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

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Date:	Mon, 17 Feb 2014 16:09:21 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Torvald Riegel <triegel@...hat.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Mon, Feb 17, 2014 at 3:17 PM, Torvald Riegel <triegel@...hat.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
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