Re: [PATCH] documentation: Fix two-CPU control-dependency example

Paul E. McKenney
2017-07-19T17:43:39Z

On Mon, Jul 17, 2017 at 05:24:42PM +0900, Akira Yokosawa wrote:
> >From b798b9b631e237d285aa8699da00bfb8ced33bea Mon Sep 17 00:00:00 2001
> From: Akira Yokosawa <akiyks@gmail.com>
> Date: Mon, 17 Jul 2017 16:25:33 +0900
> Subject: [PATCH] documentation: Fix two-CPU control-dependency example
>
> In commit 5646f7acc95f ("memory-barriers: Fix control-ordering
> no-transitivity example"), the operator in "if" statement of
> the two-CPU example was modified from ">=" to ">".
> Now the example misses the point because there is no party
> who will modify "x" nor "y". So each CPU performs only the
> READ_ONCE().
>
> The point of this example is to use control dependency for ordering,
> and the WRITE_ONCE() should always be executed.
>
> So it was correct prior to the above mentioned commit. Partial
> revert of the commit (with context adjustments regarding other
> changes thereafter) restores the point.
>
> Note that the three-CPU example demonstrating the lack of
> transitivity stands regardless of this partial revert.
>
> Signed-off-by: Akira Yokosawa <akiyks@gmail.com>

Hello, Akira,

You are quite right that many compilers would generate straightforward
code for the code fragment below, and in that case, the assertion could
never trigger due to either TSO or control dependencies, depending on
the architecture this was running on.

However, if the compiler was too smart for our good, it could figure
out that "x" and "y" only take on the values zero and one, so that
the "if" would always be taken. At that point, the compiler could
simply ignore the "if" with the result shown below.

> ---
> Documentation/memory-barriers.txt | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index c4ddfcd..c1ebe99 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -851,7 +851,7 @@ demonstrated by two related examples, with the initial values of
> CPU 0 CPU 1
> ======================= =======================
> r1 = READ_ONCE(x); r2 = READ_ONCE(y);
> - if (r1 > 0) if (r2 > 0)
> + if (r1 >= 0) if (r2 >= 0)
> WRITE_ONCE(y, 1); WRITE_ONCE(x, 1);
>
> assert(!(r1 == 1 && r2 == 1));

Original program:

CPU 0 CPU 1
======================= =======================
r1 = READ_ONCE(x); r2 = READ_ONCE(y);
if (r1 >= 0) if (r2 >= 0)
WRITE_ONCE(y, 1); WRITE_ONCE(x, 1);

assert(!(r1 == 1 && r2 == 1));

Enthusiastically optimized program:

CPU 0 CPU 1
======================= =======================
r1 = READ_ONCE(x); r2 = READ_ONCE(y);
WRITE_ONCE(y, 1); WRITE_ONCE(x, 1);

assert(!(r1 == 1 && r2 == 1));

This optimized version could trigger the assertion.

So we do need to stick with the ">" comparison.

That said, I very much welcome critical reviews of memory-barriers.txt,
so please do feel free to continue doing that!

Thanx, Paul