On Mon, 2014-02-17 at 16:09 -0800, Linus Torvalds wrote:

On Mon, Feb 17, 2014 at 3:17 PM, Torvald Riegel [off-list ref] wrote:

quoted

On Mon, 2014-02-17 at 14:32 -0800,

quoted

Stop claiming it "can return 1".. It *never* returns 1 unless you do
the load and *verify* it, or unless the load itself can be made to go
away. And with the code sequence given, that just doesn't happen. END
OF STORY.

void foo();
{
  atomic<int> x = 1;
  if (atomic_load(&x, mo_relaxed) == 1)
    atomic_store(&y, 3, mo_relaxed));
}

This is the very example I gave, where the real issue is not that "you
prove that load returns 1", you instead say "store followed by a load
can be combined".

I (in another email I just wrote) tried to show why the "prove
something is true" is a very dangerous model.  Seriously, it's pure
crap. It's broken.

I don't see anything dangerous in the example above with the language
semantics as specified: It's a well-defined situation, given the rules
of the language.  I replied to the other email you wrote with my
viewpoint on why the above is useful, how it compares to what you seem
to what, and where I think we need to start to bridge the gap.

If the C standard defines atomics in terms of "provable equivalence",
it's broken. Exactly because on a *virtual* machine you can prove
things that are not actually true in a *real* machine.

For the control dependencies you have in mind, it's actually the other
way around.  You expect the real machine's properties in a program whose
semantics only give you the virtual machine's properties.  Anything you
prove on the virtual machine will be true on the real machine (in a
correct implementation) -- but you can't expect to have real-machine
properties on language that's based on the virtual machine.

I have the
example of value speculation changing the memory ordering model of the
actual machine.

This example is not true for the language as specified.  It is true for
a modified language that you have in mind, but for this one I've just
seen pretty rough rules so far.  Please see my other reply.