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,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 A76A9C282E1 for ; Thu, 25 Apr 2019 07:34:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 758B6206BA for ; Thu, 25 Apr 2019 07:34:15 +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="kLcR4jFZ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730336AbfDYHeO (ORCPT ); Thu, 25 Apr 2019 03:34:14 -0400 Received: from merlin.infradead.org ([205.233.59.134]:47714 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727125AbfDYHeJ (ORCPT ); Thu, 25 Apr 2019 03:34:09 -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=YtZsY6i2xCXgiHs5ESM9B01vo1Wm1BqaMdwefo9q1IY=; b=kLcR4jFZEsAAzh9RjDF3H10nTs 6XiPoyLnyN9ThnCwwkAfcl1sRmbf5UCtJTGIAVYvEmwWf83jF+nos28dtSqVyykcUQLKhEh2c1lP/ tfy5SdBRhFzh2pl9Yp8tugM4ETF/ae+ZqMHt7uCBUB4GeZKa2CDLLpstoaoyQENrX42wH/kaTorOA UVApIiIsa2ey3z6wdNzQoj5cWxXgZ01UDkkJTN2OWfSvMXnkLLRzCtXOJH4QhAPeNBFMdgQOdZnw1 fj+jKJisPs5pSCaO8g/OvRTVRCqHoV5bt5OsjWMx1FnmksbqASqPBDrUMREV+rG7eXSqjUxtcROVn IQL2v2Kw==; 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 1hJYtB-0000UY-Pf; Thu, 25 Apr 2019 07:33:50 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 796CD20D0A200; Thu, 25 Apr 2019 09:33:48 +0200 (CEST) Date: Thu, 25 Apr 2019 09:33:48 +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: <20190425073348.GV11158@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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2b2b07cc.bf42.16a52dc4e4d.Coremail.huangpei@loongson.cn> 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 A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail? On Thu, Apr 25, 2019 at 12:58:50PM +0800, huangpei@loongson.cn wrote: > In my opinion. patch 2/3 is about Loongson's bug, and patch 4/5 is another theme. Agreed; it's just that looking at the MIPS code to fix 4/5 made me trip over this stuff. > 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. (I know your coherence protocol is probably more complicated than MESI, but bear with me) So A speculatively gets V's line in Exclusive mode, speculates the Lock flag is still there and completes the Store. This speculative store then leaks out and violates MESI because there _should_ only be one Exclusive owner of a line (B). Something like that? > If it is not LL/SC but other memory access from B on V, A's ll/sc can > follow the atomic semantics even if A violate the coherence protocol > in the same situation. *shudder*... C atomic-set { atomic_set(v, 1); } P1(atomic_t *v) { atomic_add_unless(v, 1, 0); } P2(atomic_t *v) { atomic_set(v, 0); } exists (v=2) So that one will still work? (that is, v=2 is forbidden) > In one word， the bug only affect local cpu‘s ll/sc operation, and > affect MP system. Because it is a coherence issue, triggered by a reorder. OK. > 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().