Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759490AbcJ1N6Z (ORCPT ); Fri, 28 Oct 2016 09:58:25 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:40060 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756260AbcJ1N6Y (ORCPT ); Fri, 28 Oct 2016 09:58:24 -0400 Date: Fri, 28 Oct 2016 15:58:17 +0200 From: Peter Zijlstra To: Tom Gundersen Cc: David Herrmann , LKML , Andy Lutomirski , Jiri Kosina , Greg KH , Hannes Reinecke , Steven Rostedt , Arnd Bergmann , Josh Triplett , Linus Torvalds , Andrew Morton Subject: Re: [RFC v1 06/14] bus1: util - queue utility library Message-ID: <20161028135817.GG3142@twins.programming.kicks-ass.net> References: <20161026191810.12275-1-dh.herrmann@gmail.com> <20161026191810.12275-7-dh.herrmann@gmail.com> <20161027164312.GI3175@twins.programming.kicks-ass.net> <20161028133335.GF3142@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1823 Lines: 37 On Fri, Oct 28, 2016 at 03:47:58PM +0200, Tom Gundersen wrote: > On Fri, Oct 28, 2016 at 3:33 PM, Peter Zijlstra wrote: > > On Fri, Oct 28, 2016 at 01:33:25PM +0200, Tom Gundersen wrote: > > And this, precisely, is what generates all the complexity found in this > > patch. You want to strictly provide more than causality, which does > > not, as per the argument above, provide this at all. > > > > You're providing a semi-global ordering of things that are themselves > > not actually ordered. > > We are providing two things: causality (as in your physics example > above), and consistency (which, I agree, is cute, but not necessarily > crucial). However, the complexity comes from causality. Consistency is > trivial. The only thing needed for consistency is to tag each message > by its sender and use this to resolve conflicts in the ordering. The > alternative would be to just let these entries order arbitrarily > instead, but conceptually it would not be simpler and it would only > save us a few lines of code. Earlier you wrote: > >> To make this work with multicast, we must stage messages first, then > >> commit on a second round. That is, we must find some way to iterate > >> over all clocks before committing, but at the same time preventing any > >> races. The multicast-stability as you just described we get for free > >> by introducing the second-level ordering via sender-address. But you don't need the two-pass thing at all for causality. The entire two-pass thing, and the serialization, is part of the consistency thing. This is not virtually free. For causality, all you need is a single iteration, delivering the message one after the other, only ever doing local clock movements. You do not need to find the max clock in the multicast set and avoid races etc..