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=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 B75B1FA372C for ; Fri, 8 Nov 2019 17:02:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8DD0621882 for ; Fri, 8 Nov 2019 17:02:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573232529; bh=wCL903ScuiGofElDf7EX9mpRBQ2yI6oD2F0VBzIhMiI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=rHDZp4uGIQNks2AGzZSltV70qrJVKW89Inw+QOtTbtKmGOu5VYFwgYnJ7h3nYWycH WRGeYiblzB+4Zwn/ufdH3fuf4JwH61dngFwat9hydoynvAyCpJ1ucIoef4wpQtxGgU G9eku48sR80RJccr/9jyQ9oMufegkeIshW941IE0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731300AbfKHRCI (ORCPT ); Fri, 8 Nov 2019 12:02:08 -0500 Received: from mail.kernel.org ([198.145.29.99]:46162 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731163AbfKHRCF (ORCPT ); Fri, 8 Nov 2019 12:02:05 -0500 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 46088222C5; Fri, 8 Nov 2019 17:02:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573232524; bh=wCL903ScuiGofElDf7EX9mpRBQ2yI6oD2F0VBzIhMiI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MAKtRC+RFnOPFfR4QyCGaTOzj4F+z/r+5QEkX66W3MQdu1xPBVYek4UJmHra0op04 e4o7aS6/9iO9gOqLrnywDYUUkfJppi3EUPo5JXIRfHpargTMHQjCYN2EdNOsQO/+3u NJ8X4Q6h7xFJrtdqq5EciOWZizRB/aadRDMwGttk= From: Will Deacon To: linux-kernel@vger.kernel.org Cc: Will Deacon , Yunjae Lee , SeongJae Park , "Paul E. McKenney" , Josh Triplett , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , Michael Ellerman , "Michael S. Tsirkin" , Jason Wang , Arnd Bergmann , Joe Perches , Boqun Feng , linux-alpha@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: [PATCH 10/13] tools/memory-model: Remove smp_read_barrier_depends() from informal doc Date: Fri, 8 Nov 2019 17:01:17 +0000 Message-Id: <20191108170120.22331-11-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191108170120.22331-1-will@kernel.org> References: <20191108170120.22331-1-will@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 'smp_read_barrier_depends()' has gone the way of mmiowb() and so many esoteric memory barriers before it. Drop the two mentions of this deceased barrier from the LKMM informal explanation document. Signed-off-by: Will Deacon --- .../Documentation/explanation.txt | 26 +++++++++---------- 1 file changed, 12 insertions(+), 14 deletions(-) diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt index 488f11f6c588..3050bf67b8d0 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -1118,12 +1118,10 @@ maintain at least the appearance of FIFO order. In practice, this difficulty is solved by inserting a special fence between P1's two loads when the kernel is compiled for the Alpha architecture. In fact, as of version 4.15, the kernel automatically -adds this fence (called smp_read_barrier_depends() and defined as -nothing at all on non-Alpha builds) after every READ_ONCE() and atomic -load. The effect of the fence is to cause the CPU not to execute any -po-later instructions until after the local cache has finished -processing all the stores it has already received. Thus, if the code -was changed to: +adds this fence after every READ_ONCE() and atomic load on Alpha. The +effect of the fence is to cause the CPU not to execute any po-later +instructions until after the local cache has finished processing all +the stores it has already received. Thus, if the code was changed to: P1() { @@ -1142,14 +1140,14 @@ READ_ONCE() or another synchronization primitive rather than accessed directly. The LKMM requires that smp_rmb(), acquire fences, and strong fences -share this property with smp_read_barrier_depends(): They do not allow -the CPU to execute any po-later instructions (or po-later loads in the -case of smp_rmb()) until all outstanding stores have been processed by -the local cache. In the case of a strong fence, the CPU first has to -wait for all of its po-earlier stores to propagate to every other CPU -in the system; then it has to wait for the local cache to process all -the stores received as of that time -- not just the stores received -when the strong fence began. +share this property: They do not allow the CPU to execute any po-later +instructions (or po-later loads in the case of smp_rmb()) until all +outstanding stores have been processed by the local cache. In the +case of a strong fence, the CPU first has to wait for all of its +po-earlier stores to propagate to every other CPU in the system; then +it has to wait for the local cache to process all the stores received +as of that time -- not just the stores received when the strong fence +began. And of course, none of this matters for any architecture other than Alpha. -- 2.24.0.rc1.363.gb1bccd3e3d-goog