From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Wed, 11 Jul 2018 12:20:38 +0200 (CEST) Received: from foss.arm.com ([217.140.101.70]:37064 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S23993849AbeGKKUbyBnH4 (ORCPT ); Wed, 11 Jul 2018 12:20:31 +0200 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 05C43ED1; Wed, 11 Jul 2018 03:20:25 -0700 (PDT) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id C9AA43F589; Wed, 11 Jul 2018 03:20:24 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id D21111AE3828; Wed, 11 Jul 2018 11:21:06 +0100 (BST) Date: Wed, 11 Jul 2018 11:21:06 +0100 From: Will Deacon To: Jiaxun Yang Cc: linux-mips@linux-mips.org, Peter Zijlstra , =?utf-8?B?6ZmI5Y2O5omN?= , Paul Burton , Ralf Baechle , James Hogan , Fuxin Zhang , wuzhangjin , stable , Alan Stern , Andrea Parri , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa , LKML Subject: Re: [PATCH V2] MIPS: implement smp_cond_load_acquire() for Loongson-3 Message-ID: <20180711102106.GG13963@arm.com> References: <1531103198-16764-1-git-send-email-chenhc@lemote.com> <20180710121727.GK2476@hirez.programming.kicks-ass.net> <5471216.FKXZRxKFUI@flygoat-ry> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5471216.FKXZRxKFUI@flygoat-ry> User-Agent: Mutt/1.5.23 (2014-03-12) Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 64779 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: will.deacon@arm.com Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On Wed, Jul 11, 2018 at 06:05:51PM +0800, Jiaxun Yang wrote: > On 2018-7-10 Tue at 20:17:27，Peter Zijlstra Wrote： > > Hi Peter > Since Huacai unable to send email via client, I'm going to reply for him > > > Sure.. we all got that far. And no, this isn't the _real_ problem. This > > is a manifestation of the problem. > > > > The problem is that your SFB is broken (per the Linux requirements). We > > require that stores will become visible. That is, they must not > > indefinitely (for whatever reason) stay in the store buffer. > > > > > I don't think this is a hardware bug, in design, SFB will flushed to > > > L1 cache in three cases: > > > > > > 1, data in SFB is full (be a complete cache line); > > > 2, there is a subsequent read access in the same cache line; > > > 3, a 'sync' instruction is executed. > > > > And I think this _is_ a hardware bug. You just designed the bug instead > > of it being by accident. > Yes, we understood that this hardware feature is not supported by LKML, > so it should be a hardware bug for LKML. > > > > It doesn't happen an _any_ other architecture except that dodgy > > ARM11MPCore part. Linux hard relies on stores to become available > > _eventually_. > > > > Still, even with the rules above, the best work-around is still the very > > same cpu_relax() hack. > > As you say, SFB makes Loongson not fully SMP-coherent. > However, modify cpu_relax can solve the current problem, > but not so straight forward. On the other hand, providing a Loongson-specific > WRITE_ONCE looks more reasonable, because it the eliminate the "non-cohrency". > So we can solve the bug from the root. Curious, but why is it not straight-forward to hack cpu_relax()? If you try to hack WRITE_ONCE, you also need to hack atomic_set, atomic64_set and all the places that should be using WRITE_ONCE but aren't ;~) Will