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=-8.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 09AC0C4727D for ; Sun, 4 Oct 2020 01:40:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D15762067C for ; Sun, 4 Oct 2020 01:40:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726227AbgJDBkY (ORCPT ); Sat, 3 Oct 2020 21:40:24 -0400 Received: from netrider.rowland.org ([192.131.102.5]:36373 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1726122AbgJDBkX (ORCPT ); Sat, 3 Oct 2020 21:40:23 -0400 Received: (qmail 332975 invoked by uid 1000); 3 Oct 2020 21:40:22 -0400 Date: Sat, 3 Oct 2020 21:40:22 -0400 From: Alan Stern To: "Paul E. McKenney" Cc: Akira Yokosawa , 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, dlustig@nvidia.com, joel@joelfernandes.org, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: [PATCH] tools: memory-model: Document that the LKMM can easily miss control dependencies Message-ID: <20201004014022.GA332600@rowland.harvard.edu> References: <20201001045116.GA5014@paulmck-ThinkPad-P72> <20201001161529.GA251468@rowland.harvard.edu> <20201001213048.GF29330@paulmck-ThinkPad-P72> <20201003132212.GB318272@rowland.harvard.edu> <045c643f-6a70-dfdf-2b1e-f369a667f709@gmail.com> <20201003171338.GA323226@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201003171338.GA323226@rowland.harvard.edu> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Add a small section to the litmus-tests.txt documentation file for the Linux Kernel Memory Model explaining that the memory model often fails to recognize certain control dependencies. Suggested-by: Akira Yokosawa Signed-off-by: Alan Stern --- tools/memory-model/Documentation/litmus-tests.txt | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) Index: usb-devel/tools/memory-model/Documentation/litmus-tests.txt =================================================================== --- usb-devel.orig/tools/memory-model/Documentation/litmus-tests.txt +++ usb-devel/tools/memory-model/Documentation/litmus-tests.txt @@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory m carrying a dependency, then the compiler can break that dependency by substituting a constant of that value. + Conversely, LKMM sometimes doesn't recognize that a particular + optimization is not allowed, and as a result, thinks that a + dependency is not present (because the optimization would break it). + The memory model misses some pretty obvious control dependencies + because of this limitation. A simple example is: + + r1 = READ_ONCE(x); + if (r1 == 0) + smp_mb(); + WRITE_ONCE(y, 1); + + There is a control dependency from the READ_ONCE to the WRITE_ONCE, + even when r1 is nonzero, but LKMM doesn't realize this and thinks + that the write may execute before the read if r1 != 0. (Yes, that + doesn't make sense if you think about it, but the memory model's + intelligence is limited.) + 2. Multiple access sizes for a single variable are not supported, and neither are misaligned or partially overlapping accesses.