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 C9552C6778A for ; Sat, 7 Jul 2018 08:09:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 81E6C213A2 for ; Sat, 7 Jul 2018 08:09:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 81E6C213A2 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 S1752836AbeGGIJC (ORCPT ); Sat, 7 Jul 2018 04:09:02 -0400 Received: from smtp2200-217.mail.aliyun.com ([121.197.200.217]:38648 "EHLO smtp2200-217.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751693AbeGGII7 (ORCPT ); Sat, 7 Jul 2018 04:08:59 -0400 X-Alimail-AntiSpam: AC=CONTINUE;BC=0.09957097|-1;CH=green;FP=0|0|0|0|0|-1|-1|-1;HT=e02c03298;MF=ren_guo@c-sky.com;NM=1;PH=DS;RN=13;RT=13;SR=0;TI=SMTPD_---.CNO7aKH_1530950927; Received: from localhost(mailfrom:ren_guo@c-sky.com fp:SMTPD_---.CNO7aKH_1530950927) by smtp.aliyun-inc.com(10.147.43.230); Sat, 07 Jul 2018 16:08:47 +0800 Date: Sat, 7 Jul 2018 16:08:47 +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: <20180707080845.GA346@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> <20180706121716.GO2512@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180706121716.GO2512@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 02:17:16PM +0200, Peter Zijlstra wrote: > > > > 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 > > Also, since you said "SYNC.IS" is a pipeline flush, those > instruction-sync primitives normally do not imply a store-buffer flush, > does yours? If not it is not a valid smp_mb() implementation. Sync.is will flush pipeline and store-buffer. "sync" means completion memory barrier. "i" means flush cpu pipeline. "s" means sharable to other cpus. > > Notably: > > CPU0 CPU1 > > WRITE_ONCE(x, 1); WRITE_ONCE(y, 1); > smp_mb(); smp_mb(); > r0 = READ_ONCE(y); r1 = READ_ONCE(x); > > must not allow: r0==0 && r1==0 > > Which would be possible with a regular instruction-sync barrier, but > must absolutely not be true with a full memory barrier. > > (and you can replace the smp_mb(); r = READ_ONCE(); with r = xchg() to > again see why you need that first smp_mb()). CPU0 CPU1 WRITE_ONCE(x, 1) WRITE_ONCE(y, 1) r0 = xchg(&y, 2) r1 = xchg(&x, 2) must not allow: r0==0 && r1==0 So we must add a smp_mb between WRITE_ONCE() and xchg(), right? Guo Ren