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 6B2B3ECAAA2 for ; Sun, 28 Aug 2022 18:22:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229747AbiH1SWJ (ORCPT ); Sun, 28 Aug 2022 14:22:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42808 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229633AbiH1SWJ (ORCPT ); Sun, 28 Aug 2022 14:22:09 -0400 Received: from mail-pf1-x435.google.com (mail-pf1-x435.google.com [IPv6:2607:f8b0:4864:20::435]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 47E5D2A430; Sun, 28 Aug 2022 11:22:05 -0700 (PDT) Received: by mail-pf1-x435.google.com with SMTP id y127so6234539pfy.5; Sun, 28 Aug 2022 11:22:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:sender:from:to:cc; bh=fDxt/23RMfjSPySyD961NLt4VpOweqOjLnL5SIVCHSw=; b=bYRe2XZSiSci2LOHFk4x2DL4UfLXQoCGgCBD1X+W6bzYL4E1x6f+eLhYmIXp7q53rW Uo4HLItQM5SCdwRKOFo/PKO99zk0ev1GsMB4n8AfLYQjMlsPBBchv0gViHGlRajCZW+K cA9OvTZId4uB8YFICMpI/1EkiEyyzvHs0SZdjd0LBmU+XWTWtTyb/6/HhwPbnmqlIRss kEbrFVTf+Jr0SVAsukYkqK/AOqGbxpalP10Wr6ELwkCpjLKMc4yVLcoMTZQnZfCCmuvf oP/IvdKyf+e3fgFbFTmyrw1IHO3wI6pgK1/WjAkAppW869soncCXuBC+5JdGn0xcr0Ni udyQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:sender:x-gm-message-state:from:to:cc; bh=fDxt/23RMfjSPySyD961NLt4VpOweqOjLnL5SIVCHSw=; b=C9BOuWz1wHzIsQ6CNyycxbFXoJn/HPEWnj+xhzGQ/X9J+naXDt85twsReXUp1GQGAK T2vqExh/+d1kUkJjm+LhMFfXCuTL8WQljZxH2LjF2iTpBIrzfij08ZDNp23qI2YMmWDl JAWOu7iuBWcETDQdtCDCGLmKteanr5tIOhiDVNOrZaqTAH/6jmBxueWle5eM5e7fQv+Y whUfnvLdmRRTQoJNorE7dsQp0qBiIYyzj595jIsSaG2Dxa2Ck5LLwLLzqXSuAR2eAaGy X1VlCwTekxR81S84AVZlJDQ9rSgSZoZlPdC3c12AkQm5K/rGCqkNQzQdYL8tvJlZtvB4 5QuA== X-Gm-Message-State: ACgBeo0LtD+Vte6JvP+aKTHh7sBoXD9EENcbSsu7pwOj9hFIfq0lrLDP krs27t8X+/FJGH+kKSui/K8= X-Google-Smtp-Source: AA6agR7tU14ikL/UQg9apn73eAvtTX2DR/yMT2CYo2OogdbUN5FCCIPInYPJGDzhUW+NCZRXkw3MpA== X-Received: by 2002:a63:c5:0:b0:40d:d290:24ef with SMTP id 188-20020a6300c5000000b0040dd29024efmr11059768pga.141.1661710924333; Sun, 28 Aug 2022 11:22:04 -0700 (PDT) Received: from localhost (2603-800c-1a02-1bae-a7fa-157f-969a-4cde.res6.spectrum.com. [2603:800c:1a02:1bae:a7fa:157f:969a:4cde]) by smtp.gmail.com with ESMTPSA id m1-20020a170902db0100b0016eef474042sm5650821plx.262.2022.08.28.11.22.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 28 Aug 2022 11:22:03 -0700 (PDT) Sender: Tejun Heo Date: Sun, 28 Aug 2022 08:22:02 -1000 From: Tejun Heo To: Gabriel Ryan Cc: Christian Brauner , Abhishek Shah , linux-kernel@vger.kernel.org, andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, cgroups@vger.kernel.org, daniel@iogearbox.net, hannes@cmpxchg.org, john.fastabend@gmail.com, kafai@fb.com, kpsingh@kernel.org, lizefan.x@bytedance.com, netdev@vger.kernel.org, songliubraving@fb.com, yhs@fb.com Subject: Re: data-race in cgroup_get_tree / proc_cgroup_show Message-ID: References: <20220819072256.fn7ctciefy4fc4cu@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Mon, Aug 22, 2022 at 01:04:58PM -0400, Gabriel Ryan wrote: > Hi Christian, > > We ran a quick test and confirm your suggestion would eliminate the > data race alert we observed. If the data race is benign (and it > appears to be), using WRITE_ONCE(cgrp_dfl_visible, true) instead of > cmpxchg in cgroup_get_tree() would probably also be ok. I don't see how the data race can lead to anything but would the following work? Thanks. diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index ffaccd6373f1e..a90fdba881bdb 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -2172,7 +2172,7 @@ static int cgroup_get_tree(struct fs_context *fc) struct cgroup_fs_context *ctx = cgroup_fc2context(fc); int ret; - cgrp_dfl_visible = true; + WRITE_ONCE(cgrp_dfl_visible, true); cgroup_get_live(&cgrp_dfl_root.cgrp); ctx->root = &cgrp_dfl_root; @@ -6056,7 +6056,7 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns, struct cgroup *cgrp; int ssid, count = 0; - if (root == &cgrp_dfl_root && !cgrp_dfl_visible) + if (root == &cgrp_dfl_root && !READ_ONCE(cgrp_dfl_visible)) continue; seq_printf(m, "%d:", root->hierarchy_id); -- tejun