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From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	Alan Stern <stern@rowland.harvard.edu>,
	Boqun Feng <boqun.feng@gmail.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Akira Yokosawa <akiyks@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>
Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo
Date: Wed, 20 Feb 2019 14:17:30 +0100	[thread overview]
Message-ID: <20190220131730.GB3215@andrea> (raw)
In-Reply-To: <20190220095659.GA17578@brain-police>

On Wed, Feb 20, 2019 at 09:57:00AM +0000, Will Deacon 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?
> 
> 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.

No no, I did mean dep (= addr | data).  As you remarked, control dep.
aren't going to work here.  I expanded on this in my reply to peterz.

  Andrea


> 
> Will

  reply	other threads:[~2019-02-20 13:17 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-19 22:57 [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo Andrea Parri
2019-02-20  2:01 ` Paul E. McKenney
2019-02-20  9:26   ` Peter Zijlstra
2019-02-20  9:57     ` Will Deacon
2019-02-20 13:17       ` Andrea Parri [this message]
2019-02-20 13:14     ` Andrea Parri
2019-02-20 13:27       ` Peter Zijlstra
2019-02-22 11:21         ` Andrea Parri
2019-02-22 13:00           ` Peter Zijlstra
2019-02-25 17:55             ` Paul E. McKenney
2019-02-26  9:21               ` Peter Zijlstra
2019-02-26  9:30               ` Peter Zijlstra
2019-02-26 10:45                 ` Peter Zijlstra
2019-02-26 11:21                   ` Peter Zijlstra
2019-02-26 11:25                     ` Peter Zijlstra
2019-02-26 11:30                       ` Peter Zijlstra
2019-02-26 11:38                         ` Borislav Petkov
2019-02-26 13:49                           ` Peter Zijlstra
2019-02-26 14:28                             ` Paul E. McKenney
2019-02-26 14:47                               ` Peter Zijlstra
2019-02-26 15:39                                 ` Paul E. McKenney
2019-02-26 14:56                               ` Akira Yokosawa
2019-02-26 15:04                                 ` Paul E. McKenney
2019-02-26 15:09                                   ` Akira Yokosawa
2019-02-26 15:04                               ` Peter Zijlstra
2019-03-06 15:46                                 ` Akira Yokosawa
2019-03-06 16:58                                   ` Peter Zijlstra
2019-03-06 17:26                                     ` Paul E. McKenney
2019-03-06 17:24                                   ` Paul E. McKenney
2019-03-02 15:27                             ` Akira Yokosawa
2019-03-04 16:09                               ` Paul E. McKenney
2019-02-20 13:41       ` Will Deacon
2019-02-20 15:30         ` Alan Stern
2019-02-20 15:30           ` Alan Stern
2019-02-20 15:22 ` Alan Stern
2019-02-20 15:22   ` Alan Stern

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