From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753434AbeBVKUp (ORCPT ); Thu, 22 Feb 2018 05:20:45 -0500 Received: from merlin.infradead.org ([205.233.59.134]:49438 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753386AbeBVKUm (ORCPT ); Thu, 22 Feb 2018 05:20:42 -0500 Date: Thu, 22 Feb 2018 11:20:26 +0100 From: Peter Zijlstra To: Daniel Lustig Cc: Boqun Feng , "Paul E. McKenney" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, stern@rowland.harvard.edu, parri.andrea@gmail.com, will.deacon@arm.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, nborisov@suse.com, Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman Subject: Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based external-view litmus test Message-ID: <20180222102026.GT25235@hirez.programming.kicks-ass.net> References: <20180220232405.GA19274@linux.vnet.ibm.com> <1519169112-20593-10-git-send-email-paulmck@linux.vnet.ibm.com> <20180222032349.klcuiq23f52sfop6@tardis> <20180222041357.GB2855@linux.vnet.ibm.com> <20180222052746.vofmqbpnmfahck3z@tardis> <20180222100636.GP25201@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180222100636.GP25201@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 22, 2018 at 11:06:36AM +0100, Peter Zijlstra wrote: > On Wed, Feb 21, 2018 at 09:42:08PM -0800, Daniel Lustig wrote: > > And yes, if we go with a purely RCpc interpretation of acquire and > > release, then I don't believe the writes in the previous critical > > section would be ordered with the writes in the subsequent critical > > section. > > Excuse my ignorance (also jumping in the middle of things), but how can > this be? > > spin_unlock() is a store-release, this means the write to the lock word > must happen after any stores inside the critical section. > > spin_lock() is a load-acquire + test-and-set-ctrl-dep, we'll only > proceed with the critical section if we observe the lock 'unlocked', > which also means we must observe the stores prior to the unlock. > > And both the ctrl-dep and the ACQUIRE ensure future stores cannot happen > before. > > So while the lock store and subsequent critical section stores are > unordered, I don't see how it would be possible to not be ordered > against stores from a previous critical section. > Or are we talking about a third party observing while not partaking in the lock-chain? Then I agree, the stores can be observed out of order by this 3rd actor.