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=-2.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 3E206CA9EBC for ; Thu, 24 Oct 2019 13:05:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 13BFD2166E for ; Thu, 24 Oct 2019 13:05:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="exxfDv/k" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393480AbfJXNFM (ORCPT ); Thu, 24 Oct 2019 09:05:12 -0400 Received: from mail-wm1-f66.google.com ([209.85.128.66]:51158 "EHLO mail-wm1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730937AbfJXNFL (ORCPT ); Thu, 24 Oct 2019 09:05:11 -0400 Received: by mail-wm1-f66.google.com with SMTP id q13so2769170wmj.0; Thu, 24 Oct 2019 06:05:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=sc0yPCVfEQ0pK0/NGjx6W8CYcehYA6aagiijMrOtEy4=; b=exxfDv/km68V7c4r9FPU52WfWb76KUvX5zkVz2NErtmjm6rlB8EFH1DOXat1A4mcph QrQmRA39YPKwmyIv9FZY5HvuzdDxYgN2hKhNVeKGcwF6sEGHcJmRAfxUIZ0Waw1vNTwa UhWNW9PZxwPi7HkiK0tkHEyAgl3vBf5FAIoDIXcgnI2fEA6Ypkkn+gu6+HTb46pm7kdP k+mgG2U9QwyI4Irzp6ZrLwwSoSarNb2IOVMc3OrAnkqz92JXiUW+SGV0z9VYnKXCw9dc 4v/iWnH2NAA0ZSFqcmZ75TVo4toh5rl2X7Z4NXLep/HXAz/QDySZBThD4Nfzj1Bvl30t TbkQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=sc0yPCVfEQ0pK0/NGjx6W8CYcehYA6aagiijMrOtEy4=; b=WU3uHJa8bbMFOap5m3Dp3myLlz3hOpeORlOeLp9F2xxa/SgPXctLI2cSrCa36nxYDu adi/0L8oBWxq45gL+p12h6SWXbj/cm2mNmoSmJ4jIwogl/0V+rabvXBbk7Tr8eSC3cYV KnOzR6jCscMe6LXiQwcYqLyFfDlKLqQTc9Bsd5oA4+bpjqiA1pWPKJBB62Duqj879Sgr wMR6XGdNyczWVTuNWbu3y3UFZ4NzraGFStDN2tw5xosQmgvR/Uby65LnUuTRphaHrbct 7FSFPQ4iybuKnhYHXv62K9DCl6epgT8vrcskCMk9XOhdHmi6v02IR7+Qk6b1MbGG1HOo tI/Q== X-Gm-Message-State: APjAAAX9s3XOavZ5EgGsL4PMw15mueT1EtXMXueQ0PcORhrUDUdOLxq5 CAgdmwLxSmQZsuabp2ODOSU= X-Google-Smtp-Source: APXvYqy3mBItBc7WBTEJWCzdSzT7R8VrpWyTlH9x1LISMYKss+/Eg/CcOUqTfkhwDq/TtrKFIpfAwg== X-Received: by 2002:a7b:cb05:: with SMTP id u5mr4534526wmj.36.1571922308834; Thu, 24 Oct 2019 06:05:08 -0700 (PDT) Received: from andrea.guest.corp.microsoft.com ([2a01:110:8012:1010:e187:86b0:69d4:5ba5]) by smtp.gmail.com with ESMTPSA id l14sm10298010wrr.37.2019.10.24.06.05.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Oct 2019 06:05:07 -0700 (PDT) Date: Thu, 24 Oct 2019 15:05:02 +0200 From: Andrea Parri To: Dmitry Vyukov Cc: Christian Brauner , Will Deacon , LKML , bsingharora@gmail.com, Marco Elver , stable , syzbot , syzkaller-bugs Subject: Re: [PATCH v6] taskstats: fix data-race Message-ID: <20191024130502.GA11335@andrea.guest.corp.microsoft.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) 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 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. ;-) 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