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

Paul E. McKenney
2017-07-19T21:56:07Z

On Thu, Jul 20, 2017 at 06:33:26AM +0900, Akira Yokosawa wrote:
> On 2017/07/20 2:43, Paul E. McKenney wrote:
> > 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.
>
> Well but,
>
> Current example:
>
> 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));
>
> Such a clever compiler might be able to prove that "x" and "y"
> are never modified and end up in the following:
>
> CPU 0 CPU 1
> ======================= =======================
> r1 = READ_ONCE(x); r2 = READ_ONCE(y);
>
> assert(!(r1 == 1 && r2 == 1));
>
> This means it is impossible to describe this example in C,
> doesn't it?

That is a matter of some debate, but it has gotten increasingly more
difficult to get C to do one's bidding over the decades.

> What am I missing here?

The compiler has to work harder in your example case, so it is probably
just a matter of time. In the original with ">=", all the compiler needed
to do was look at all the assignments and the initial value. In the
original, it also had to do reasoning based on control dependencies
(which are not yet part of the C standard).

But yes, the degree to which a compiler can optimize atomics is a matter
of some debate. Here is a proposal to let the developer choose:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0062r1.html

What are your thoughts on this?

Thanx, Paul

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