_cgroup_select_victim_node+0xb5/0x3d0 mm/memcontrol.c:1686 > > > > try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376 > > > > reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349 > > > > mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430 > > > > tracehook_notify_resume include/linux/tracehook.h:197 [inline] > > > > exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163 > > > > prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194 > > > > swapgs_restore_regs_and_return_to_usermode+0x0/0x40 > > > > > > > > read to 0xffff88809fade9b0 of 4 bytes by task 7290 on cpu 1: > > > > mem_cgroup_select_victim_node+0x92/0x3d0 mm/memcontrol.c:1675 > > > > try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376 > > > > reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349 > > > > mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430 > > > > tracehook_notify_resume include/linux/tracehook.h:197 [inline] > > > > exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163 > > > > prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194 > > > > swapgs_restore_regs_and_return_to_usermode+0x0/0x40 > > > > > > > > mem_cgroup_select_victim_node() can be called concurrently which reads > > > > and modifies memcg->last_scanned_node without any synchrnonization. So, > > > > read and modify memcg->last_scanned_node with READ_ONCE()/WRITE_ONCE() > > > > to stop potential reordering. > > Strictly speaking, READ_ONCE/WRITE_ONCE alone avoid various bad compiler > optimizations, including store tearing, load tearing, etc. This does not > add memory barriers to constrain memory ordering. (If this code needs > some memory ordering guarantees w.r.t. previous loads/stores then this > alone is not enough.) > > > > I am sorry but I do not understand the problem and the fix. Why does the > > > race happen and why does _ONCE fixes it? There is still no > > > synchronization. Do you want to prevent from memcg->last_scanned_node > > > reloading? > > > > > > > The problem is memcg->last_scanned_node can read and modified > > concurrently. Though to me it seems like a tolerable race and not > > worth to add an explicit lock. My aim was to make KCSAN happy here to > > look elsewhere for the concurrency bugs. However I see that it might > > complain next on memcg->scan_nodes. > > The plain concurrent reads/writes are a data race, which may manifest in > various undefined behaviour due to compiler optimizations. The _ONCE > will prevent these (KCSAN only reports data races). Note that, "data > race" does not necessarily imply "race condition"; some data races are > race conditions (usually the more interesting bugs) -- but not *all* > data races are race conditions. If there is no race condition here that > warrants heavier synchronization (locking etc.), then this patch is all > that should be needed. > > I can't comment on the rest. > Thanks Marco for the explanation.