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 90BE8C54FCC for ; Mon, 20 Apr 2020 23:12:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 68E3920BED for ; Mon, 20 Apr 2020 23:12:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587424366; bh=prcsJ07hDip1xTde/WHBahDCQn8U4Cvvsfn0XdiVO80=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=ZhbrmmNyJ2DID0XvmVVPwqhXd9S1PyEn2g4AWoNOntGVcsrOqoFFQfKwHHZn3bgkV f2bneLiaLuPLn04gB5dW/nDVVY4gc4r5VodO848a2vZhke+JvlzDfieer8pWEMxVOq 7PLaRoNuh4UBNd9XOhAuIABIJijv7RsQU/1e9NO0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727075AbgDTXMp (ORCPT ); Mon, 20 Apr 2020 19:12:45 -0400 Received: from mail.kernel.org ([198.145.29.99]:39184 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726006AbgDTXMp (ORCPT ); Mon, 20 Apr 2020 19:12:45 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 6BB1C20B1F; Mon, 20 Apr 2020 23:12:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587424364; bh=prcsJ07hDip1xTde/WHBahDCQn8U4Cvvsfn0XdiVO80=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=SE9RHwqQbGZk0A3Q+St4Mh2IYkKdWVFA4lBrTtID2ZrL+OEQpTvr0iXK6TLIn5sqA ElVGxlpUSdVRsIMk4EbltbLkdxVUFkM7AKgsuCucxokKuu/iVX7lJSHLv4SBOPhEOM RLBuX+s10xt1rzylHHwWsi52cD2GyCcMBEKTpxNE= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 3C2353523411; Mon, 20 Apr 2020 16:12:44 -0700 (PDT) Date: Mon, 20 Apr 2020 16:12:44 -0700 From: "Paul E. McKenney" To: Marco Elver Cc: David Laight , 'Petko Manolov' , LKML Subject: Re: [RFC] WRITE_ONCE_INC() and friends Message-ID: <20200420231244.GK17661@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200419094439.GA32841@carbon> <491f0b0bc9e4419d93a78974fd7f44c7@AcuMS.aculab.com> <20200419182957.GA36919@carbon> <8e5a0283ed76465aac19a2b97a27ff15@AcuMS.aculab.com> <20200420150545.GB17661@paulmck-ThinkPad-P72> <20200420225715.GA176156@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420225715.GA176156@google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 21, 2020 at 12:57:15AM +0200, Marco Elver wrote: > On Mon, 20 Apr 2020, Paul E. McKenney wrote: > > > On Sun, Apr 19, 2020 at 09:37:10PM +0000, David Laight wrote: > > > From: Petko Manolov > > > > Sent: 19 April 2020 19:30 > > > > > > > > On 20-04-19 18:02:50, David Laight wrote: > > > > > From: Petko Manolov > > > > > > Sent: 19 April 2020 10:45 > > > > > > Recently I started reading up on KCSAN and at some point I ran into stuff like: > > > > > > > > > > > > WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1); > > > > > > WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); > > > > > > > > > > If all the accesses use READ/WRITE_ONCE() why not just mark the structure > > > > > field 'volatile'? > > > > > > > > This is a bit heavy. I guess you've read this one: > > > > > > > > https://lwn.net/Articles/233479/ > > > > > > I remember reading something similar before. > > > I also remember a very old gcc (2.95?) that did a readback > > > after every volatile write on sparc (to flush the store buffer). > > > That broke everything. > > > > > > I suspect there is a lot more code that is attempting to be lockless > > > these days. > > > Ring buffers (one writer and one reader) are a typical example where > > > you don't need locks but do need to use a consistent value. > > > > > > Now you may also need ordering between accesses - which I think needs > > > more than volatile. > > > > In Petko's patch, all needed ordering is supplied by the fact that it > > is the same variable being read and written. But yes, in many other > > cases, more ordering is required. > > > > > > And no, i am not sure all accesses are through READ/WRITE_ONCE(). If, for > > > > example, all others are from withing spin_lock/unlock pairs then we _may_ not > > > > need READ/WRITE_ONCE(). > > > > > > The cost of volatile accesses is probably minimal unless the > > > code is written assuming the compiler will only access things once. > > > > And there are variables marked as volatile, for example, jiffies. > > > > But one downside of declaring variables volatile is that it can prevent > > KCSAN from spotting violations of the concurrency design for those > > variables. > > Note that, KCSAN currently treats volatiles not as special, except a > list of some known global volatiles (like jiffies). This means, that > KCSAN will tell us about data races involving unmarked volatiles (unless > they're in the list). > > As far as I can tell, this is what we want. At least according to LKMM. > > If, for whatever reason, volatiles should be treated differently, we'll > have to modify the compilers to emit different instrumentation for the > kernel. I stand corrected, then, thank you! In the current arrangement, declaring a variable volatile will cause KCSAN to generate lots of false positives. I don't currently have a strong feeling on changing the current situation with respect to volatile variables. Is there a strong reason to change? The general view of the community, as you say, has been that you don't use the volatile keyword outside of exceptions such as jiffies, atomic_read(), atomic_set(), READ_ONCE(), WRITE_ONCE() and perhaps a few others. Thoughts? Thanx, Paul