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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,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 D4D18C41604 for ; Sat, 3 Oct 2020 13:22:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C919207FB for ; Sat, 3 Oct 2020 13:22:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725788AbgJCNWN (ORCPT ); Sat, 3 Oct 2020 09:22:13 -0400 Received: from netrider.rowland.org ([192.131.102.5]:38425 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1725781AbgJCNWN (ORCPT ); Sat, 3 Oct 2020 09:22:13 -0400 Received: (qmail 319931 invoked by uid 1000); 3 Oct 2020 09:22:12 -0400 Date: Sat, 3 Oct 2020 09:22:12 -0400 From: Alan Stern To: "Paul E. McKenney" Cc: parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: Litmus test for question from Al Viro Message-ID: <20201003132212.GB318272@rowland.harvard.edu> References: <20201001045116.GA5014@paulmck-ThinkPad-P72> <20201001161529.GA251468@rowland.harvard.edu> <20201001213048.GF29330@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201001213048.GF29330@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org To expand on my statement about the LKMM's weakness regarding control constructs, here is a litmus test to illustrate the issue. You might want to add this to one of the archives. Alan C crypto-control-data (* * LB plus crypto-control-data plus data * * Expected result: allowed * * This is an example of OOTA and we would like it to be forbidden. * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level) * control-dependent on the preceding READ_ONCE. But the dependencies are * hidden by the form of the conditional control construct, hence the * name "crypto-control-data". The memory model doesn't recognize them. *) {} P0(int *x, int *y) { int r1; r1 = 1; if (READ_ONCE(*x) == 0) r1 = 0; WRITE_ONCE(*y, r1); } P1(int *x, int *y) { WRITE_ONCE(*x, READ_ONCE(*y)); } exists (0:r1=1)