From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo Date: Wed, 20 Feb 2019 09:57:00 +0000 Message-ID: <20190220095659.GA17578@brain-police> References: <1550617057-4911-1-git-send-email-andrea.parri@amarulasolutions.com> <20190220020117.GD11787@linux.ibm.com> <20190220092604.GD32494@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190220092604.GD32494@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: "Paul E. McKenney" , Andrea Parri , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Alan Stern , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig List-Id: linux-arch.vger.kernel.org On Wed, Feb 20, 2019 at 10:26:04AM +0100, Peter Zijlstra wrote: > On Tue, Feb 19, 2019 at 06:01:17PM -0800, Paul E. McKenney wrote: > > On Tue, Feb 19, 2019 at 11:57:37PM +0100, Andrea Parri wrote: > > > Remove this subtle (and, AFAICT, unused) ordering: we can add it back, > > > if necessary, but let us not encourage people to rely on this thing. > > > > > > For example, the following "exists" clause can be satisfied with this > > > change: > > > > > > C dep-rfi > > > > > > { } > > > > > > P0(int *x, int *y) > > > { > > > WRITE_ONCE(*x, 1); > > > smp_store_release(y, 1); > > > } > > > > > > P1(int *x, int *y, int *z) > > > { > > > int r0; > > > int r1; > > > int r2; > > > > > > r0 = READ_ONCE(*y); > > > WRITE_ONCE(*z, r0); > > > r1 = smp_load_acquire(z); > > > r2 = READ_ONCE(*x); > > > } > > > > > > exists (1:r0=1 /\ 1:r2=0) > > > > Any objections? If I don't hear any in a couple days, I will apply this. > > IIUC you cannot build hardware that allows the above, so why would we > allow it? Agreed. Maybe the intention was to make the dependency between the read of *y and the write of *z on P1 a control dependency instead? That's certainly allowed on arm64. Will From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.101.70]:54978 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726989AbfBTJ5J (ORCPT ); Wed, 20 Feb 2019 04:57:09 -0500 Date: Wed, 20 Feb 2019 09:57:00 +0000 From: Will Deacon Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo Message-ID: <20190220095659.GA17578@brain-police> References: <1550617057-4911-1-git-send-email-andrea.parri@amarulasolutions.com> <20190220020117.GD11787@linux.ibm.com> <20190220092604.GD32494@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190220092604.GD32494@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: "Paul E. McKenney" , Andrea Parri , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Alan Stern , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig Message-ID: <20190220095700.RFOl0Lg7AGKuBk-159RvFmbeh21V2rbh1G5gXi8tUgw@z> On Wed, Feb 20, 2019 at 10:26:04AM +0100, Peter Zijlstra wrote: > On Tue, Feb 19, 2019 at 06:01:17PM -0800, Paul E. McKenney wrote: > > On Tue, Feb 19, 2019 at 11:57:37PM +0100, Andrea Parri wrote: > > > Remove this subtle (and, AFAICT, unused) ordering: we can add it back, > > > if necessary, but let us not encourage people to rely on this thing. > > > > > > For example, the following "exists" clause can be satisfied with this > > > change: > > > > > > C dep-rfi > > > > > > { } > > > > > > P0(int *x, int *y) > > > { > > > WRITE_ONCE(*x, 1); > > > smp_store_release(y, 1); > > > } > > > > > > P1(int *x, int *y, int *z) > > > { > > > int r0; > > > int r1; > > > int r2; > > > > > > r0 = READ_ONCE(*y); > > > WRITE_ONCE(*z, r0); > > > r1 = smp_load_acquire(z); > > > r2 = READ_ONCE(*x); > > > } > > > > > > exists (1:r0=1 /\ 1:r2=0) > > > > Any objections? If I don't hear any in a couple days, I will apply this. > > IIUC you cannot build hardware that allows the above, so why would we > allow it? Agreed. Maybe the intention was to make the dependency between the read of *y and the write of *z on P1 a control dependency instead? That's certainly allowed on arm64. Will