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=-11.3 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 6FF5FC4363C for ; Mon, 5 Oct 2020 02:38:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 382B5207BC for ; Mon, 5 Oct 2020 02:38:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725846AbgJECir (ORCPT ); Sun, 4 Oct 2020 22:38:47 -0400 Received: from netrider.rowland.org ([192.131.102.5]:36005 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1725841AbgJECir (ORCPT ); Sun, 4 Oct 2020 22:38:47 -0400 Received: (qmail 362162 invoked by uid 1000); 4 Oct 2020 22:38:46 -0400 Date: Sun, 4 Oct 2020 22:38:46 -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: <20201005023846.GA359428@rowland.harvard.edu> References: <20201001045116.GA5014@paulmck-ThinkPad-P72> <20201001161529.GA251468@rowland.harvard.edu> <20201001213048.GF29330@paulmck-ThinkPad-P72> <20201003132212.GB318272@rowland.harvard.edu> <20201004233146.GP29330@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201004233146.GP29330@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 Sun, Oct 04, 2020 at 04:31:46PM -0700, Paul E. McKenney wrote: > Nice simple example! How about like this? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit c964f404eabe4d8ce294e59dda713d8c19d340cf > Author: Alan Stern > Date: Sun Oct 4 16:27:03 2020 -0700 > > manual/kernel: Add a litmus test with a hidden dependency > > This commit adds a litmus test that has a data dependency that can be > hidden by control flow. In this test, both the taken and the not-taken > branches of an "if" statement must be accounted for in order to properly > analyze the litmus test. But herd7 looks only at individual executions > in isolation, so fails to see the dependency. > > Signed-off-by: Alan Stern > Signed-off-by: Paul E. McKenney > > diff --git a/manual/kernel/crypto-control-data.litmus b/manual/kernel/crypto-control-data.litmus > new file mode 100644 > index 0000000..6baecf9 > --- /dev/null > +++ b/manual/kernel/crypto-control-data.litmus > @@ -0,0 +1,31 @@ > +C crypto-control-data > +(* > + * LB plus crypto-control-data plus data > + * > + * Result: Sometimes > + * > + * 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) Considering the bug in herd7 pointed out by Akira, we should rewrite P1 as: P1(int *x, int *y) { int r2; r = READ_ONCE(*y); WRITE_ONCE(*x, r2); } Other than that, this is fine. Alan