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 37E24C33CAC for ; Thu, 6 Feb 2020 17:10:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 07099214AF for ; Thu, 6 Feb 2020 17:10:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="IlZd8Ec2" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727791AbgBFRKh (ORCPT ); Thu, 6 Feb 2020 12:10:37 -0500 Received: from mail-pj1-f65.google.com ([209.85.216.65]:56067 "EHLO mail-pj1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727440AbgBFRKh (ORCPT ); Thu, 6 Feb 2020 12:10:37 -0500 Received: by mail-pj1-f65.google.com with SMTP id d5so235870pjz.5; Thu, 06 Feb 2020 09:10:36 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=7+lAwSYC8iTX7zrUHCptKd74zLe4G6IoJxg7yC+WSMw=; b=IlZd8Ec2CeQUIMo7JKROxECgERL2W7KqbQy9dwxtSVyxmj93wckSPexLQn8vAZfnxu J7/RGwzUnnmkGEhvN1mxMbYEd1f+Frd39o+YPP5MeafHniRmjuqpp6FsfEkijzMMvAZR l+jyyy0UUcVjpLktAWrHKC0uPiVJ7wnZfnXOaFaTgXlUhgBEWCCEbwF+Lob5Tt7aclu2 T8pu+TsX/ciwqv78yDab1ogSeoRSYgDgleCDxEUTu+ASdaOnoUQxURCA10PrV1Xkn9Uu gaarc6K9R3nMu3GsPseBdYC8NQxpW5saqmouwXb7FfCIFWUJ7e4JwEpduQ2kvPYMjwax J85Q== 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=7+lAwSYC8iTX7zrUHCptKd74zLe4G6IoJxg7yC+WSMw=; b=nkcvPTRKUE8mAL419gJCPXRcAc2PnTrtQVY6b84QJdy5q3seHO1piMStjGRB2X9kR/ rxF1fbFxow96+FQIK9fSyWx9xBwhomRd+ut+/E2UNXO3WB3RNFBPbMX2R5qoei7vU+d5 A2QS0dWPXP2ys/mbQR9cWXs4sCA+6ESUVazhFBvGIgL1Jz0KqnKpqVBJLIVa4bbyqH9O fTBZ5WKYoKBDx1jEGsLFG0wqUUJmXVuJdtwEhixpqeilDL16jCOdHKFZvHU+V9PovCWT Nm6exgx5h2u/z+lStBybWnzYF+HZkee74gGUzkbXQN5Zib9Zxd8pn1JIZASNVJIPXaU/ 6YJQ== X-Gm-Message-State: APjAAAVi2nAQhKQBaXemSiPSubmcENP3Zv9qOQxvAv5KUx6wBS/mWjTa 5vA4QYvvyop3odksDQOp1O0= X-Google-Smtp-Source: APXvYqwp6YfS5qUU3b7hVaaDpvcsi7uYY+1y8VNhzJSn4CjyUw2P62KTa/pOo0hsVTWBAqMoXCS/gw== X-Received: by 2002:a17:90a:7307:: with SMTP id m7mr5699761pjk.75.1581009036305; Thu, 06 Feb 2020 09:10:36 -0800 (PST) Received: from ?IPv6:2620:15c:2c1:200:55c7:81e6:c7d8:94b? ([2620:15c:2c1:200:55c7:81e6:c7d8:94b]) by smtp.gmail.com with ESMTPSA id d4sm4131894pjg.19.2020.02.06.09.10.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Feb 2020 09:10:35 -0800 (PST) Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len() To: "Jason A. Donenfeld" , eric.dumazet@gmail.com Cc: cai@lca.pw, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Marco Elver References: <1580841629-7102-1-git-send-email-cai@lca.pw> <20200206163844.GA432041@zx2c4.com> From: Eric Dumazet Message-ID: <453212cf-8987-9f05-ceae-42a4fc3b0876@gmail.com> Date: Thu, 6 Feb 2020 09:10:34 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20200206163844.GA432041@zx2c4.com> Content-Type: text/plain; charset=utf-8 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 2/6/20 8:38 AM, Jason A. Donenfeld wrote: > Hi Eric, > > On Tue, Feb 04, 2020 at 01:40:29PM -0500, Qian Cai wrote: >> - list->qlen--; >> + WRITE_ONCE(list->qlen, list->qlen - 1); > > Sorry I'm a bit late to the party here, but this immediately jumped out. > This generates worse code with a bigger race in some sense: > > list->qlen-- is: > > 0: 83 6f 10 01 subl $0x1,0x10(%rdi) > > whereas WRITE_ONCE(list->qlen, list->qlen - 1) is: > > 0: 8b 47 10 mov 0x10(%rdi),%eax > 3: 83 e8 01 sub $0x1,%eax > 6: 89 47 10 mov %eax,0x10(%rdi) > > Are you sure that's what we want? > > Jason > Unfortunately we do not have ADD_ONCE() or something like that. Sure, on x86 we could get much better code generation. If we agree a READ_ONCE() was needed at the read side, then a WRITE_ONCE() is needed as well on write sides. If we believe load-tearing and/or write-tearing must not ever happen, then we must document this.