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=DKIM_INVALID,DKIM_SIGNED, 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 B86A9C10F03 for ; Thu, 25 Apr 2019 09:13:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 857FB218B0 for ; Thu, 25 Apr 2019 09:13:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="kTozG7yh" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727757AbfDYJNO (ORCPT ); Thu, 25 Apr 2019 05:13:14 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:52192 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726044AbfDYJNN (ORCPT ); Thu, 25 Apr 2019 05:13:13 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Transfer-Encoding :Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help: List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=tmzeAVH5tFNv7q674hWZHV06NgF23KzCSrnTktTl8Sw=; b=kTozG7yhieSXurc7vFwZ9AFYWI 0t6+VEDYgWCPnIo3Gjm/fkTXJqmuDxzfsXI53CR0yf5HiDYl7rwgbcASGtpDbGttAfmWyKZWhmV6R Suf7dlybJ/ZUuPJcW6E1waX9kO3DacyExF+XstsKRrHZYhUy8pNWuI2kzzyYpnxq8dfGw5GNomM7/ bHEOQpUgbRnDonG0ZxqP7GOgL1U/+NvUgH8TZqgMBY7oBi9tsSDiu/SBMDObVzhOtTVv2de2mdr1y N65971pC9bC1rkvVpk/5Gxt2LMCtjNBVBS4wGHh6fq6DqX/0HdxqQQLq7JN9JxEAwNS9CcEbfzi3e QgBBSe8A==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1hJaRA-0000RT-Qx; Thu, 25 Apr 2019 09:13:00 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id DDDF529BC31D8; Thu, 25 Apr 2019 11:12:58 +0200 (CEST) Date: Thu, 25 Apr 2019 11:12:58 +0200 From: Peter Zijlstra To: huangpei@loongson.cn Cc: Paul Burton , "stern@rowland.harvard.edu" , "akiyks@gmail.com" , "andrea.parri@amarulasolutions.com" , "boqun.feng@gmail.com" , "dlustig@nvidia.com" , "dhowells@redhat.com" , "j.alglave@ucl.ac.uk" , "luc.maranget@inria.fr" , "npiggin@gmail.com" , "paulmck@linux.ibm.com" , "will.deacon@arm.com" , "linux-kernel@vger.kernel.org" , "torvalds@linux-foundation.org" , Huacai Chen Subject: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage Message-ID: <20190425091258.GC14281@hirez.programming.kicks-ass.net> References: <20190424123656.484227701@infradead.org> <20190424124421.636767843@infradead.org> <20190424211759.52xraajqwudc2fza@pburton-laptop> <2b2b07cc.bf42.16a52dc4e4d.Coremail.huangpei@loongson.cn> <20190425073348.GV11158@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190425073348.GV11158@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 25, 2019 at 09:33:48AM +0200, Peter Zijlstra wrote: > > Let me explain the bug more specific: > > > > the bug ONLY matters in following situation: > > > > #. more than one cpu (assume cpu A and B) doing ll/sc on same shared > > var V > > > > #. speculative memory access from A cause A erroneously succeed sc > > operation, since the erroneously successful sc operation violate the > > coherence protocol. (here coherence protocol means the rules that CPU > > follow to implement ll/sc right) > > > > #. B succeed sc operation too, but this sc operation is right both > > logically and follow the coherence protocol, and makes A's sc wrong > > logically since only ONE sc operation can succeed. > > In one word， the bug only affect local cpu‘s ll/sc operation, and > > affect MP system. > > PS: > > > > If local_t is only ll/sc manipulated by current CPU， then no need fix it. > > It _should_ be CPU local, but this was not at all clear from reading the > original changelog nor the comment with loongson_llsc_mb(). However, if it is a coherence issue, the thing is at the cacheline level, and there is nothing that says the line isn't shared, just the one variable isn't. Ideally there should be minimal false sharing, but....