From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-6.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 0580B7D08A for ; Mon, 7 Jan 2019 13:36:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727204AbfAGNgo (ORCPT ); Mon, 7 Jan 2019 08:36:44 -0500 Received: from mx1.redhat.com ([209.132.183.28]:35098 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726886AbfAGNgo (ORCPT ); Mon, 7 Jan 2019 08:36:44 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8B9CE7F3EA; Mon, 7 Jan 2019 13:36:43 +0000 (UTC) Received: from redhat.com (ovpn-122-143.rdu2.redhat.com [10.10.122.143]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9AF155D9CD; Mon, 7 Jan 2019 13:36:37 +0000 (UTC) Date: Mon, 7 Jan 2019 08:36:36 -0500 From: "Michael S. Tsirkin" To: Peter Zijlstra Cc: Jason Wang , linux-kernel@vger.kernel.org, Alan Stern , Andrea Parri , Will Deacon , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa , Daniel Lustig , linux-arch@vger.kernel.org, netdev@vger.kernel.org, virtualization@lists.linux-foundation.org, Jonathan Corbet , Richard Henderson , Ivan Kokshaysky , Matt Turner , Arnd Bergmann , Luc Van Oostenryck , linux-doc@vger.kernel.org, linux-alpha@vger.kernel.org, linux-sparse@vger.kernel.org Subject: Re: [PATCH RFC 3/4] barriers: convert a control to a data dependency Message-ID: <20190107082223-mutt-send-email-mst@kernel.org> References: <20190102205715.14054-1-mst@redhat.com> <20190102205715.14054-4-mst@redhat.com> <86023cbe-d1ae-a0d6-7b75-26556f1a0c1f@redhat.com> <20190106231756-mutt-send-email-mst@kernel.org> <20190107094610.GA2861@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190107094610.GA2861@worktop.programming.kicks-ass.net> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 07 Jan 2019 13:36:44 +0000 (UTC) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Mon, Jan 07, 2019 at 10:46:10AM +0100, Peter Zijlstra wrote: > On Sun, Jan 06, 2019 at 11:23:07PM -0500, Michael S. Tsirkin wrote: > > On Mon, Jan 07, 2019 at 11:58:23AM +0800, Jason Wang wrote: > > > On 2019/1/3 上午4:57, Michael S. Tsirkin wrote: > > > > > +#if defined(COMPILER_HAS_OPTIMIZER_HIDE_VAR) && \ > > > > + !defined(ARCH_NEEDS_READ_BARRIER_DEPENDS) > > > > + > > > > +#define dependent_ptr_mb(ptr, val) ({ \ > > > > + long dependent_ptr_mb_val = (long)(val); \ > > > > + long dependent_ptr_mb_ptr = (long)(ptr) - dependent_ptr_mb_val; \ > > > > + \ > > > > + BUILD_BUG_ON(sizeof(val) > sizeof(long)); \ > > > > + OPTIMIZER_HIDE_VAR(dependent_ptr_mb_val); \ > > > > + (typeof(ptr))(dependent_ptr_mb_ptr + dependent_ptr_mb_val); \ > > > > +}) > > > > + > > > > +#else > > > > + > > > > +#define dependent_ptr_mb(ptr, val) ({ mb(); (ptr); }) > > > > > > > > > So for the example of patch 4, we'd better fall back to rmb() or need a > > > dependent_ptr_rmb()? > > > > > > Thanks > > > > You mean for strongly ordered architectures like Intel? > > Yes, maybe it makes sense to have dependent_ptr_smp_rmb, > > dependent_ptr_dma_rmb and dependent_ptr_virt_rmb. > > > > mb variant is unused right now so I'll remove it. > > How about naming the thing: dependent_ptr() ? That is without any (r)mb > implications at all. The address dependency is strictly weaker than an > rmb in that it will only order the two loads in qestion and not, like > rmb, any prior to any later load. So I'm fine with this as it's enough for virtio, but I would like to point out two things: 1. E.g. on x86 both SMP and DMA variants can be NOPs but the madatory one can't, so assuming we do not want it to be stronger than rmp then either we want smp_dependent_ptr(), dma_dependent_ptr(), dependent_ptr() or we just will specify that dependent_ptr() works for both DMA and SMP. 2. Down the road, someone might want to order a store after a load. Address dependency does that for us too. Assuming we make dependent_ptr a NOP on x86, we will want an mb variant which isn't a NOP on x86. Will we want to rename dependent_ptr to dependent_ptr_rmb at that point? Thanks, -- MST