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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 D54BEC35242 for ; Mon, 17 Feb 2020 03:25:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AC18D2072C for ; Mon, 17 Feb 2020 03:25:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727901AbgBQDZH (ORCPT ); Sun, 16 Feb 2020 22:25:07 -0500 Received: from helcar.hmeau.com ([216.24.177.18]:48208 "EHLO deadmen.hmeau.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726742AbgBQDZH (ORCPT ); Sun, 16 Feb 2020 22:25:07 -0500 Received: from gondobar.mordor.me.apana.org.au ([192.168.128.4] helo=gondobar) by deadmen.hmeau.com with esmtps (Exim 4.89 #2 (Debian)) id 1j3X1r-0003Fn-Gw; Mon, 17 Feb 2020 11:25:03 +0800 Received: from herbert by gondobar with local (Exim 4.89) (envelope-from ) id 1j3X1m-0003P1-Vz; Mon, 17 Feb 2020 11:24:59 +0800 Date: Mon, 17 Feb 2020 11:24:58 +0800 From: Herbert Xu To: "Jason A. Donenfeld" Cc: eric.dumazet@gmail.com, cai@lca.pw, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len() Message-ID: <20200217032458.kwatitz3pvxeb25w@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200206163844.GA432041@zx2c4.com> X-Newsgroups: apana.lists.os.linux.kernel,apana.lists.os.linux.netdev Organization: Core User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Fixing these KCSAN warnings is actively making the kernel worse. Why are we still doing this? Thanks, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt