Re: [RFC][PATCH 0/5] arch: atomic rework

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Feb 17, 2014 at 3:17 PM, Torvald Riegel <triegel@redhat.com> wrote:
> On Mon, 2014-02-17 at 14:32 -0800,
>
>> 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.

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

See?

                Linus