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=-5.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 AA330CA9EAF for ; Mon, 21 Oct 2019 12:19:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 81A0620873 for ; Mon, 21 Oct 2019 12:19:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=rasmusvillemoes.dk header.i=@rasmusvillemoes.dk header.b="UNEfoqJS" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728569AbfJUMTH (ORCPT ); Mon, 21 Oct 2019 08:19:07 -0400 Received: from mail-lf1-f67.google.com ([209.85.167.67]:46946 "EHLO mail-lf1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728521AbfJUMTG (ORCPT ); Mon, 21 Oct 2019 08:19:06 -0400 Received: by mail-lf1-f67.google.com with SMTP id t8so9885202lfc.13 for ; Mon, 21 Oct 2019 05:19:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=rasmusvillemoes.dk; s=google; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=JNOM4Pg0n8Q4UqLt3Kh0p/niTCV+End7za47n4/6Ktg=; b=UNEfoqJSjHmG0+iNd+CHQt8KffLkhlNrReI8ER0SN3uGr4sAv282oSQFllCK1GufFa zhyeCzPVuEfYjLiYUz65wwf4KY0lcSxLiBZfhClN0nM4F+d1yXzwkBXnW56H1nx2orKR 9lHhBafiS+qfKOKzY/Sz5Y0Vu8kmm9SZea1bw= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=JNOM4Pg0n8Q4UqLt3Kh0p/niTCV+End7za47n4/6Ktg=; b=UZgxaZBEwZSrAjLRldUsMuO7SeENfr7hQaBKpFIlO+MMdEHXdSXX9Lgo86AUfW8Fkr DT4VtBlmKmEJVantqj6pLwS5MnE37wwpl+Axsh/uyOKML6ILycPh8SFRPal4DsKHYbQS gV0QexcMJag5LDdjoFXQ3ANyPBjjpcw8wPDVxXms/0qseQEdOtWhDUWDhZz4K89WsyBt suBOIvENxiAQpRru7AGswMteqsQLF17FbIfEEV8PYTM4whAaReAwJ+qTwOdICTGI4SSk 8YawV++SqB63WOGItgUmXvQi2Lw7+7al4qO5Ds6Zp2nCCsIvs+zbU6h2RqDsO2CgF1sX shxQ== X-Gm-Message-State: APjAAAVbK5gYZfQnX4VNPq6Xw2OtdqeMkZh+n0knh+RzTnTgMmihzADZ U8L3IFYofeJUlqCmcRPjrO0N1Q== X-Google-Smtp-Source: APXvYqwt2eM1kZ85F15jYxbBeCLzCU2TXQeJLonX186jbNS9UUkduoKUyAQHXJVB71RH+jbnqv8GOQ== X-Received: by 2002:ac2:484e:: with SMTP id 14mr8022102lfy.184.1571660343384; Mon, 21 Oct 2019 05:19:03 -0700 (PDT) Received: from [172.16.11.28] ([81.216.59.226]) by smtp.gmail.com with ESMTPSA id r19sm6810718ljd.95.2019.10.21.05.19.02 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 21 Oct 2019 05:19:02 -0700 (PDT) Subject: Re: [PATCH v6] taskstats: fix data-race To: Christian Brauner , Will Deacon , linux-kernel@vger.kernel.org Cc: bsingharora@gmail.com, dvyukov@google.com, elver@google.com, parri.andrea@gmail.com, stable@vger.kernel.org, syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com References: <20191009114809.8643-1-christian.brauner@ubuntu.com> <20191021113327.22365-1-christian.brauner@ubuntu.com> From: Rasmus Villemoes Message-ID: Date: Mon, 21 Oct 2019 14:19:01 +0200 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: <20191021113327.22365-1-christian.brauner@ubuntu.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 21/10/2019 13.33, Christian Brauner wrote: > The first approach used smp_load_acquire() and smp_store_release(). > However, after having discussed this it seems that the data dependency > for kmem_cache_alloc() would be fixed by WRITE_ONCE(). > Furthermore, the smp_load_acquire() would only manage to order the stats > check before the thread_group_empty() check. So it seems just using > READ_ONCE() and WRITE_ONCE() will do the job and I wanted to bring this > up for discussion at least. > > /* v6 */ > - Christian Brauner : > - bring up READ_ONCE()/WRITE_ONCE() approach for discussion > --- > kernel/taskstats.c | 26 +++++++++++++++----------- > 1 file changed, 15 insertions(+), 11 deletions(-) > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > index 13a0f2e6ebc2..111bb4139aa2 100644 > --- a/kernel/taskstats.c > +++ b/kernel/taskstats.c > @@ -554,25 +554,29 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info) > 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 WRITE_ONCE() below. */ > + stats = READ_ONCE(sig->stats); > + if (stats || thread_group_empty(tsk)) > + return stats; > > /* No problem if kmem_cache_zalloc() fails */ > - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > spin_lock_irq(&tsk->sighand->siglock); > - if (!sig->stats) { > - sig->stats = stats; > - stats = NULL; > + if (!stats) { > + stats = stats_new; > + /* Pairs with READ_ONCE() above. */ > + WRITE_ONCE(sig->stats, stats_new); > + stats_new = NULL; No idea about the memory ordering issues, but don't you need to load/check sig->stats again? Otherwise it seems that two threads might both see !sig->stats, both allocate a stats_new, and both unconditionally in turn assign their stats_new to sig->stats. Then the first assignment ends up becoming a memory leak (and any writes through that pointer done by the caller end up in /dev/null...) Rasmus