From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757440AbdJKQMg (ORCPT ); Wed, 11 Oct 2017 12:12:36 -0400 Received: from merlin.infradead.org ([205.233.59.134]:50992 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752022AbdJKQMe (ORCPT ); Wed, 11 Oct 2017 12:12:34 -0400 Date: Wed, 11 Oct 2017 18:12:20 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: David Howells , Will Deacon , linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, linux-arch@vger.kernel.org, Jonathan Corbet , Alexander Kuleshov , dvyukov@google.com Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Message-ID: <20171011161220.zqkdhynxerrcmvdd@hirez.programming.kicks-ass.net> References: <20171011122217.GD11106@arm.com> <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> <6309.1507735045@warthog.procyon.org.uk> <20171011155948.GE3521@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171011155948.GE3521@linux.vnet.ibm.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 11, 2017 at 08:59:48AM -0700, Paul E. McKenney wrote: > On Wed, Oct 11, 2017 at 04:17:25PM +0100, David Howells wrote: > > I will, however, quibble with the appropriateness of the name READ_ONCE()... > > I still think it's not sufficiently obvious that this is a barrier and the > > barrier is after. Maybe READ_AND_BARRIER()? > > Linus was unhappy with READ_ONCE_CTRL() to tag control dependencies, but > indicated that he might consider it if it helped code-analysis tools. > Adding Dmitry Vyukov for his thoughts on whether tagging READ_ONCE() > for dependencies would help. Me, I would suggest READ_ONCE_DEP(), but > let's figure out if the bikeshed needs to be painted before arguing over > the color. ;-) Count me one vote for the READ_ONCE() name. This is about dependent reads, which are nothing special on anything except Alpha. We want to remove the exception/specialness from the memory model; and therefore have to fix up all primitives that could possibly be used for these reads to unconditionally issue the barrier (on Alpha). The alternative is: rm -rf arch/alpha. Adding something like READ_ONCE_DEP() does not rid us of the idea that dependent reads are special and thus defeats the purpose, we might as well retain lockless_dereference(). Now; any user of dependent reads must use READ_ONCE() in any case, to avoid load tearing and reloads. So using READ_ONCE() for the dependent reads is not extra or additional (note we'll also have to add the barrier to all our relaxed and release atomics and anything else that implies READ_ONCE and doesn't already imply smp_mb() after).