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=-13.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 14171CA9EAF for ; Thu, 24 Oct 2019 13:14:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DA12E20659 for ; Thu, 24 Oct 2019 13:14:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="msl1HfiQ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2502435AbfJXNOE (ORCPT ); Thu, 24 Oct 2019 09:14:04 -0400 Received: from mail-qk1-f195.google.com ([209.85.222.195]:33355 "EHLO mail-qk1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2502405AbfJXNOE (ORCPT ); Thu, 24 Oct 2019 09:14:04 -0400 Received: by mail-qk1-f195.google.com with SMTP id 71so19468922qkl.0 for ; Thu, 24 Oct 2019 06:14:02 -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=JQHcyUnSIp339K9FKmfDYw9UCbM8Q9Ch8P2/s1FBU9s=; b=msl1HfiQYnIpkl7hU80XjsF7bZtllIkrpxf3WO7plSCw75gbGg9RfOzqf2ThJZSI9i hySC5uA444uiteodVBFS1PuxXkqcIBEAGVnS9rhaFOgYInfKZb2HE4tv+ORlaR3bd5V7 6bULjoWlbDAk3UE14BgBJOacrxfygYoQHHuPXVPNi0LoRJcSqnb/qH5qPywqpal6Veo4 nSXSq5u5REQ7ylpz/K9g4INW6rc8Emxwne+UJN6j7alhtV4PJe+7UK7ju1lsMeLtp7eY G1B/60+BQkYSYGxZlGD9iZnpGHT9I2XFZhaoNBH2iEhi+A4e9IbyLeZNH/UH0opzzvl3 E/Eg== 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=JQHcyUnSIp339K9FKmfDYw9UCbM8Q9Ch8P2/s1FBU9s=; b=GD9qlSxr009oDJ5ypE+FQyGn299H7ACw6zEBWNm/OhSd8XedgTIAyw7hRh3WUwydbR KEK6XV4n0DrYH1VxgCIsSNJRy0oirK7oNvVtljPRxxPUecnM7+arD+uhF7ebWovtSgDU lM212OBPQrJPk4KoWWXX6cmnNmqBWQYWS44AnO0Ht90tWCuWo6MLssOwN9lxcXze+2/U LYeeZafw2kfT0zUE0a8XG0VxC2NbA3cjFKFIXBhenFoIz6j4tz9qDhxwhGOhB/03QMAY m38zHE7ND4DOmxn2aG73FW9Yo71+yied71wvecN18ob46ite4L/vpwITvp+2xvnLbkg6 yfHQ== X-Gm-Message-State: APjAAAVrfce+jNwIflWg4f8kD0Gvj83LXsGEm+5T04VOWZ7umzFQTkNS YRcYgHGjVz1u1vm7FcxS5U7igU7KlZi7mXS6vq58bQ== X-Google-Smtp-Source: APXvYqwZ7apFX/YX+sP24Otc+tiWUNJLzt1FSVCurx1ko1dlteIYXugGs1AhlFccwEmG7l/NhWDo20oDb1SkZ5RCcGw= X-Received: by 2002:a05:620a:a11:: with SMTP id i17mr3508638qka.8.1571922840980; Thu, 24 Oct 2019 06:14:00 -0700 (PDT) MIME-Version: 1.0 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> In-Reply-To: <20191024130502.GA11335@andrea.guest.corp.microsoft.com> From: Dmitry Vyukov Date: Thu, 24 Oct 2019 15:13:48 +0200 Message-ID: Subject: Re: [PATCH v6] taskstats: fix data-race To: Andrea Parri Cc: Christian Brauner , Will Deacon , LKML , bsingharora@gmail.com, Marco Elver , stable , syzbot , syzkaller-bugs 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 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 Am I missing something? Why do we need acquire with address dependency? > IAC, v6 looks more like: > > addr = ptr.load(memory_order_consume); > if (!!addr) > *ptr = 1; > data = *ptr; > > to me (hence my comments/questions ...). > > Thanks, > Andrea