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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 E8FEBC6778F for ; Sat, 7 Jul 2018 20:10:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0FD2A20E20 for ; Sat, 7 Jul 2018 20:10:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amarulasolutions.com header.i=@amarulasolutions.com header.b="S8V+RVYl" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0FD2A20E20 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=amarulasolutions.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 S1754392AbeGGUKa (ORCPT ); Sat, 7 Jul 2018 16:10:30 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:50509 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754187AbeGGUK2 (ORCPT ); Sat, 7 Jul 2018 16:10:28 -0400 Received: by mail-wm0-f66.google.com with SMTP id v25-v6so17663315wmc.0 for ; Sat, 07 Jul 2018 13:10:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amarulasolutions.com; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=XqMzAm+XcOKx02bKhnu4nkl3n3rW6S/mXcuAgvYzmLk=; b=S8V+RVYlcRcSAqtcUkMR0FVk/RVVQCrpd1mAOAO/XoJKtHqe+i2Y21Ju4HTC0vNShJ eGM8xPUH2x8JvvUWaZKv+vzPH8LLX3eBewQnN1ICxrTZ6wRybTu/FAat1W9irduaeuBp nwG+ZFhFwxDmRbP9Ayw8wz0OB9kazJitXfN/E= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=XqMzAm+XcOKx02bKhnu4nkl3n3rW6S/mXcuAgvYzmLk=; b=hKbp5H9WXnxsKHloF4Fnjc+0Rl201azJMBY/XEJf7eYQxQD7Pk0tOq0gqsX+fVJBPg UGPZTQKLcYQBdAxjKd+ac5lcEDAQCvH+zOjekfOPM+crGtaS0v8+ACr6+nGN7CQzDtZG s0hZbOBUMG+MQ+6k4RN5jXDtNeR5RjXejzcyjJhq6+424Ef5IA+2IgoMT8Jp9peOrUqm 1vnGoCU0qDSJ5nPez+V7LLKaY6foqY40pOCkSSyTDQHDbD1ofsoAQlGqa0QkxzD2BSKh iErtx05s06sr7vbDvT1dLYOUssICDLjZ8WdovltV1aA04CDMkaD6y4XMeHjRyaTZ2uzZ hiYw== X-Gm-Message-State: APt69E1PWAR9BsLVBuuI1+221XBYu+li8QFDMoz1kmfI3LMTEbk1ju8+ KIefq9Lfb3LdSf7SVGPXCf4kUA== X-Google-Smtp-Source: AAOMgpdcAoCDeII2JyKfhvxJ2Q7GW/yI/Q2WhyU1tszekNfLASKJEAH9NYEjW7/T8+Q+9DLhm0b25w== X-Received: by 2002:a1c:4004:: with SMTP id n4-v6mr8540740wma.83.1530994227210; Sat, 07 Jul 2018 13:10:27 -0700 (PDT) Received: from andrea ([94.230.152.15]) by smtp.gmail.com with ESMTPSA id s184-v6sm9623616wmf.5.2018.07.07.13.10.25 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 07 Jul 2018 13:10:26 -0700 (PDT) Date: Sat, 7 Jul 2018 22:10:24 +0200 From: Andrea Parri To: Guo Ren Cc: Peter Zijlstra , 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: <20180707201024.GB3225@andrea> 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> <20180707080845.GA346@guoren> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180707080845.GA346@guoren> 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 Sat, Jul 07, 2018 at 04:08:47PM +0800, Guo Ren wrote: > On Fri, Jul 06, 2018 at 02:17:16PM +0200, Peter Zijlstra wrote: > 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? The state (r0==0 && r1==0) _must_ not be allowed in the above snippet (so, even without the additional smp_mb() between WRITE_ONCE() and xchg()). In informal terms, xchg() provides the smp_mb(). Compare implementations of xchg() and xchg_relaxed(). The following could also be helpful (in addition to the references pointed out earlier): Documentation/atomic_t.txt Andrea > > Guo Ren >