From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752455AbcBBDzH (ORCPT ); Mon, 1 Feb 2016 22:55:07 -0500 Received: from e32.co.us.ibm.com ([32.97.110.150]:56303 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751972AbcBBDzF (ORCPT ); Mon, 1 Feb 2016 22:55:05 -0500 X-IBM-Helo: d03dlp01.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Mon, 1 Feb 2016 19:54:58 -0800 From: "Paul E. McKenney" To: Will Deacon Cc: Peter Zijlstra , "Maciej W. Rozycki" , David Daney , =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= , Ralf Baechle , linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, boqun.feng@gmail.com Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock() Message-ID: <20160202035458.GF6719@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20160127114348.GF2390@arm.com> <20160127145421.GT6357@twins.programming.kicks-ass.net> <20160127152158.GJ2390@arm.com> <20160127233836.GQ4503@linux.vnet.ibm.com> <20160128095718.GC30928@arm.com> <20160128223131.GV4503@linux.vnet.ibm.com> <20160129095958.GA4541@arm.com> <20160129102253.GG4503@linux.vnet.ibm.com> <20160201135621.GD6828@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160201135621.GD6828@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16020203-0005-0000-0000-00001BFC03AA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 01, 2016 at 01:56:22PM +0000, Will Deacon wrote: > On Fri, Jan 29, 2016 at 02:22:53AM -0800, Paul E. McKenney wrote: > > On Fri, Jan 29, 2016 at 09:59:59AM +0000, Will Deacon wrote: > > > On Thu, Jan 28, 2016 at 02:31:31PM -0800, Paul E. McKenney wrote: > > > > [ . . . ] > > > > > > For Linux in general, this is a question: How strict do we want to be > > > > about matching the type of write with the corresponding read? My > > > > default approach is to initially be quite strict and loosen as needed. > > > > Here "quite strict" might mean requiring an rcu_assign_pointer() for > > > > the write and rcu_dereference() for the read, as opposed to (say) > > > > ACCESS_ONCE() for the read. (I am guessing that this would be too > > > > tight, but it makes a good example.) > > > > > > > > Thoughts? > > > > > > That sounds broadly sensible to me and allows rcu_assign_pointer and > > > rcu_dereference to be used as drop-in replacements for release/acquire > > > where local transitivity isn't required. However, I don't think we can > > > rule out READ_ONCE/WRITE_ONCE interactions as they seem to be used > > > already in things like the osq_lock (albeit without the address > > > dependency). > > > > Agreed. So in the most strict case that I can imagine anyone putting > > up with, we have the following pairings: > > I think we can group these up: > > Locally transitive: > > > o smp_store_release() -> smp_load_acquire() (locally transitive) > > Locally transitive chain termination: > > (i.e. these can't be used to extend a chain) Agreed. > > o smp_store_release() -> lockless_dereference() (???) > > o rcu_assign_pointer() -> rcu_dereference() > > o smp_store_release() -> READ_ONCE(); if I am OK with the first and last, but I believe that the middle one has real use cases. So the rcu_assign_pointer() -> rcu_dereference() case needs to be locally transitive. > Globally transitive: > > > o smp_mb(); WRITE_ONCE() -> READ_ONCE(); (globally transitive) > > o synchronize_rcu(); WRITE_ONCE() -> READ_ONCE(); (globally transitive) > > RCU: > > > o synchronize_rcu(); WRITE_ONCE() -> rcu_read_lock(); READ_ONCE() > > (strange and wonderful properties) Agreed. > > Seem reasonable, or am I missing some? > > Looks alright to me. So I have some litmus tests to generate. ;-) Thnax, Paul