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=-11.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 10257C4363C for ; Wed, 7 Oct 2020 19:40:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BACBE2083B for ; Wed, 7 Oct 2020 19:40:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728392AbgJGTkv (ORCPT ); Wed, 7 Oct 2020 15:40:51 -0400 Received: from netrider.rowland.org ([192.131.102.5]:43061 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728186AbgJGTkv (ORCPT ); Wed, 7 Oct 2020 15:40:51 -0400 Received: (qmail 470110 invoked by uid 1000); 7 Oct 2020 15:40:50 -0400 Date: Wed, 7 Oct 2020 15:40:50 -0400 From: Alan Stern To: "Paul E. McKenney" Cc: Luc Maranget , 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, dlustig@nvidia.com, joel@joelfernandes.org, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Message-ID: <20201007194050.GC468921@rowland.harvard.edu> References: <20201003171338.GA323226@rowland.harvard.edu> <20201005151557.4bcxumreoekgwmsa@yquem.inria.fr> <20201005155310.GH376584@rowland.harvard.edu> <20201005165223.GB29330@paulmck-ThinkPad-P72> <20201005181949.GA387079@rowland.harvard.edu> <20201005191801.GF29330@paulmck-ThinkPad-P72> <20201005194834.GB389867@rowland.harvard.edu> <20201006163954.GM29330@paulmck-ThinkPad-P72> <20201006170525.GA423499@rowland.harvard.edu> <20201007175040.GQ29330@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201007175040.GQ29330@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Wed, Oct 07, 2020 at 10:50:40AM -0700, Paul E. McKenney wrote: > And here is the updated version. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit b7cd60d4b41ad56b32b36b978488f509c4f7e228 > Author: Alan Stern > Date: Tue Oct 6 09:38:37 2020 -0700 > > manual/kernel: Add LB+mb+data litmus test Let's change this to: manual/kernel: Add LB data dependency test with no intermediate variable Without that extra qualification, people reading just the title would wonder why we need a simple LB litmus test in the archive. > > Test whether herd7 can detect a data dependency when there is no > intermediate local variable, as in WRITE_ONCE(*x, READ_ONCE(*y)). > Commit 0f3f8188a326 in herdtools fixed an oversight which caused such > dependencies to be missed. > > Signed-off-by: Alan Stern > Signed-off-by: Paul E. McKenney > > diff --git a/manual/kernel/C-LB+mb+data.litmus b/manual/kernel/C-LB+mb+data.litmus > new file mode 100644 > index 0000000..0cf9a7a > --- /dev/null > +++ b/manual/kernel/C-LB+mb+data.litmus > @@ -0,0 +1,27 @@ > +C LB+mb+data > +(* > + * Result: Never > + * > + * Test whether herd7 can detect a data dependency when there is no > + * intermediate local variable, as in WRITE_ONCE(*x, READ_ONCE(*y)). > + * Commit 0f3f8188a326 in herdtools fixed an oversight which caused such > + * dependencies to be missed. You changed this comment! It should have remained the way it was: + * Versions of herd7 prior to commit 0f3f8188a326 ("[herd] Fix dependency + * definition") recognize data dependencies only when they flow through + * an intermediate local variable. Since the dependency in P1 doesn't, + * those versions get the wrong answer for this test. > + *) > + > +{} > + > +P0(int *x, int *y) > +{ > + int r1; > + > + r1 = READ_ONCE(*x); > + smp_mb(); > + WRITE_ONCE(*y, r1); > +} > + > +P1(int *x, int *y) > +{ > + WRITE_ONCE(*x, READ_ONCE(*y)); > +} > + > +exists (0:r1=1) Alan