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,URIBL_BLOCKED,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 0F675C433DF for ; Tue, 30 Jun 2020 17:38:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DF1DA20826 for ; Tue, 30 Jun 2020 17:38:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593538716; bh=qoEfU3b1y4/xeEqvyt+YVXOv4viPb6BcJ15Sl3Ij7bg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=Nn8Zjqv051wYe4gRIR8DtOaWm7lvvukMprquT2Rd1XDjPz1hWTmF/27Dg3ZlrBxXY kpFj8Nqp5q41LzkznsQ5zp9DiaUGKmMVPOrczKlxjudkWIFQSRhqbhbemcR2WHzrU3 X8mwGfTaolIcE71xFq1M34aFgckfZ3tHcH40uupA= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390420AbgF3Rif (ORCPT ); Tue, 30 Jun 2020 13:38:35 -0400 Received: from mail.kernel.org ([198.145.29.99]:51552 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388211AbgF3Ria (ORCPT ); Tue, 30 Jun 2020 13:38:30 -0400 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 7D7A220829; Tue, 30 Jun 2020 17:38:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593538710; bh=qoEfU3b1y4/xeEqvyt+YVXOv4viPb6BcJ15Sl3Ij7bg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=q0keKuHsfsfAgKC1LtX9sks7jRJHDQZZ1ezB73t7AIUcDYxJr7j4+9TF1D35lERby Nj+pl0uHaslag6jrJXtgEsQmSbKqbY/SCiZy0EA85EepoyuxPrS4RPnoAn9tocRTT0 faKEZLt+fjG+6/EJXnJJbTOWCm/N3A81o5uuyGgU= From: Will Deacon To: linux-kernel@vger.kernel.org Cc: Will Deacon , Sami Tolvanen , Nick Desaulniers , Kees Cook , Marco Elver , "Paul E. McKenney" , Josh Triplett , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , "Michael S. Tsirkin" , Jason Wang , Arnd Bergmann , Boqun Feng , Catalin Marinas , Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-alpha@vger.kernel.org, virtualization@lists.linux-foundation.org, kernel-team@android.com Subject: [PATCH 11/18] tools/memory-model: Remove smp_read_barrier_depends() from informal doc Date: Tue, 30 Jun 2020 18:37:27 +0100 Message-Id: <20200630173734.14057-12-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200630173734.14057-1-will@kernel.org> References: <20200630173734.14057-1-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. Acked-by: Alan Stern Acked-by: Paul E. McKenney 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 e91a2eb19592..01adf9e0ebac 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -1122,12 +1122,10 @@ 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() { @@ -1146,14 +1144,14 @@ 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. -- 2.27.0.212.ge8ba1cc988-goog