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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 90AF1C35247 for ; Mon, 3 Feb 2020 16:02:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 642CA21741 for ; Mon, 3 Feb 2020 16:02:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580745754; bh=ng4f3LBqSvC9IA75BiWlMTBK0oaU1qkTurbY5hi6jmU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=UjfA6kGgzAUSh47goXcV18nIfDlGz/cWWssHzWMfemfsM/wT+9XOVlwreI38fwpXk B3w2ZaYWcDR8FlOdzrtPZuPCKowDo+eNqDNsq8u+VvLr9erqu8cJ1z9eepzZPg62Kg d2eJJIQ2DU68Smvl4iQZWibhWEtt0iZFmIyRw6Zk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728594AbgBCQCd (ORCPT ); Mon, 3 Feb 2020 11:02:33 -0500 Received: from mail.kernel.org ([198.145.29.99]:56408 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727869AbgBCQCd (ORCPT ); Mon, 3 Feb 2020 11:02:33 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6A6D120721; Mon, 3 Feb 2020 16:02:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580745752; bh=ng4f3LBqSvC9IA75BiWlMTBK0oaU1qkTurbY5hi6jmU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=doY9Zt0A1fwTGeIE+xiu6Je684fdhKakhzIz5vjAegJNO84761iCkOGDNr2CnJ7RO 7RTKo8x0i5b+gFRhnzEkyVONsAqVsjD07QnYJDRvYAaiIQx8HHonZr2ApJS00tziUG yYqIxmsb5fQ8jX15h5CWOY4ylOYo3tqOPWIupZZE= Date: Mon, 3 Feb 2020 16:02:28 +0000 From: Will Deacon To: "Paul E. McKenney" Cc: David Laight , 'Eric Dumazet' , Peter Zijlstra , Thomas Gleixner , the arch/x86 maintainers , LKML , Marco Elver Subject: Re: Confused about hlist_unhashed_lockless() Message-ID: <20200203160227.GA7274@willie-the-truck> References: <20200131164308.GA5175@willie-the-truck> <20200131184322.GA11457@worktop.programming.kicks-ass.net> <26258e70c35e4c108173a27317e64a0b@AcuMS.aculab.com> <20200203155839.GK2935@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200203155839.GK2935@paulmck-ThinkPad-P72> 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 Mon, Feb 03, 2020 at 07:58:39AM -0800, Paul E. McKenney wrote: > On Mon, Feb 03, 2020 at 03:45:54PM +0000, David Laight wrote: > > From: Eric Dumazet > > > Sent: 31 January 2020 18:53 > > > > > > On Fri, Jan 31, 2020 at 10:48 AM Eric Dumazet wrote: > > > > > > > > > > > This is nice, now with have data_race() > > > > > > > > Remember these patches were sent 2 months ago, at a time we were > > > > trying to sort out things. > > > > > > > > data_race() was merged a few days ago. > > > > > > Well, actually data_race() is not there yet anyway. > > > > Shouldn't it be NO_DATA_RACE() ?? > > No, because you use data_race() when there really are data races, but you > want KCSAN to ignore them. For example, diagnostic code that doesn't > participate in the actual concurrency design and that doesn't run all > that often might use data_race(). For another example, if a developer > knew that data races existed, but that the compiler could not reasonably > do anything untoward with those data races, that developer might well > choose to use data_race() instead of READ_ONCE(). Especially if the > access in question was on a fastpath where helpful compiler optimizations > would be prohibited by use of READ_ONCE(). Yes, and in this particular case I think we can remove some WRITE_ONCE()s from the non-RCU hlist code too (similarly for hlist_nulls). Will