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 6322BC04AB4 for ; Tue, 14 May 2019 15:58:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 337922085A for ; Tue, 14 May 2019 15:58:50 +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="q+LAgf1K" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726296AbfENP6t (ORCPT ); Tue, 14 May 2019 11:58:49 -0400 Received: from merlin.infradead.org ([205.233.59.134]:38414 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725980AbfENP6s (ORCPT ); Tue, 14 May 2019 11:58:48 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=uEG6RAbenetSOMt2p9wGwy8CK7MGZhuTPCJ+MQ39rqU=; b=q+LAgf1KxBqb6/SCaU1L7GtjIF kDReAoucGpNYabwwnltrdstTqskmLN4GIHlOa3uY8I35Rk8/VcqoQHpx5Vm/XZo0pW3GrcSqMAZpI nk9185FKy/dY5nH594dnb3Jetb8Q4vGQXnLNTPfrcl/9alvgWxYyHUjvW0IJscOGVsYL9oxN7QmCR 5cQkgJmCnBswxrl2Amz9FDI4g9RJV0/QGdA5lzwIS/GGucJv4xcuPfI5StaCxCrf166f1pC+/FJ7k +6X7mLZmjTh/j43/jyvh7wieNqyCRXxsPLnzKFrO8885cv683+/ZVJd65Y412ChiSV5Oq5L1ADWCR QhXusegg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1hQZol-0000Wc-Bp; Tue, 14 May 2019 15:58:15 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 9AF8F2029F877; Tue, 14 May 2019 17:58:13 +0200 (CEST) Date: Tue, 14 May 2019 17:58:13 +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: <20190514155813.GG2677@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> <20190425091258.GC14281@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: <20190425091258.GC14281@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 I think this thread got 'lost' On Thu, Apr 25, 2019 at 11:12:58AM +0200, Peter Zijlstra wrote: > 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.... So if two variables share a line, and one is local while the other is shared atomic, can contention on the line, but not the variable, cause issues for the local variable? If not; why not? Because so far the issue is line granular due to the coherence aspect.