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=-9.8 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 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 7EC6FC4363A for ; Mon, 5 Oct 2020 08:36:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 49BC220795 for ; Mon, 5 Oct 2020 08:36:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725909AbgJEIg4 convert rfc822-to-8bit (ORCPT ); Mon, 5 Oct 2020 04:36:56 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:25968 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725880AbgJEIg4 (ORCPT ); Mon, 5 Oct 2020 04:36:56 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-232-gvT30tU6O0i1Mz5IyqnV4A-1; Mon, 05 Oct 2020 09:36:52 +0100 X-MC-Unique: gvT30tU6O0i1Mz5IyqnV4A-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 5 Oct 2020 09:36:51 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 5 Oct 2020 09:36:51 +0100 From: David Laight To: "'paulmck@kernel.org'" , Alan Stern 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 Thread-Topic: Litmus test for question from Al Viro Thread-Index: AQHWmqaOWthdViPieEmyGwyo/H2Pw6mIrfkA Date: Mon, 5 Oct 2020 08:36:51 +0000 Message-ID: <5cf6b793978e4cd8ae10344336c13adb@AcuMS.aculab.com> 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> In-Reply-To: <20201004233146.GP29330@paulmck-ThinkPad-P72> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org From: Paul E. McKenney > Sent: 05 October 2020 00:32 ... > 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); > +} Hmmm.... the compiler will semi-randomly transform that to/from: if (READ_ONCE(*x) == 0) r1 = 0; else r1 = 1; and r1 = READ_ONCE(*x) != 0; Both of which (probably) get correctly detected as a write to *y that is dependant on *x - so is 'problematic' with P1() which does the opposite assignment. Which does rather imply that hurd is a bit broken. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)