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

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Date:	Tue, 18 Feb 2014 16:46:57 +0100
From:	Torvald Riegel <triegel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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, 2014-02-17 at 16:09 -0800, Linus Torvalds wrote:
> 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.

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.

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