From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757216AbdJKMUE (ORCPT ); Wed, 11 Oct 2017 08:20:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36490 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751958AbdJKMUC (ORCPT ); Wed, 11 Oct 2017 08:20:02 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com B3813C059B62 Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=dhowells@redhat.com Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20171010155042.GD3521@linux.vnet.ibm.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> To: paulmck@linux.vnet.ibm.com Cc: dhowells@redhat.com, linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, linux-arch@vger.kernel.org, peterz@infradead.org, will.deacon@arm.com, Jonathan Corbet , Alexander Kuleshov Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <26454.1507724399.1@warthog.procyon.org.uk> Date: Wed, 11 Oct 2017 13:19:59 +0100 Message-ID: <26455.1507724399@warthog.procyon.org.uk> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Wed, 11 Oct 2017 12:20:02 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). David