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 EEB46ECE58C for ; Mon, 7 Oct 2019 14:18:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C58EA206BB for ; Mon, 7 Oct 2019 14:18:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="BLIJFSTB" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727903AbfJGOSj (ORCPT ); Mon, 7 Oct 2019 10:18:39 -0400 Received: from mail-qt1-f195.google.com ([209.85.160.195]:34061 "EHLO mail-qt1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727490AbfJGOSj (ORCPT ); Mon, 7 Oct 2019 10:18:39 -0400 Received: by mail-qt1-f195.google.com with SMTP id 3so19378469qta.1 for ; Mon, 07 Oct 2019 07:18:38 -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=L0aGd5iGzy7oTR3viDB2jBKpSZXW/T11Qmx88FocAp0=; b=BLIJFSTB/G0hTxSyuUcChHQTUG4TktVTAMCyTFc/UM7op4sD0B69rpMXeOIiGf6kRI Z1zzsxjlfbahk/UR802rSVa+3rdsfua1CFa44/pN48dPKOoNoEx8PZVnmS4QBu1yA53o 4HvslXk7uZ8b75qpxBOGtgVZLEuVRfY1ZoBu/2z3PxABOv3H3/r0ZIqoMMaJr85jAPxI al7Mm3ECcO+MVagXddXp6mQFUludyxrKGHF6IqEhA5qe8VwsK5a/NOOUtYcl+Hv6KVAl 6gvDtZsTbtfIH7ZBl5+HUZtwiwVEjYu6kSq9BRnUvKZzNcRksoDGCvQBYI/dJkAulJjV 6z1Q== 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=L0aGd5iGzy7oTR3viDB2jBKpSZXW/T11Qmx88FocAp0=; b=pm2xYGlE64t/yx+IMm7oK0N8bP39KHQj1FUVLmBJ8dDrblRjflCiXWhlkDw969EJoA 8OI8wlXdKnb/pwY9CG7TnBKt8wXADTk7RqKVgwAy/TBICy9eMwPOhJOtyfnnnhWmO488 HgMsBiARNmEVbkuiPtdJ9Tto69ufenznQlLbAVi3Iio/CsAhb7rwhzKsoDw6AUFaDmVb AD/HEIoF3TEIqYN8aEdKJ7H//WjttqDcj+eVSImHrHzg0uPrAVIWIu/g+eL8n+kmn1Mi xDepfjrpLG1z72SW0FnXnR6AKeD+xQ3DdKOUF922wP2ygfuVuoae+UTW7oq0BU2OD81C pItA== X-Gm-Message-State: APjAAAVceGfrSFol17LkgJep6rOq4W+xJPPhIJAjCgy/yS4N7j5xe0wb +Z2CsytveV6+9JdBbse64usvpABF5fogIVVYFCwmbQ== X-Google-Smtp-Source: APXvYqyCkMof+lfWuP5KA6qlI2H3Fjv46/wLvsY4rIyhiAcrcHG0FOMjuLe5Y/7mZugp/EJEMPLLjieK3rPinWTysy4= X-Received: by 2002:ac8:7646:: with SMTP id i6mr30548962qtr.50.1570457917819; Mon, 07 Oct 2019 07:18:37 -0700 (PDT) MIME-Version: 1.0 References: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> <20191007110117.1096-1-christian.brauner@ubuntu.com> <20191007131804.GA19242@andrea.guest.corp.microsoft.com> <20191007141432.GA22083@andrea.guest.corp.microsoft.com> In-Reply-To: <20191007141432.GA22083@andrea.guest.corp.microsoft.com> From: Dmitry Vyukov Date: Mon, 7 Oct 2019 16:18:26 +0200 Message-ID: Subject: Re: [PATCH v2] taskstats: fix data-race To: Andrea Parri Cc: Christian Brauner , bsingharora@gmail.com, Marco Elver , LKML , syzbot , syzkaller-bugs , stable Content-Type: text/plain; charset="UTF-8" Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Mon, Oct 7, 2019 at 4:14 PM Andrea Parri wrote: > > > > > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > > > > { > > > > struct signal_struct *sig = tsk->signal; > > > > - struct taskstats *stats; > > > > + struct taskstats *stats_new, *stats; > > > > > > > > - if (sig->stats || thread_group_empty(tsk)) > > > > - goto ret; > > > > + /* Pairs with smp_store_release() below. */ > > > > + stats = READ_ONCE(sig->stats); > > > > > > This pairing suggests that the READ_ONCE() is heading an address > > > dependency, but I fail to identify it: what is the target memory > > > access of such a (putative) dependency? > > > > I would assume callers of this function access *stats. So the > > dependency is between loading stats and accessing *stats. > > AFAICT, the only caller of the function in 5.4-rc2 is taskstats_exit(), > which 'casts' the return value to a boolean (so I really don't see how > any address dependency could be carried over/relied upon here). This does not make sense. But later taskstats_exit does: memcpy(stats, tsk->signal->stats, sizeof(*stats)); Perhaps it's supposed to use stats returned by taskstats_tgid_alloc?