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=-8.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 27090C433FF for ; Sun, 28 Jul 2019 15:35:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E1B4B2077C for ; Sun, 28 Jul 2019 15:35:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="ARdtilcr" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726180AbfG1Pfr (ORCPT ); Sun, 28 Jul 2019 11:35:47 -0400 Received: from mail-pl1-f196.google.com ([209.85.214.196]:39218 "EHLO mail-pl1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726032AbfG1Pfr (ORCPT ); Sun, 28 Jul 2019 11:35:47 -0400 Received: by mail-pl1-f196.google.com with SMTP id b7so26565633pls.6 for ; Sun, 28 Jul 2019 08:35:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=Pm6pwIPdlV9wRk+JitjPlIfT8uYdgqGZKgDDx5JXMvo=; b=ARdtilcr0o+k6afqdq26dB6uiPQmwsK/dIg5CCUfize/+2sZckkq9SBfSdltYDqdvK CmUecGvwbkSdnSWtSvlXRjuxZcdFejEOeqPY3zzYmNgSuYBBQd3SV4PGISic/QDX6sxV tPS9OrxdPUdOe16wYiDYpxXi115+Z/pjxkEZU= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=Pm6pwIPdlV9wRk+JitjPlIfT8uYdgqGZKgDDx5JXMvo=; b=Uld8qJxHpGnMyaeKgZzC61qh8VRGDDOXBDBUwEmGBq7iZN3D0ICHYOhRh5J6QYOPZs 5cu0Bue1ErV0VSjCFxRo75JEaaBaHK2of59B8uZY6NNPyk4PZtTsn9AU/18c1lq1QdVI MMnL/lOiU4TTgkAKQ7OLweheFb1VFT1Ass6L8kRXTjJKYiCmx/W3CmWiCIOYzwDEu9r0 44by2nYlF/Mi5xWm2PyeoQDU2clWTwpAHpDok7lFTwi5j/Zngnk9RNZYYdKtz6W5JQal 6mWZNps+ad4q2lJYgNgjru0gA++SD2FKFYHFjVgzY0i/u09OFAzkXJAhEMVds2AlZkWN PIsA== X-Gm-Message-State: APjAAAXNfrLhSKQdkGA4tcru579jPO3/b9Z5rNiYrwWfZazwtYaQlyK4 6fwae7Z9H3UoQIzznq8714U= X-Google-Smtp-Source: APXvYqwlh8jFqpMNB2vT4vmBRbLrB7P3r3gqG6V5tDxzBhF0Er/215XImhQMOAIPhhTKSVtXcooiBA== X-Received: by 2002:a17:902:b702:: with SMTP id d2mr109216801pls.259.1564328146304; Sun, 28 Jul 2019 08:35:46 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id x24sm55904341pgl.84.2019.07.28.08.35.45 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Sun, 28 Jul 2019 08:35:45 -0700 (PDT) Date: Sun, 28 Jul 2019 11:35:44 -0400 From: Joel Fernandes To: Boqun Feng Cc: Alan Stern , linux-kernel@vger.kernel.org, kernel-team@android.com, Akira Yokosawa , Andrea Parri , Daniel Lustig , David Howells , Ingo Molnar , Jade Alglave , linux-arch@vger.kernel.org, Luc Maranget , Nicholas Piggin , "Paul E. McKenney" , Peter Zijlstra , Will Deacon Subject: Re: [PATCH v2] lkmm/docs: Correct ->prop example with additional rfe link Message-ID: <20190728153544.GA87531@google.com> References: <20190728031303.164545-1-joel@joelfernandes.org> <20190728151959.GA82871@google.com> <20190728152806.GB26905@tardis> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190728152806.GB26905@tardis> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 28, 2019 at 11:28:06PM +0800, Boqun Feng wrote: > On Sun, Jul 28, 2019 at 11:19:59AM -0400, Joel Fernandes wrote: > > On Sun, Jul 28, 2019 at 10:48:51AM -0400, Alan Stern wrote: > > > On Sat, 27 Jul 2019, Joel Fernandes (Google) wrote: > > > > > > > The lkmm example about ->prop relation should describe an additional rfe > > > > link between P1's store to y and P2's load of y, which should be > > > > critical to establishing the ordering resulting in the ->prop ordering > > > > on P0. IOW, there are 2 rfe links, not one. > > > > > > > > Correct these in the docs to make the ->prop ordering on P0 more clear. > > > > > > > > Cc: kernel-team@android.com > > > > Reviewed-by: Boqun Feng > > > > Signed-off-by: Joel Fernandes (Google) > > > > --- > > > > > > This is not a good update. See below... > > > > No problem, thanks for the feedback. I am new to the LKMM so please bear > > with me.. I should have tagged this RFC. > > > > > > .../memory-model/Documentation/explanation.txt | 17 ++++++++++------- > > > > 1 file changed, 10 insertions(+), 7 deletions(-) > > > > > > > > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt > > > > index 68caa9a976d0..aa84fce854cc 100644 > > > > --- a/tools/memory-model/Documentation/explanation.txt > > > > +++ b/tools/memory-model/Documentation/explanation.txt > > > > @@ -1302,8 +1302,8 @@ followed by an arbitrary number of cumul-fence links, ending with an > > > > rfe link. You can concoct more exotic examples, containing more than > > > > one fence, although this quickly leads to