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 4B294C43381 for ; Tue, 26 Feb 2019 18:34:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1B0D52173C for ; Tue, 26 Feb 2019 18:34:00 +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="NgsAe6kj" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729174AbfBZSd6 (ORCPT ); Tue, 26 Feb 2019 13:33:58 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:49780 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728736AbfBZSd6 (ORCPT ); Tue, 26 Feb 2019 13:33:58 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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=yMjr9P9q7f4UeMLgQtoS/kYz9gHPzsPW36yOTRN+6zs=; b=NgsAe6kjMFgvzv8B7lIgBbeib 51KkbiWjh24fY41XtyeSI4GggO8dJPb8oXh+LmLfZWMV/I8Rdawikou3209ihcL/KSaBGo+y9c3+9 dTXKN1YpPpoYQ7AGhCX5VR11JX4UpASsp/lkjVkTmL/5os59Aq3+Z3FewZV0nLA5vQztacD5Karah GI/NroIH5Ype+qylijX7sealpi9ef4LEjRPddI2mBeNaDt2mKeVxrxgXYgB8KS4h10e39I3m0y+bh +WuNTI9J6jFmSi2RN9IjC9wfHj1s1TAndM5S8xEMSDbU1zlxLZ7TErJZLFXqUtPR1MSHoKXhWXRGA qhwNqYr8w==; Received: from [188.207.104.89] (helo=worktop.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gyhXq-0005Jw-CF; Tue, 26 Feb 2019 18:33:34 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id 7490E984351; Tue, 26 Feb 2019 19:33:35 +0100 (CET) Date: Tue, 26 Feb 2019 19:33:35 +0100 From: Peter Zijlstra To: Will Deacon Cc: Linus Torvalds , linux-arch , Linux List Kernel Mailing , "Paul E. McKenney" , Benjamin Herrenschmidt , Michael Ellerman , Arnd Bergmann , Andrea Parri , Palmer Dabbelt , Daniel Lustig , David Howells , Alan Stern , "Maciej W. Rozycki" , Paul Burton , Ingo Molnar , Yoshinori Sato , Rich Felker , Tony Luck Subject: Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking Message-ID: <20190226183335.GN2861@worktop.programming.kicks-ass.net> References: <20190222185026.10973-1-will.deacon@arm.com> <20190222185026.10973-2-will.deacon@arm.com> <20190226182624.GC28709@fuggles.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190226182624.GC28709@fuggles.cambridge.arm.com> 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 Tue, Feb 26, 2019 at 06:26:24PM +0000, Will Deacon wrote: > On Fri, Feb 22, 2019 at 01:49:32PM -0800, Linus Torvalds wrote: > So I *am* using __this_cpu_xchg() here, which means the architecture can > get away with plain old loads and stores (which is what RISC-V does, for > example), but I see that's not the case on e.g. x86 so I'll rework using > read() and write() because it doesn't hurt. Right, so the problem on x86 is that XCHG has an implicit LOCK prefix, so there no !atomic variant. So even the cpu-local xchg gets the LOCK prefix penalty, even though all we really wanted is a single instruction. Arguably we could fix that for __this_cpu_xchg(), which isn't IRQ-safe.