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

Boqun Feng
2017-07-20T01:30:41Z

On Wed, Jul 19, 2017 at 02:56:02PM -0700, Paul E. McKenney wrote:
> 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:
> >

Hi Akira,

I wouldn't call that compiler a clever one, it's a broken one ;-)

So here is the thing: READ_ONCE() is a *volatile* load, which means the
compiler has to generate code that actually does a load, so the values
of r1 and r2 depend on the loads, therefore, a sane compiler will not
optimize the if()s out because the volatile semantics of READ_ONCE().
Otherwise, I think we have a lot more to worry about than this case.

> > 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:
>

Hi Paul,

I know the compiler could optimize atomics in very interesting ways, but
this case is about volatile, so I guess our case is still fine? ;-)

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

Great material to wake up mind in the morning! Thanks.

Regards,
Boqun

> 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
> > >
> > >
> >
>