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From: Alan Stern <stern@rowland.harvard.edu>
To: Will Deacon <will@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	parri.andrea@gmail.com, peterz@infradead.org,
	boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
	j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com,
	dlustig@nvidia.com, joel@joelfernandes.org,
	viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org
Subject: Re: Litmus test for question from Al Viro
Date: Mon, 5 Oct 2020 10:16:28 -0400	[thread overview]
Message-ID: <20201005141628.GA376584@rowland.harvard.edu> (raw)
In-Reply-To: <20201005082002.GA23216@willie-the-truck>

On Mon, Oct 05, 2020 at 09:20:03AM +0100, Will Deacon wrote:
> On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote:
> > Considering the bug in herd7 pointed out by Akira, we should rewrite P1 as:
> > 
> > P1(int *x, int *y)
> > {
> > 	int r2;
> > 
> > 	r = READ_ONCE(*y);
> 
> (r2?)

Oops, yes, thank you.

> > 	WRITE_ONCE(*x, r2);
> > }
> > 
> > Other than that, this is fine.
> 
> But yes, module the typo, I agree that this rewrite is much better than the
> proposal above. The definition of control dependencies on arm64 (per the Arm
> ARM [1]) isn't entirely clear that it provides order if the WRITE is
> executed on both paths of the branch, and I believe there are ongoing
> efforts to try to tighten that up.

Do you mean that people aren't yet in agreement about what the answer 
should be?

How can the CPU tell whether a write is executed on both branches?  
Speculatively execute both and see if they both reach the same write 
instruction?  What if the same instruction is reached on both branches 
but the values written or the addresses written to are different?

>  I'd rather keep _that_ topic separate
> from the "bug in herd" topic to avoid extra confusion.

Indeed.

Alan

  parent reply	other threads:[~2020-10-05 14:16 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-01  4:51 Litmus test for question from Al Viro Paul E. McKenney
2020-10-01 16:15 ` Alan Stern
2020-10-01 16:36   ` Al Viro
2020-10-01 18:39     ` Alan Stern
2020-10-01 19:29       ` Al Viro
2020-10-01 21:30   ` Paul E. McKenney
2020-10-03  2:01     ` Alan Stern
2020-10-03 13:22     ` Alan Stern
2020-10-03 15:16       ` Akira Yokosawa
2020-10-03 17:13         ` Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Alan Stern
2020-10-03 22:50           ` Akira Yokosawa
2020-10-04  1:40           ` [PATCH] tools: memory-model: Document that the LKMM can easily miss control dependencies Alan Stern
2020-10-04 21:07             ` joel
2020-10-04 23:12               ` Paul E. McKenney
2020-10-05 15:15           ` Bug in herd7 [Was: Re: Litmus test for question from Al Viro] Luc Maranget
2020-10-05 15:53             ` Alan Stern
2020-10-05 16:52               ` Paul E. McKenney
2020-10-05 18:19                 ` Alan Stern
2020-10-05 19:18                   ` Paul E. McKenney
2020-10-05 19:48                     ` Alan Stern
2020-10-06 16:39                       ` Paul E. McKenney
2020-10-06 17:05                         ` Alan Stern
2020-10-07 17:50                           ` Paul E. McKenney
2020-10-07 19:40                             ` Alan Stern
2020-10-07 22:38                               ` Paul E. McKenney
2020-10-08  2:25                                 ` Alan Stern
2020-10-08  2:50                                   ` Paul E. McKenney
2020-10-08 14:01                                     ` Alan Stern
2020-10-08 18:32                                       ` Paul E. McKenney
2020-10-05 15:54             ` Paul E. McKenney
2020-10-04 23:31       ` Litmus test for question from Al Viro Paul E. McKenney
2020-10-05  2:38         ` Alan Stern
2020-10-05  8:20           ` Will Deacon
2020-10-05  9:12             ` Will Deacon
2020-10-05 14:01               ` Paul E. McKenney
2020-10-05 14:23               ` Alan Stern
2020-10-05 15:13                 ` Will Deacon
2020-10-05 15:16                   ` Alan Stern
2020-10-05 15:35                     ` Peter Zijlstra
2020-10-05 15:49                       ` Paul E. McKenney
2020-10-05 14:16             ` Alan Stern [this message]
2020-10-05 14:03           ` Paul E. McKenney
2020-10-05 14:24             ` Alan Stern
2020-10-05 14:44             ` joel
2020-10-05 15:55               ` Paul E. McKenney
2020-10-05  8:36         ` David Laight
2020-10-05 13:59           ` Paul E. McKenney
2020-10-03 16:08     ` joel
2020-10-03 16:11       ` joel
2020-10-04 23:13         ` Paul E. McKenney
2020-10-03  2:35   ` Jon Masters
2020-10-04 23:32     ` Paul E. McKenney

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