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

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Date:	Mon, 17 Feb 2014 20:55:47 +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 Sat, 2014-02-15 at 10:49 -0800, Linus Torvalds wrote:
> On Sat, Feb 15, 2014 at 9:45 AM, Torvald Riegel <triegel@...hat.com> wrote:
> >
> > I think a major benefit of C11's memory model is that it gives a
> > *precise* specification for how a compiler is allowed to optimize.
> 
> Clearly it does *not*. This whole discussion is proof of that. It's
> not at all clear,

It might not be an easy-to-understand specification, but as far as I'm
aware it is precise.  The Cambridge group's formalization certainly is
precise.  From that, one can derive (together with the usual rules for
as-if etc.) what a compiler is allowed to do (assuming that the standard
is indeed precise).  My replies in this discussion have been based on
reasoning about the standard, and not secret knowledge (with the
exception of no-out-of-thin-air, which is required in the standard's
prose but not yet formalized).

I agree that I'm using the formalization as a kind of placeholder for
the standard's prose (which isn't all that easy to follow for me
either), but I guess there's no way around an ISO standard using prose.

If you see a case in which the standard isn't precise, please bring it
up or open a C++ CWG issue for it.

> and the standard apparently is at least debatably
> allowing things that shouldn't be allowed.

Which example do you have in mind here?  Haven't we resolved all the
debated examples, or did I miss any?

> It's also a whole lot more
> complicated than "volatile", so the likelihood of a compiler writer
> actually getting it right - even if the standard does - is lower.

It's not easy, that's for sure, but none of the high-performance
alternatives are easy either.  There are testing tools out there based
on the formalization of the model, and we've found bugs with them.

And the alternative of using something not specified by the standard is
even worse, I think, because then you have to guess what a compiler
might do, without having any constraints; IOW, one is resorting to "no
sane compiler would do that", and that doesn't seem to very robust
either.

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