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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 C19ABC35247 for ; Fri, 7 Feb 2020 00:27:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 885A62064C for ; Fri, 7 Feb 2020 00:27:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="S2cuqs7j" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726956AbgBGA1j (ORCPT ); Thu, 6 Feb 2020 19:27:39 -0500 Received: from hqnvemgate25.nvidia.com ([216.228.121.64]:3017 "EHLO hqnvemgate25.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726509AbgBGA1j (ORCPT ); Thu, 6 Feb 2020 19:27:39 -0500 Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate25.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Thu, 06 Feb 2020 16:27:13 -0800 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate101.nvidia.com (PGP Universal service); Thu, 06 Feb 2020 16:27:38 -0800 X-PGP-Universal: processed; by hqpgpgate101.nvidia.com on Thu, 06 Feb 2020 16:27:38 -0800 Received: from [10.110.48.28] (10.124.1.5) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Fri, 7 Feb 2020 00:27:35 +0000 Subject: Re: [PATCH] mm: fix a data race in put_page() To: Qian Cai CC: Jan Kara , David Hildenbrand , , , , , , References: <20200206145501.GD26114@quack2.suse.cz> <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com> <235ACF21-35BE-4EDA-BA64-9553DA53BF12@lca.pw> From: John Hubbard X-Nvconfidentiality: public Message-ID: <90ab0b09-0f70-fe6d-259e-f529f4ef9174@nvidia.com> Date: Thu, 6 Feb 2020 16:27:34 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.2 MIME-Version: 1.0 In-Reply-To: <235ACF21-35BE-4EDA-BA64-9553DA53BF12@lca.pw> X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL105.nvidia.com (172.20.187.12) To HQMAIL107.nvidia.com (172.20.187.13) Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1581035233; bh=pB+e0J8CDnr1RA7mN63T5ahx5bEiD5ff9DW/6lG1XnU=; h=X-PGP-Universal:Subject:To:CC:References:From:X-Nvconfidentiality: Message-ID:Date:User-Agent:MIME-Version:In-Reply-To: X-Originating-IP:X-ClientProxiedBy:Content-Type:Content-Language: Content-Transfer-Encoding; b=S2cuqs7jxI+Yz2WpcZsM50KxZjg3hS9AyvT0oVMZ2cnpQANtBqBm0zXp50hPdyFne ACRVBJR2K16wvgfAJhlHAs23BYy6bcJUgosLeSf8YsH1uJgoR1MCnd40x7xQt5GvXi 7L7OE5RrS29BrdAEloemxLdPjkE5zuLYwAARPWFLlLhcLHq+tIa8ffcOdVCVgGNRqx v166Z9J+14Y9nrqOm2uI+V9CUEXlqzDjfONv2qklVvXQcFQe8II8Dls41h8AaxNE8m hUMYvggOel0fyy3lBlJWoarQ0w6UrRT/cvR3jo5yZmSILDj66SAxvwSJMJpa5A+7eZ i4Ybo85P6ShXQ== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/6/20 4:18 PM, Qian Cai wrote: >> On Feb 6, 2020, at 6:34 PM, John Hubbard wrote: >> On 2/6/20 7:23 AM, Qian Cai wrote: >>>> On Feb 6, 2020, at 9:55 AM, Jan Kara wrote: >>>> I don't think the problem is real. The question is how to make KCSAN happy >>>> in a way that doesn't silence other possibly useful things it can find and >>>> also which makes it most obvious to the reader what's going on... IMHO >>>> using READ_ONCE() fulfills these targets nicely - it is free >>>> performance-wise in this case, it silences the checker without impacting >>>> other races on page->flags, its kind of obvious we don't want the load torn >>>> in this case so it makes sense to the reader (although a comment may be >>>> nice). >>> >>> Actually, use the data_race() macro there fulfilling the same purpose too, i.e, silence the splat here but still keep searching for other races. >>> >> >> Yes, but both READ_ONCE() and data_race() would be saying untrue things about this code, >> and that somewhat offends my sense of perfection... :) >> >> * READ_ONCE(): this field need not be restricted to being read only once, so the >> name is immediately wrong. We're using side effects of READ_ONCE(). >> >> * data_race(): there is no race on the N bits worth of page zone number data. There >> is only a perceived race, due to tools that look at word-level granularity. >> >> I'd propose one or both of the following: >> >> a) Hope that Marco (I've fixed the typo in his name. --jh) has an idea to enhance KCSAN so as to support this model of >> access, and/or > > A similar thing was brought up before, i.e., anything compared to zero is immune to load-tearing > issues, but it is rather difficult to implement it in the compiler, so it was settled to use data_race(), > > https://lore.kernel.org/lkml/CANpmjNN8J1oWtLPHTgCwbbtTuU_Js-8HD=cozW5cYkm8h-GTBg@mail.gmail.com/#r > Thanks for that link to the previous discussion, good context. >> >> b) Add a new, better-named macro to indicate what's going on. Initial bikeshed-able >> candidates: >> >> READ_RO_BITS() >> READ_IMMUTABLE_BITS() >> ...etc... >> > > Actually, Linus might hate those kinds of complication rather than a simple data_race() macro, > > https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/ > Another good link. However, my macros above haven't been proposed yet, and I'm perfectly comfortable proposing something that Linus *might* (or might not!) hate. No point in guessing about it, IMHO. If you want, I'll be happy to put on my flame suit and post a patchset proposing READ_IMMUTABLE_BITS() (or a better-named thing, if someone has another name idea). :) thanks, -- John Hubbard NVIDIA