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[209.132.180.67]) by mx.google.com with ESMTP id i27si20140327pgl.305.2019.04.25.01.39.50; Thu, 25 Apr 2019 01:40:04 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=fail header.i=@infradead.org header.s=merlin.20170209 header.b=al7AleF3; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730102AbfDYHPv (ORCPT + 99 others); Thu, 25 Apr 2019 03:15:51 -0400 Received: from merlin.infradead.org ([205.233.59.134]:47468 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726474AbfDYHPu (ORCPT ); Thu, 25 Apr 2019 03:15:50 -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-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding: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=eG7Po0z2bfo1lvBmirguFsmzJiBnDOcOQs8lZ9FhK6o=; b=al7AleF3OAPX9ZnPucLi6hTty S6mmZK3SjaOxPJzaxOCBIkwAQ8SmyO5hp12tdJ5atPIF7fR2KFFcD6/HkIfnafjhMT15x9BTIeWVS j29OX8f2MoB9eY242AZMoI+vkaKkWl8jIoim/RrhPjIPyUYjbRtoTZtY+sQfx8ean/Qnyyv+QaTRa yoEUqoVnIWqNFbgVQlJWeo8MlgQuX4Pbd7YzIUZ3ak1vImyRBVJ4BAVITFZxklMg67euQolVubvEb WMu94Xm3PAS4iL2y3XG7B6w74UfBxC8kI0w7LDUGOXLP4k7TTtRpEPNsbOOb+bWw+CuZXg2/zRJLD h/veBsR1Q==; 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 1hJYbR-0000Eh-GG; Thu, 25 Apr 2019 07:15:29 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 26BCB203E8871; Thu, 25 Apr 2019 09:15:28 +0200 (CEST) Date: Thu, 25 Apr 2019 09:15:28 +0200 From: Peter Zijlstra To: Paul Burton Cc: "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 , Huang Pei Subject: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage Message-ID: <20190425071528.GU11158@hirez.programming.kicks-ass.net> References: <20190424123656.484227701@infradead.org> <20190424124421.636767843@infradead.org> <20190424211759.52xraajqwudc2fza@pburton-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190424211759.52xraajqwudc2fza@pburton-laptop> 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 Wed, Apr 24, 2019 at 09:18:04PM +0000, Paul Burton wrote: > Hi Peter, > > On Wed, Apr 24, 2019 at 02:36:58PM +0200, Peter Zijlstra wrote: > > The comment describing the loongson_llsc_mb() reorder case doesn't > > make any sense what so ever. Instruction re-ordering is not an SMP > > artifact, but rather a CPU local phenomenon. This means that _every_ > > LL/SC loop needs this barrier right in front to avoid the CPU from > > leaking a memop inside it. > > Does it? It does, however.. > The Loongson bug being described here causes an sc to succeed > erroneously if certain loads or stores are executed between the ll & > associated sc, including speculatively. On a UP system there's no code > running on other cores to race with us & cause our sc to fail - ie. sc > should always succeed anyway, so if the bug hits & the sc succeeds > what's the big deal? It would have succeeded anyway. At least that's my > understanding based on discussions with Loongson engineers a while ago. Ah! So that wasn't spelled out as such. This basically says that: Yes, it also screws with SC on UP, however the failure case is harmless. (Also the comment with loongson_llsc_mb() seems incomplete in that it doesn't mention the SC can also erroneously fail; typically that isn't a problem because we'll just get an extra loop around and succeed eventually.) That said; I'm not entirely sure. The reason we use LL/SC even for CPU-local variables is because of interrupts and the like. Would not a false positive be a problem if it _should_ fail because of an interrupt? > Having said that, if you have a strong preference for adding the barrier > in UP systems anyway then I don't really object. It's not like anyone's > likely to want to run a UP kernel on the affected systems, nevermind > care about a miniscule performance impact. It mostly all didn't make sense to me; and having a consistent recipie for LL/SC loops is easier on the person occasionally looking at all this (me, mostly :-). (also, you should probably have a look at include/asm-generic/bitops/atomic.h) > One possibility your change could benefit would be if someone ran Linux > on a subset of cores & some non-Linux code on other cores, in which case > there could be something to cause the sc to fail. I've no idea if that's > something these Loongson systems ever do though. Or a bunch of UP guests ? > > For the branch speculation case; if futex_atomic_cmpxchg_inatomic() > > needs one at the bne branch target, then surely the normal > > __cmpxch_asmg() implementation does too. We cannot rely on the > > s/cmpxch_asmg/cmpxchg_asm/ Typing hard :-)