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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED,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 5F0A4C6778A for ; Sat, 7 Jul 2018 07:42:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2329C21766 for ; Sat, 7 Jul 2018 07:42:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2329C21766 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=c-sky.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 S1753765AbeGGHm0 (ORCPT ); Sat, 7 Jul 2018 03:42:26 -0400 Received: from smtp2200-217.mail.aliyun.com ([121.197.200.217]:39589 "EHLO smtp2200-217.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750861AbeGGHmZ (ORCPT ); Sat, 7 Jul 2018 03:42:25 -0400 X-Alimail-AntiSpam: AC=CONTINUE;BC=0.08227099|-1;CH=green;FP=0|0|0|0|0|-1|-1|-1;HT=e02c03271;MF=ren_guo@c-sky.com;NM=1;PH=DS;RN=13;RT=13;SR=0;TI=SMTPD_---.CNNgBZy_1530949330; Received: from localhost(mailfrom:ren_guo@c-sky.com fp:SMTPD_---.CNNgBZy_1530949330) by smtp.aliyun-inc.com(10.147.41.178); Sat, 07 Jul 2018 15:42:11 +0800 Date: Sat, 7 Jul 2018 15:42:10 +0800 From: Guo Ren To: Peter Zijlstra Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, daniel.lezcano@linaro.org, jason@lakedaemon.net, arnd@arndb.de, c-sky_gcc_upstream@c-sky.com, gnu-csky@mentor.com, thomas.petazzoni@bootlin.com, wbx@uclibc-ng.org, green.hu@gmail.com, Will Deacon Subject: Re: [PATCH V2 11/19] csky: Atomic operations Message-ID: <20180707074209.GA32147@guoren> References: <860b8db036b33d7b3648cb1f4ec827a53dc1a01b.1530465326.git.ren_guo@c-sky.com> <20180705175059.GE2530@hirez.programming.kicks-ass.net> <20180706110129.GC8707@guoren> <20180706115614.GV2476@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180706115614.GV2476@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 06, 2018 at 01:56:14PM +0200, Peter Zijlstra wrote: > That's how LL/SC works. What I was asking is if they have any effect on > memory ordering. Some architectures have LL/SC imply memory ordering, > most do not. > > Going by your spinlock implementation they don't imply any memory > ordering. ldex/stex don't imply any memory ordering. > > > > The mandated semantics for xchg() / cmpxchg() is an effective smp_mb() > > > before _and_ after. > > > > switch (size) { \ > > case 4: \ > > smp_mb(); \ > > asm volatile ( \ > > "1: ldex.w %0, (%3) \n" \ > > " mov %1, %2 \n" \ > > " stex.w %1, (%3) \n" \ > > " bez %1, 1b \n" \ > > : "=&r" (__ret), "=&r" (tmp) \ > > : "r" (__new), "r"(__ptr) \ > > : "memory"); \ > > smp_mb(); \ > > break; \ > > Hmm? > > But I couldn't undertand what's wrong without the 1th smp_mb()? > > 1th smp_mb will make all ld/st finish before ldex.w. Is it necessary? > > Yes. > > CPU0 CPU1 > > r1 = READ_ONCE(x); WRITE_ONCE(y, 1); > r2 = xchg(&y, 2); smp_store_release(&x, 1); > > must not allow: r1==1 && r2==0 CPU1 smp_store_release could be finished before WRITE_ONCE, so r1=1 && r2=0? > > > The above implementation suggests LDEX implies a SYNC.IS, is this > > > correct? > > No, ldex doesn't imply a sync.is. > > Right, as per the spinlock emails, then your proposed primitives are > incorrect. Yes, approve. Guo Ren