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

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Message-ID: <20140207120124.GF5976@mudshark.cambridge.arm.com>
Date:	Fri, 7 Feb 2014 12:01:25 +0000
From:	Will Deacon <will.deacon@....com>
To:	Torvald Riegel <triegel@...hat.com>
Cc:	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

Hello Torvald,

It looks like Paul clarified most of the points I was trying to make
(thanks Paul!), so I won't go back over them here.

On Thu, Feb 06, 2014 at 09:09:25PM +0000, Torvald Riegel wrote:
> Are you familiar with the formalization of the C11/C++11 model by Batty
> et al.?
> http://www.cl.cam.ac.uk/~mjb220/popl085ap-sewell.pdf
> http://www.cl.cam.ac.uk/~mjb220/n3132.pdf
> 
> They also have a nice tool that can run condensed examples and show you
> all allowed (and forbidden) executions (it runs in the browser, so is
> slow for larger examples), including nice annotated graphs for those:
> http://svr-pes20-cppmem.cl.cam.ac.uk/cppmem/

Thanks for the link, that's incredibly helpful. I've used ppcmem and armmem
in the past, but I didn't realise they have a version for C++11 too.
Actually, the armmem backend doesn't implement our atomic instructions or
the acquire/release accessors, so it's not been as useful as it could be.
I should probably try to learn OCaml...

> IMHO, one thing worth considering is that for C/C++, the C11/C++11 is
> the only memory model that has widespread support.  So, even though it's
> a fairly weak memory model (unless you go for the "only seq-cst"
> beginners advice) and thus comes with a higher complexity, this model is
> what likely most people will be familiar with over time.  Deviating from
> the "standard" model can have valid reasons, but it also has a cost in
> that new contributors are more likely to be familiar with the "standard"
> model.

Indeed, I wasn't trying to write-off the C11 memory model as something we
can never use in the kernel. I just don't think the current situation is
anywhere close to usable for a project such as Linux. If a greater
understanding of the memory model does eventually manifest amongst C/C++
developers (by which I mean, the beginners advice is really treated as
such and there is a widespread intuition about ordering guarantees, as
opposed to the need to use formal tools), then surely the tools and libraries
will stabilise and provide uniform semantics across the 25+ architectures
that Linux currently supports. If *that* happens, this discussion is certainly
worth having again.

Will
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