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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 17F18C43143 for ; Tue, 2 Oct 2018 10:13:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CF9E620878 for ; Tue, 2 Oct 2018 10:12:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CF9E620878 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zytor.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727239AbeJBQz2 (ORCPT ); Tue, 2 Oct 2018 12:55:28 -0400 Received: from terminus.zytor.com ([198.137.202.136]:43987 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726244AbeJBQz2 (ORCPT ); Tue, 2 Oct 2018 12:55:28 -0400 Received: from terminus.zytor.com (localhost [127.0.0.1]) by terminus.zytor.com (8.15.2/8.15.2) with ESMTPS id w92ACHum1916508 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Tue, 2 Oct 2018 03:12:17 -0700 Received: (from tipbot@localhost) by terminus.zytor.com (8.15.2/8.15.2/Submit) id w92ACHqQ1916505; Tue, 2 Oct 2018 03:12:17 -0700 Date: Tue, 2 Oct 2018 03:12:17 -0700 X-Authentication-Warning: terminus.zytor.com: tipbot set sender to tipbot@zytor.com using -f From: "tip-bot for Paul E. McKenney" Message-ID: Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, eranian@google.com, acme@redhat.com, mingo@kernel.org, andrea.parri@amarulasolutions.com, jolsa@redhat.com, tglx@linutronix.de, paulmck@linux.vnet.ibm.com, vincent.weaver@maine.edu, alexander.shishkin@linux.intel.com, hpa@zytor.com, peterz@infradead.org Reply-To: alexander.shishkin@linux.intel.com, hpa@zytor.com, vincent.weaver@maine.edu, peterz@infradead.org, paulmck@linux.vnet.ibm.com, tglx@linutronix.de, jolsa@redhat.com, andrea.parri@amarulasolutions.com, acme@redhat.com, mingo@kernel.org, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, eranian@google.com In-Reply-To: <20180926182920.27644-4-paulmck@linux.ibm.com> References: <20180926182920.27644-4-paulmck@linux.ibm.com> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] tools/memory-model: Add more LKMM limitations Git-Commit-ID: d8fa25c4efde0e5f31a427202e583d73d3f021c4 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: d8fa25c4efde0e5f31a427202e583d73d3f021c4 Gitweb: https://git.kernel.org/tip/d8fa25c4efde0e5f31a427202e583d73d3f021c4 Author: Paul E. McKenney AuthorDate: Wed, 26 Sep 2018 11:29:19 -0700 Committer: Ingo Molnar CommitDate: Tue, 2 Oct 2018 10:28:04 +0200 tools/memory-model: Add more LKMM limitations This commit adds more detail about compiler optimizations and not-yet-modeled Linux-kernel APIs. Signed-off-by: Paul E. McKenney Reviewed-by: Andrea Parri Cc: Alexander Shishkin Cc: Arnaldo Carvalho de Melo Cc: Jiri Olsa Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Cc: akiyks@gmail.com Cc: boqun.feng@gmail.com Cc: dhowells@redhat.com Cc: j.alglave@ucl.ac.uk Cc: linux-arch@vger.kernel.org Cc: luc.maranget@inria.fr Cc: npiggin@gmail.com Cc: parri.andrea@gmail.com Cc: stern@rowland.harvard.edu Cc: will.deacon@arm.com Link: http://lkml.kernel.org/r/20180926182920.27644-4-paulmck@linux.ibm.com Signed-off-by: Ingo Molnar --- tools/memory-model/README | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) diff --git a/tools/memory-model/README b/tools/memory-model/README index ee987ce20aae..acf9077cffaa 100644 --- a/tools/memory-model/README +++ b/tools/memory-model/README @@ -171,6 +171,12 @@ The Linux-kernel memory model has the following limitations: particular, the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING" sections). + Note that this limitation in turn limits LKMM's ability to + accurately model address, control, and data dependencies. + For example, if the compiler can deduce the value of some variable + carrying a dependency, then the compiler can break that dependency + by substituting a constant of that value. + 2. Multiple access sizes for a single variable are not supported, and neither are misaligned or partially overlapping accesses. @@ -190,6 +196,36 @@ The Linux-kernel memory model has the following limitations: However, a substantial amount of support is provided for these operations, as shown in the linux-kernel.def file. + a. When rcu_assign_pointer() is passed NULL, the Linux + kernel provides no ordering, but LKMM models this + case as a store release. + + b. The "unless" RMW operations are not currently modeled: + atomic_long_add_unless(), atomic_add_unless(), + atomic_inc_unless_negative(), and + atomic_dec_unless_positive(). These can be emulated + in litmus tests, for example, by using atomic_cmpxchg(). + + c. The call_rcu() function is not modeled. It can be + emulated in litmus tests by adding another process that + invokes synchronize_rcu() and the body of the callback + function, with (for example) a release-acquire from + the site of the emulated call_rcu() to the beginning + of the additional process. + + d. The rcu_barrier() function is not modeled. It can be + emulated in litmus tests emulating call_rcu() via + (for example) a release-acquire from the end of each + additional call_rcu() process to the site of the + emulated rcu-barrier(). + + e. Sleepable RCU (SRCU) is not modeled. It can be + emulated, but perhaps not simply. + + f. Reader-writer locking is not modeled. It can be + emulated in litmus tests using atomic read-modify-write + operations. + The "herd7" tool has some additional limitations of its own, apart from the memory model: @@ -204,3 +240,6 @@ the memory model: Some of these limitations may be overcome in the future, but others are more likely to be addressed by incorporating the Linux-kernel memory model into other tools. + +Finally, please note that LKMM is subject to change as hardware, use cases, +and compilers evolve.