From nobody Sat May 2 08:58:24 2026 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 2302BC43334 for ; Mon, 13 Jun 2022 15:15:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1387544AbiFMPPF (ORCPT ); Mon, 13 Jun 2022 11:15:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36270 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1387207AbiFMPNv (ORCPT ); Mon, 13 Jun 2022 11:13:51 -0400 Received: from mailout1.rbg.tum.de (mailout1.rbg.tum.de [131.159.0.201]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 46AABB6E; Mon, 13 Jun 2022 05:28:25 -0700 (PDT) Received: from mailrelay1.rbg.tum.de (mailrelay1.in.tum.de [131.159.254.14]) by mailout1.rbg.tum.de (Postfix) with ESMTPS id 895E54D; Mon, 13 Jun 2022 14:28:20 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=in.tum.de; s=20220209; t=1655123300; bh=Fy/GQ0kuNY2dgt6/NaElsORQBYUxRHvsByxnFzQadfw=; h=From:To:Cc:Subject:Date:From; b=qm01H+Guv33t7GmzpIwNun8w0ScI8wczrt31RkjIBbbD+NR+ViDZnnRkHphKSyqle iaq+2v8W6y8gapcRjwZj64HYNe4C4KuM3GzAjoK/gcfK9uKFzTuZBg9bQruSiNohPD KyVLlSyAc/JcNv7aU3W8s5EQyyXXYXU2m9M2HMc85r6xQ1c4wmnsRCx4VX0N2nbovj Izs50u97ViKHQaGWmaLtlKzc3uak2ptoyMQvw6Ctf4f37VwzOrlT3CmdjTSSRGxTH9 /fpTuYaskC2WD6Li/64gqcWpwlYoE8nUMF6yzyrOhSSjrk2ZRnIYK/n6NwKGhpWde8 yO6Q0osrw6cFQ== Received: by mailrelay1.rbg.tum.de (Postfix, from userid 112) id 846E428B; Mon, 13 Jun 2022 14:28:20 +0200 (CEST) Received: from mailrelay1.rbg.tum.de (localhost [127.0.0.1]) by mailrelay1.rbg.tum.de (Postfix) with ESMTP id 527C428A; Mon, 13 Jun 2022 14:28:20 +0200 (CEST) Received: from mail.in.tum.de (mailproxy.in.tum.de [IPv6:2a09:80c0::78]) by mailrelay1.rbg.tum.de (Postfix) with ESMTPS id 47EB4286; Mon, 13 Jun 2022 14:28:20 +0200 (CEST) Received: by mail.in.tum.de (Postfix, from userid 112) id 411744A03AB; Mon, 13 Jun 2022 14:28:20 +0200 (CEST) Received: (Authenticated sender: heidekrp) by mail.in.tum.de (Postfix) with ESMTPSA id CE83B4A02E6; Mon, 13 Jun 2022 14:28:19 +0200 (CEST) (Extended-Queue-bit xtech_cr@fff.in.tum.de) From: =?UTF-8?q?Paul=20Heidekr=C3=BCger?= To: Alan Stern , Andrea Parri , Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa , Daniel Lustig , Joel Fernandes , =?UTF-8?q?Paul=20Heidekr=C3=BCger?= , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Cc: Marco Elver , Charalampos Mainas , Pramod Bhatotia , Soham Chakraborty , Martin Fink Subject: [PATCH] tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt Date: Mon, 13 Jun 2022 12:27:44 +0000 Message-Id: <20220613122744.373516-1-paul.heidekrueger@in.tum.de> X-Mailer: git-send-email 2.35.1 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org As discussed, clarify LKMM not recognizing certain kinds of orderings. In particular, highlight the fact that LKMM might deliberately make weaker guarantees than compilers and architectures. Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.= de/T/#u Signed-off-by: Paul Heidekr=C3=BCger Cc: Marco Elver Cc: Charalampos Mainas Cc: Pramod Bhatotia Cc: Soham Chakraborty Cc: Martin Fink --- .../Documentation/litmus-tests.txt | 29 ++++++++++++------- 1 file changed, 19 insertions(+), 10 deletions(-) diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memo= ry-model/Documentation/litmus-tests.txt index 8a9d5d2787f9..623059eff84e 100644 --- a/tools/memory-model/Documentation/litmus-tests.txt +++ b/tools/memory-model/Documentation/litmus-tests.txt @@ -946,22 +946,31 @@ Limitations of the Linux-kernel memory model (LKMM) i= nclude: carrying a dependency, then the compiler can break that dependency by substituting a constant of that value. =20 - 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: + Conversely, LKMM will sometimes overstate the amount of reordering + done by architectures and compilers, leading it to missing some + pretty obvious orderings. A simple example is: =20 r1 =3D READ_ONCE(x); if (r1 =3D=3D 0) smp_mb(); WRITE_ONCE(y, 1); =20 - 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 !=3D 0. (Yes, that - doesn't make sense if you think about it, but the memory model's - intelligence is limited.) + There is no dependency from the WRITE_ONCE() to the READ_ONCE(), + and as a result, LKMM does not assume ordering. However, the + smp_mb() in the if branch will prevent architectures from + reordering the WRITE_ONCE() ahead of the READ_ONCE() but only if r1 + is 0. This, by definition, is not a control dependency, yet + ordering is guaranteed in some cases, depending on the READ_ONCE(), + which LKMM doesn't recognize. + + It is clear that it is not dangerous in the slightest for LKMM to + make weaker guarantees than architectures. In fact, it is + desirable, as it gives compilers room for making optimizations. + For instance, because a value of 0 triggers undefined behavior + elsewhere, a clever compiler might deduce that r1 can never be 0 in + the if condition. As a result, said clever compiler might deem it + safe to optimize away the smp_mb(), eliminating the branch and + any ordering an architecture would guarantee otherwise. =20 2. Multiple access sizes for a single variable are not supported, and neither are misaligned or partially overlapping accesses. --=20 2.35.1