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=-7.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, 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 AF57CCA9EBC for ; Thu, 24 Oct 2019 13:21:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8FAA72084C for ; Thu, 24 Oct 2019 13:21:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2502511AbfJXNVo (ORCPT ); Thu, 24 Oct 2019 09:21:44 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:38776 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730061AbfJXNVo (ORCPT ); Thu, 24 Oct 2019 09:21:44 -0400 Received: from [213.220.153.21] (helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1iNd3Z-0002J4-Se; Thu, 24 Oct 2019 13:21:37 +0000 Date: Thu, 24 Oct 2019 15:21:37 +0200 From: Christian Brauner To: Dmitry Vyukov Cc: Andrea Parri , Will Deacon , LKML , bsingharora@gmail.com, Marco Elver , stable , syzbot , syzkaller-bugs Subject: Re: [PATCH v6] taskstats: fix data-race Message-ID: <20191024132136.jknzt7rgjssgv5b6@wittgenstein> References: <20191009114809.8643-1-christian.brauner@ubuntu.com> <20191021113327.22365-1-christian.brauner@ubuntu.com> <20191023121603.GA16344@andrea.guest.corp.microsoft.com> <20191024113155.GA7406@andrea.guest.corp.microsoft.com> <20191024130502.GA11335@andrea.guest.corp.microsoft.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 24, 2019 at 03:13:48PM +0200, Dmitry Vyukov wrote: > On Thu, Oct 24, 2019 at 3:05 PM Andrea Parri wrote: > > > > On Thu, Oct 24, 2019 at 01:51:20PM +0200, Dmitry Vyukov wrote: > > > On Thu, Oct 24, 2019 at 1:32 PM Andrea Parri wrote: > > > > > > > > > How these later loads can be completely independent of the pointer > > > > > value? They need to obtain the pointer value from somewhere. And this > > > > > can only be done by loaded it. And if a thread loads a pointer and > > > > > then dereferences that pointer, that's a data/address dependency and > > > > > we assume this is now covered by READ_ONCE. > > > > > > > > The "dependency" I was considering here is a dependency _between the > > > > load of sig->stats in taskstats_tgid_alloc() and the (program-order) > > > > later loads of *(sig->stats) in taskstats_exit(). Roughly speaking, > > > > such a dependency should correspond to a dependency chain at the asm > > > > or registers level from the first load to the later loads; e.g., in: > > > > > > > > Thread [register r0 contains the address of sig->stats] > > > > > > > > A: LOAD r1,[r0] // LOAD_ACQUIRE sig->stats > > > > ... > > > > B: LOAD r2,[r0] // LOAD *(sig->stats) > > > > C: LOAD r3,[r2] > > > > > > > > there would be no such dependency from A to C. Compare, e.g., with: > > > > > > > > Thread [register r0 contains the address of sig->stats] > > > > > > > > A: LOAD r1,[r0] // LOAD_ACQUIRE sig->stats > > > > ... > > > > C: LOAD r3,[r1] // LOAD *(sig->stats) > > > > > > > > AFAICT, there's no guarantee that the compilers will generate such a > > > > dependency from the code under discussion. > > > > > > Fixing this by making A ACQUIRE looks like somewhat weird code pattern > > > to me (though correct). B is what loads the address used to read > > > indirect data, so B ought to be ACQUIRE (or LOAD-DEPENDS which we get > > > from READ_ONCE). > > > > > > What you are suggesting is: > > > > > > addr = ptr.load(memory_order_acquire); > > > if (addr) { > > > addr = ptr.load(memory_order_relaxed); > > > data = *addr; > > > } > > > > > > whereas the canonical/non-convoluted form of this pattern is: > > > > > > addr = ptr.load(memory_order_consume); > > > if (addr) > > > data = *addr; > > > > No, I'd rather be suggesting: > > > > addr = ptr.load(memory_order_acquire); > > if (addr) > > data = *addr; > > > > since I'd not expect any form of encouragement to rely on "consume" or > > on "READ_ONCE() + true-address-dependency" from myself. ;-) > > But why? I think kernel contains lots of such cases and it seems to be > officially documented by the LKMM: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt > address dependencies and ppo You mean this section: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt#n955 and specifically: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt#n982 ?