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=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 7A07DC4727E for ; Mon, 5 Oct 2020 08:20:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3768D20776 for ; Mon, 5 Oct 2020 08:20:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601886011; bh=a6m4Vgw8XbmSh9LXZ+VfNeL4cVShtjQyaGRZrfHilhc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=GtOE4eam6yD/w/vHfjkll9xOSZgH3ad3+PmUlhrq904c13sIX4MR3eifHHIOiAATG 0g+Rm0ihfsDvS+c8W7+A/wJpe72krwJKwXkGCh+e1fo4rl4YGzZJP3KU1gSuA21H6+ 4xZxepGGTef6J9DnxTSAmOgB6cMTvdc0HzQowDZM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726124AbgJEIUK (ORCPT ); Mon, 5 Oct 2020 04:20:10 -0400 Received: from mail.kernel.org ([198.145.29.99]:50586 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725885AbgJEIUJ (ORCPT ); Mon, 5 Oct 2020 04:20:09 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 7D7142075A; Mon, 5 Oct 2020 08:20:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601886009; bh=a6m4Vgw8XbmSh9LXZ+VfNeL4cVShtjQyaGRZrfHilhc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=1u9JuD93Gti/Wj9WmMCkK1I5DynXywWoeymlZnmS3QtvSYAOHKp77ZAR+Lvx1TEqI CFA+5p2rl3Mo5KfjljtwtcnEnFHYFndesdQK8ElTumaeDnHL7noZFb0Qy1AsH3qohB UNciyCmUIkp4yzcFA2p0/pX2YGkzrg7niKNvGeVE= Date: Mon, 5 Oct 2020 09:20:03 +0100 From: Will Deacon To: Alan Stern Cc: "Paul E. McKenney" , parri.andrea@gmail.com, 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: <20201005082002.GA23216@willie-the-truck> 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> <20201005023846.GA359428@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201005023846.GA359428@rowland.harvard.edu> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote: > 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); (r2?) > WRITE_ONCE(*x, r2); > } > > Other than that, this is fine. But yes, module the typo, I agree that this rewrite is much better than the proposal above. The definition of control dependencies on arm64 (per the Arm ARM [1]) isn't entirely clear that it provides order if the WRITE is executed on both paths of the branch, and I believe there are ongoing efforts to try to tighten that up. I'd rather keep _that_ topic separate from the "bug in herd" topic to avoid extra confusion. Will [1] https://developer.arm.com/documentation/ddi0487/fc/