From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752811AbdJKOSQ (ORCPT ); Wed, 11 Oct 2017 10:18:16 -0400 Received: from foss.arm.com ([217.140.101.70]:33522 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751554AbdJKOSO (ORCPT ); Wed, 11 Oct 2017 10:18:14 -0400 Date: Wed, 11 Oct 2017 15:18:17 +0100 From: Will Deacon To: "Paul E. McKenney" Cc: David Howells , linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, linux-arch@vger.kernel.org, peterz@infradead.org, Jonathan Corbet , Alexander Kuleshov Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Message-ID: <20171011141817.GG11106@arm.com> References: <20171010155042.GD3521@linux.vnet.ibm.com> <1507594969-8347-12-git-send-email-paulmck@linux.vnet.ibm.com> <20171010001951.GA6476@linux.vnet.ibm.com> <8079.1507628146@warthog.procyon.org.uk> <26455.1507724399@warthog.procyon.org.uk> <20171011122217.GD11106@arm.com> <20171011125451.GW3521@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171011125451.GW3521@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 11, 2017 at 05:54:51AM -0700, Paul E. McKenney wrote: > On Wed, Oct 11, 2017 at 01:22:17PM +0100, Will Deacon wrote: > > On Wed, Oct 11, 2017 at 01:19:59PM +0100, David Howells wrote: > > > Paul E. McKenney wrote: > > > > > > > - node = result.terminal_node.node; > > > > - smp_read_barrier_depends(); > > > > + node = READ_ONCE(result.terminal_node.node); /* Address dependency. */ > > > > > > The main problem I have with this method of annotation is that it's not > > > obvious there's a barrier there or which side the barrier is. > > > > > > I think one of the trickiest issues is that a barrier is typically between two > > > things and we're not making it clear what those two things actually are. > > > > > > Also, I would say that the most natural interpretation of READ_ONCE() is that > > > the implicit barrier comes after the read, e.g.: > > > > > > f = READ_ONCE(stuff->foo); > > > /* Implied barrier */ > > > look_at(f->a); > > > look_at(f->b); > > > > > > I.e. READ_ONCE() prevents stuff->foo from being reread whilst you access f and > > > orders LOAD(stuff->foo) before LOAD(f->a) and LOAD(f->b). > > > > FWIW, that's exactly what my patches do, this fixup looks a bit weird > > because it removes a prior barrier which suggests that either (a) it's in > > the wrong place to start with, or (b) we're annotating the wrong load. > > You lost me on this one. Here is the side-by-side change, minus the > comment: > > node = result.terminal_node.node; node = READ_ONCE(result.terminal_node.node); > smp_read_barrier_depends(); > > The barrier was after the load that got annotated. Yes, sorry, I completely lost my ability to read diff. Looking again, I don't actually know what's being ordered by the smp_read_barrier_depends() in the snippet above, given that assigning "node" is a load from the stack afaict. Will