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=-0.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 E9798C33CAC for ; Thu, 6 Feb 2020 16:38:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C297221775 for ; Thu, 6 Feb 2020 16:38:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="riCCmfUW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727703AbgBFQiv (ORCPT ); Thu, 6 Feb 2020 11:38:51 -0500 Received: from frisell.zx2c4.com ([192.95.5.64]:38113 "EHLO frisell.zx2c4.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727358AbgBFQiu (ORCPT ); Thu, 6 Feb 2020 11:38:50 -0500 Received: by frisell.zx2c4.com (ZX2C4 Mail Server) with ESMTP id 122bd2e9; Thu, 6 Feb 2020 16:37:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=zx2c4.com; h=date:from:to :cc:subject:message-id:references:mime-version:content-type :in-reply-to; s=mail; bh=zYzK/NEhj71GVhP3Ol6c67NfALM=; b=riCCmfU Wh7M6FwzcPNAwIkELDWRwzGmxOaomzdKYXpzYSu11gUGkpbNlfhfr0ZMimpRPvhU WwXawBd9puKQLo1+lJPndZasMhdfjCuAb4t50LGAaFdCw8gsjNaIliIQN6WxvNgL RTC6BKVCahber8XtQxTOn5a1PFB0NMCq5Ns9af7+v6iMmVQ4TbU/hvODfAuqN0O4 j/6NzoevKpSFF8RRQ0YwJ6yrNP73wDCq0f3ld436w10f72qaezs8NDtAlokjKd2U tRs2rUEigiJK0kJutwGZmeAQP/iHVlOxJaB6KK6glQml44nTLhiqIjzulh7fkIJc RRB+mbpkLrhR1Xw== Received: by frisell.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 062996d6 (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256:NO); Thu, 6 Feb 2020 16:37:41 +0000 (UTC) Date: Thu, 6 Feb 2020 17:38:44 +0100 From: "Jason A. Donenfeld" To: eric.dumazet@gmail.com Cc: cai@lca.pw, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len() Message-ID: <20200206163844.GA432041@zx2c4.com> References: <1580841629-7102-1-git-send-email-cai@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1580841629-7102-1-git-send-email-cai@lca.pw> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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