From patchwork Tue Jun 30 17:37:27 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Will Deacon X-Patchwork-Id: 11634455 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 26D8B60D for ; Tue, 30 Jun 2020 17:41:27 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0098F20775 for ; Tue, 30 Jun 2020 17:41:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="V33k4kU4"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="q0keKuHs" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0098F20775 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To:Message-Id:Date: Subject:To:From:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=gB9dHQhKp1dtqOtObbyHIhHRYvHK4Z5SrowO+Y5tvrg=; b=V33k4kU4PQGAg3+wdbcjfyLcD 8Jb0j8Gzrm9IjSzoFb/hNSi7aOZ7WQUeLu0EuZdGylLw5iZ+QXQagibNGDwqjuMAvvaCnIsijEAoN Wv5bwyz5K8g4GtJ+rI9/WgkEFRHg7LHNkA6edJX/I4XafH39OOwtgBkpymxoku2TMim2wbPK7AGYu gbawPNhe0xQYV/+SzM3cYV0pSQQgmjqrCU/HnhWQIf+w2uhpkuuYxt/lQlnoi1zuGR9dU7XDv0FLZ q8W8scgmG2dNuczfOTlNgsSOMdP2XE7oboQ8qjLioB5X70q8PwGMZtyH0L4a18DvOKH31uXFweQ+D 1lp4Wfw0Q==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jqKEE-0001oT-Sq; Tue, 30 Jun 2020 17:39:30 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jqKDG-0001Jg-Pk for linux-arm-kernel@lists.infradead.org; Tue, 30 Jun 2020 17:38:34 +0000 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 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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200630_133831_123654_08C6F475 X-CRM114-Status: GOOD ( 14.62 ) X-Spam-Score: -5.2 (-----) X-Spam-Report: SpamAssassin version 3.4.4 on merlin.infradead.org summary: Content analysis details: (-5.2 points) pts rule name description ---- ---------------------- -------------------------------------------------- -5.0 RCVD_IN_DNSWL_HI RBL: Sender listed at https://www.dnswl.org/, high trust [198.145.29.99 listed in list.dnswl.org] 0.0 SPF_HELO_NONE SPF: HELO does not publish an SPF Record -0.0 SPF_PASS SPF: sender matches SPF record -0.1 DKIM_VALID_EF Message has a valid DKIM or DK signature from envelope-from domain -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.0 DKIMWL_WL_HIGH DKIMwl.org - Whitelisted High sender X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Will Deacon , Arnd Bergmann , Alan Stern , Sami Tolvanen , Matt Turner , kernel-team@android.com, Marco Elver , Kees Cook , "Paul E. McKenney" , Boqun Feng , Josh Triplett , Ivan Kokshaysky , linux-arm-kernel@lists.infradead.org, Richard Henderson , Nick Desaulniers , linux-alpha@vger.kernel.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.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.