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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 44A08C4332F for ; Thu, 8 Dec 2022 21:37:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229793AbiLHVhp (ORCPT ); Thu, 8 Dec 2022 16:37:45 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45278 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229678AbiLHVhl (ORCPT ); Thu, 8 Dec 2022 16:37:41 -0500 Received: from netrider.rowland.org (netrider.rowland.org [192.131.102.5]) by lindbergh.monkeyblade.net (Postfix) with SMTP id 1890E10B6 for ; Thu, 8 Dec 2022 13:37:36 -0800 (PST) Received: (qmail 744325 invoked by uid 1000); 8 Dec 2022 16:37:35 -0500 Date: Thu, 8 Dec 2022 16:37:35 -0500 From: "stern@rowland.harvard.edu" To: Boqun Feng Cc: Jonas Oberhauser , "paulmck@kernel.org" , "parri.andrea@gmail.com" , "will@kernel.org" , "peterz@infradead.org" , "npiggin@gmail.com" , "dhowells@redhat.com" , "j.alglave@ucl.ac.uk" , "luc.maranget@inria.fr" , "akiyks@gmail.com" , "dlustig@nvidia.com" , "joel@joelfernandes.org" , "urezki@gmail.com" , "quic_neeraju@quicinc.com" , "frederic@kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v2] tools: memory-model: Make plain accesses carry dependencies Message-ID: References: <20221203204405.GW4001@paulmck-ThinkPad-P17-Gen-1> <20221203231122.GZ4001@paulmck-ThinkPad-P17-Gen-1> <43c7ea9ebdd14497b85633950b014240@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 06, 2022 at 12:52:47PM -0800, Boqun Feng wrote: > On Tue, Dec 06, 2022 at 12:46:58PM -0800, Boqun Feng wrote: > > Thank you, Alan! One question though, can a "smart" compiler optimize > > out the case below, with the same logic? > > > > void P0(int *x, int *y, int *a) > > { > > int r1, r2; > > > > r1 = READ_ONCE(*x); // A > > > > *a = r1 & 0xffff; // B > > > > r2 = *a & 0xffff0000; // C > > > > WRITE_ONCE(*y, r2); // D > > > > } > > > > I think we have A ->data B ->rfi C ->data D, however a "smart" compiler > > can figure out that r2 is actually zero, right? And the code get > > optimized to: > > > > r1 = READ_ONCE(*x); > > r2 = 0; > > WRITE_ONCE(*y, r2); > > *a = r1 & 0xffff; > > > > and break the dependency. Yes, that could happen. > > I know that our memory model is actually unware of the differences of > > syntatics dependencies vs semantics syntatics, so one may argue that in > > the (data; rfi) example above the compiler optimization is outside the > > scope of LKMM, but won't the same reasoning apply to the (addr; rfi) > > example from you? The WRITE_ONCE() _syntatically_ depends on load of > > a[r1], therefore even a "smart" compiler can figure out the value, LKMM > > I guess it should be that r2 (i.e. the load of a[r1]) _syntatically_ > depends on the value of r1. Yes. But more to the point, the LKMM already has this problem for ordinary dependencies. If you do: r1 = READ_ONCE(*x); r2 = r1 & 0x0000ffff; r3 = r2 & 0xffff0000; WRITE_ONCE(*y, r3); then the LKMM will think there is a dependency (because there is a _syntactic_ dependency), but the compiler is likely to realize that there isn't a _semantic_ dependency and will destroy the ordering. We warn people about this possibility, and the same warning applies to dependencies carried by plain accesses. So I don't think this is a reason to object to Jonas's carries-dep relation. Alan