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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 B0B9DC433F4 for ; Fri, 31 Aug 2018 16:06:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 735F720658 for ; Fri, 31 Aug 2018 16:06:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 735F720658 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729216AbeHaUOj (ORCPT ); Fri, 31 Aug 2018 16:14:39 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:32872 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727286AbeHaUOj (ORCPT ); Fri, 31 Aug 2018 16:14:39 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0D3D1ED1; Fri, 31 Aug 2018 09:06:28 -0700 (PDT) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id D2E253F5BD; Fri, 31 Aug 2018 09:06:27 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id 83C791AE30C7; Fri, 31 Aug 2018 17:06:40 +0100 (BST) Date: Fri, 31 Aug 2018 17:06:40 +0100 From: Will Deacon To: Alan Stern Cc: Andrea Parri , "Paul E. McKenney" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com Subject: Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Message-ID: <20180831160640.GG30626@arm.com> References: <20180831091641.GA3634@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 31, 2018 at 10:52:54AM -0400, Alan Stern wrote: > On Fri, 31 Aug 2018, Andrea Parri wrote: > > On Thu, Aug 30, 2018 at 05:31:32PM -0400, Alan Stern wrote: > > > On Thu, 30 Aug 2018, Andrea Parri wrote: > > > > (Remark: ordinary release/acquire are building blocks for code such as > > > > qspinlock, (q)rwlock, mutex, rwsem, ... and what else??). > > > > > > But are these building blocks used the same way for all architectures? > > > > The more, the better! (because then we have the LKMM tools) > > > > We already discussed the "fast path" example: the fast paths of the > > above all resemble: > > > > *_lock(s): atomic_cmpxchg_acquire(&s->val, UNLOCKED_VAL, LOCKED_VAL) ... > > *_unlock(s): ... atomic_set_release(&s->val, UNLOCKED_VAL) > > > > When I read this code, I think "Of course." (unless some arch. has > > messed the implementation of cmpxchg_* up, which can happen...); but > > then I read the subject line of this patch and I think "Wait, what?". > > > > You can argue that this is not generic code, sure; but why on Earth > > would you like to do so?! > > Because the code might not work! On RISC-V, for example, the > implementation of ordinary release/acquire is currently not as strong > as atomic release/acquire. > > Yes, it's true that implementing locks with atomic_cmpxchg_acquire > should be correct on all existing architectures. And Paul has invited > a patch to modify the LKMM accordingly. If you feel that such a change > would be a useful enhancement to the LKMM's applicability, please write > it. Yes, please! That would be the "RmW" discussion which Andrea partially quoted earlier on, so getting that going independently from this patch sounds like a great idea to me. Cheers, Will