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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A5F39C10F1E for ; Tue, 20 Dec 2022 15:10:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233760AbiLTPKx (ORCPT ); Tue, 20 Dec 2022 10:10:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55726 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233257AbiLTPKt (ORCPT ); Tue, 20 Dec 2022 10:10:49 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5C53A62EA; Tue, 20 Dec 2022 07:10:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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; bh=FFNeZkVqnyoGozFMwV8suZuO15qmhw0RI08pyhsvplQ=; b=aj3xUuejuA9YH7oE/tMQbHxDrC 5p9yKMxF2BMunnN+3aFfpuq4xvQtgL6Mtrf776Ph7v2oiGV9BjVApUkzIjk/WfaM1JFNiL/d3uNFt Cv73DfWGBBk8PaAlwJVSwfYr3hfq3P/tA8V8DL1kjpeeNx7zhonlT8kvfC88Q40rDC5AGMpqylyTa hPk5u9253JZPrw4gklKKy0rEjWbNLpr/vX9MLoGk1h4JsNN6FMYf6ULtZxxqymlZtiTpJmvNfyRE+ Vda14xeiFfnUBjl0zdQiemuszxdTFOeh76ZedxboR2+ROwt3pEguOkI7xDP9FcJk8q2/NS7Axhz1z OnVJkmTA==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1p7eFb-001u7k-I3; Tue, 20 Dec 2022 15:09:52 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 3BE3F300193; Tue, 20 Dec 2022 16:09:38 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 0457E223694AF; Tue, 20 Dec 2022 16:09:37 +0100 (CET) Date: Tue, 20 Dec 2022 16:09:37 +0100 From: Peter Zijlstra To: Linus Torvalds Cc: Boqun Feng , corbet@lwn.net, will@kernel.org, mark.rutland@arm.com, catalin.marinas@arm.com, dennis@kernel.org, tj@kernel.org, cl@linux.com, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, Herbert Xu , davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, joro@8bytes.org, suravee.suthikulpanit@amd.com, robin.murphy@arm.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, Arnd Bergmann , penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, Andrew Morton , vbabka@suse.cz, roman.gushchin@linux.dev, 42.hyeyoo@gmail.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-s390@vger.kernel.org, linux-crypto@vger.kernel.org, iommu@lists.linux.dev, linux-arch@vger.kernel.org Subject: Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}() Message-ID: References: <20221219153525.632521981@infradead.org> <20221219154119.154045458@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Tue, Dec 20, 2022 at 08:31:19AM -0600, Linus Torvalds wrote: > On Tue, Dec 20, 2022 at 5:09 AM Peter Zijlstra wrote: > > > > On Mon, Dec 19, 2022 at 12:07:25PM -0800, Boqun Feng wrote: > > > > > > I wonder whether we should use "(*(u128 *)ptr)" instead of "(*(unsigned > > > long *) ptr)"? Because compilers may think only 64bit value pointed by > > > "ptr" gets modified, and they are allowed to do "useful" optimization. > > > > In this I've copied the existing cmpxchg_double() code; I'll have to let > > the arch folks speak here, I've no clue. > > It does sound like the right thing to do. I doubt it ends up making a > difference in practice, but yes, the asm doesn't have a memory > clobber, so the input/output types should be the right ones for the > compiler to not possibly do something odd and cache the part that it > doesn't see as being accessed. Right, and x86 does just *ptr, without trying to cast away the volatile even. I've pushed out a *(u128 *)ptr variant for arm64 and s390, then at least we'll know if the compiler objects.