From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752581AbeBZKjQ (ORCPT ); Mon, 26 Feb 2018 05:39:16 -0500 Received: from foss.arm.com ([217.140.101.70]:47950 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752218AbeBZKjP (ORCPT ); Mon, 26 Feb 2018 05:39:15 -0500 Date: Mon, 26 Feb 2018 10:39:16 +0000 From: Will Deacon To: Daniel Lustig Cc: Peter Zijlstra , "Paul E. McKenney" , Andrea Parri , linux-kernel@vger.kernel.org, Palmer Dabbelt , Albert Ou , Alan Stern , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Ingo Molnar , Linus Torvalds , linux-riscv@lists.infradead.org Subject: Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock() Message-ID: <20180226103915.GA8736@arm.com> References: <1519301990-11766-1-git-send-email-parri.andrea@gmail.com> <20180222134004.GN25181@hirez.programming.kicks-ass.net> <20180222141249.GA14033@andrea> <82beae6a-2589-6136-b563-3946d7c4fc60@nvidia.com> <20180222181317.GI2855@linux.vnet.ibm.com> <20180222182717.GS25181@hirez.programming.kicks-ass.net> <563431d0-4fb5-9efd-c393-83cc5197e934@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <563431d0-4fb5-9efd-c393-83cc5197e934@nvidia.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 22, 2018 at 11:47:57AM -0800, Daniel Lustig wrote: > On 2/22/2018 10:27 AM, Peter Zijlstra wrote: > > On Thu, Feb 22, 2018 at 10:13:17AM -0800, Paul E. McKenney wrote: > >> So we have something that is not all that rare in the Linux kernel > >> community, namely two conflicting more-or-less concurrent changes. > >> This clearly needs to be resolved, either by us not strengthening the > >> Linux-kernel memory model in the way we were planning to or by you > >> strengthening RISC-V to be no weaker than PowerPC for these sorts of > >> externally viewed release-acquire situations. > >> > >> Other thoughts? > > > > Like said in the other email, I would _much_ prefer to not go weaker > > than PPC, I find that PPC is already painfully weak at times. > > Sure, and RISC-V could make this work too by using RCsc instructions > and/or by using lightweight fences instead. It just wasn't clear at > first whether smp_load_acquire() and smp_store_release() were RCpc, > RCsc, or something else, and hence whether RISC-V would actually need > to use something stronger than pure RCpc there. Likewise for > spin_unlock()/spin_lock() and everywhere else this comes up. > > As Paul's email in the other thread observed, RCpc seems to be > OK for smp_load_acquire()/smp_store_release() at least according > to the current LKMM herd spec. Unlock/lock are stronger already > I guess. But if there's an active proposal to strengthen them all > to something stricter than pure RCpc, then that's good to know. > > My understanding from earlier discussions is that ARM has no plans > to use their own RCpc instruction for smp_load_acquire() instead > of their RCsc instructions. Is that still true? If they were to > use the RCpc load there, that would cause them to have the same > problem we're discussing here, right? Just checking. Agreed. No plans to use the LDAPR instruction in Linux. Will