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 13:41:06 +0000 Message-ID: <20190220134106.GA7523@fuggles.cambridge.arm.com> References: <1550617057-4911-1-git-send-email-andrea.parri@amarulasolutions.com> <20190220020117.GD11787@linux.ibm.com> <20190220092604.GD32494@hirez.programming.kicks-ass.net> <20190220131456.GA3215@andrea> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190220131456.GA3215@andrea> Sender: linux-kernel-owner@vger.kernel.org To: Andrea Parri Cc: Peter Zijlstra , "Paul E. McKenney" , 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 02:14:56PM +0100, Andrea Parri wrote: > 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? > > The change/simplification was mainly intended as precautionary measure > (hence the "we can add it back, ..."): I do agree that it shouldn't be > possible to realize the above state; OTOH, you really don't need to be > too "creative" to imagine possible mis-uses/mis-interpretations of the > (dep ; rfi) term ("forget" ONCEs, trick herd7 with "false dependencies" > or simply wrongly assume that control dependencies are part this "dep", > what else? ...). So, no, I'm not that fond to this term; why should I > be? or you are simply suggesting to expand the changelog? Simplification can mean different things to different people. Whilst I completely agree that relying on the ordering provided by "dep ; rfi" is subtle and error prone, having it forbid the outcome above appeals to a hardware-based mindset of how memory ordering works. In the kernel community, I would posit that the majority of developers are writing code with the underlying hardware in mind and so allowing behaviours in the memory model which are counter to how a real machine operates is likely to make things more confusing, rather than simplifying them! IIRC, herd has a feature where you can "flag" the result of a litmus test to highlight certain internal constraint violations (e.g. warning that a data race is present in a concurrent C11 program). How about we preserve the existing semantics, but flag any use of "dep; rfi" to indicate that the ordering guarantees being relied upon are subtle and error-prone, and therefore should only be considered for fast-path code? Will From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:57920 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725836AbfBTNlO (ORCPT ); Wed, 20 Feb 2019 08:41:14 -0500 Date: Wed, 20 Feb 2019 13:41:06 +0000 From: Will Deacon Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo Message-ID: <20190220134106.GA7523@fuggles.cambridge.arm.com> References: <1550617057-4911-1-git-send-email-andrea.parri@amarulasolutions.com> <20190220020117.GD11787@linux.ibm.com> <20190220092604.GD32494@hirez.programming.kicks-ass.net> <20190220131456.GA3215@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190220131456.GA3215@andrea> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Andrea Parri Cc: Peter Zijlstra , "Paul E. McKenney" , 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: <20190220134106.XClND3UTgtZ5XK3faIz6h-0PHM0z1m2nqtV7UoqsiNE@z> On Wed, Feb 20, 2019 at 02:14:56PM +0100, Andrea Parri wrote: > 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? > > The change/simplification was mainly intended as precautionary measure > (hence the "we can add it back, ..."): I do agree that it shouldn't be > possible to realize the above state; OTOH, you really don't need to be > too "creative" to imagine possible mis-uses/mis-interpretations of the > (dep ; rfi) term ("forget" ONCEs, trick herd7 with "false dependencies" > or simply wrongly assume that control dependencies are part this "dep", > what else? ...). So, no, I'm not that fond to this term; why should I > be? or you are simply suggesting to expand the changelog? Simplification can mean different things to different people. Whilst I completely agree that relying on the ordering provided by "dep ; rfi" is subtle and error prone, having it forbid the outcome above appeals to a hardware-based mindset of how memory ordering works. In the kernel community, I would posit that the majority of developers are writing code with the underlying hardware in mind and so allowing behaviours in the memory model which are counter to how a real machine operates is likely to make things more confusing, rather than simplifying them! IIRC, herd has a feature where you can "flag" the result of a litmus test to highlight certain internal constraint violations (e.g. warning that a data race is present in a concurrent C11 program). How about we preserve the existing semantics, but flag any use of "dep; rfi" to indicate that the ordering guarantees being relied upon are subtle and error-prone, and therefore should only be considered for fast-path code? Will