diminishing returns in terms > > > > of complexity. For instance, here's an example containing a coe link > > > > -followed by two fences and an rfe link, utilizing the fact that > > > > -release fences are A-cumulative: > > > > +followed by a fence, an rfe link, another fence and and a final rfe link, > > > ^---^ > > > > +utilizing the fact that release fences are A-cumulative: > > > > > > I don't like this, for two reasons. First is the repeated "and" typo. > > > > Will fix the trivial typo, sorry about that. > > > > > More importantly, it's not necessary to go into this level of detail; a > > > better revision would be: > > > > > > +followed by two cumul-fences and an rfe link, utilizing the fact that > > > > > > This is appropriate because the cumul-fence relation is defined to > > > contain the rfe link which you noticed wasn't mentioned explicitly. > > > > No, I am talking about the P1's store to Y and P2's load of Y. That is not > > through a cumul-fence so I don't understand what you meant? That _is_ missing > > the rfe link I am referring to, that is left out. > > > > The example says r2 = 1 and then we work backwards from that. r2 could very > > well have been 0, there's no fence or anything involved, it just happens to > > be the executation pattern causing r2 = 1 and hence the rfe link. Right? > > > > > > int x, y, z; > > > > > > > > @@ -1334,11 +1334,14 @@ If x = 2, r0 = 1, and r2 = 1 after this code runs then there is a prop > > > > link from P0's store to its load. This is because P0's store gets > > > > overwritten by P1's store since x = 2 at the end (a coe link), the > > > > smp_wmb() ensures that P1's store to x propagates to P2 before the > > > > -store to y does (the first fence), the store to y propagates to P2 > > > > -before P2's load and store execute, P2's smp_store_release() > > > > -guarantees that the stores to x and y both propagate to P0 before the > > > > -store to z does (the second fence), and P0's load executes after the > > > > -store to z has propagated to P0 (an rfe link). > > > > +store to y does (the first fence), P2's store to y happens before P2's > > > ---------------------------------------^ > > > > > > This makes no sense, since P2 doesn't store to y. You meant P1's store > > > to y. Also, the use of "happens before" is here unnecessarily > > > ambiguous (is it an informal usage or does it refer to the formal > > > happens-before relation?). The original "propagates to" is better. > > > > Will reword this. > > > > > > +load of y (rfe link), P2's smp_store_release() ensures that P2's load > > > > +of y executes before P2's store to z (second fence), which implies that > > > > +that stores to x and y propagate to P2 before the smp_store_release(), which > > > > +means that P2's smp_store_release() will propagate stores to x and y to all > > > > +CPUs before the store to z propagates (A-cumulative property of this fence). > > > > +Finally P0's load of z executes after P2's store to z has propagated to > > > > +P0 (rfe link). > > > > > > Again, a better change would be simply to replace the two instances of > > > "fence" in the original text with "cumul-fence". > > > > Ok that's fine. But I still feel the rfe is not a part of the cumul-fence. > > The fences have nothing to do with the rfe. Or, I am missing something quite > > badly. > > > > Boqun, did you understand what Alan is saying? > > > > I think 'cumul-fence' that Alan mentioned is not a fence, but a > relation, which could be the result of combining a rfe relation and a > A-cumulative fence relation. Please see the section "PROPAGATION ORDER > RELATION: cumul-fence" or the definition of cumul-fence in > linux-kernel.cat. > > Did I get you right, Alan? If so, your suggestion is indeed a better > change. To be frank, I don't think it is better if that's what Alan meant. It is better to be explicit about the ->rfe so that the reader walking through the example can clearly see the ordering and make sense of it. Just saying 'cumul-fence' and then hoping the reader sees the light is quite a big assumption and makes the document less readable. I mean the fact that you are asking Alan for clarification, means that it is not that obvious ;) thanks, - Joel