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

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Message-ID: <1392671158.18779.6909.camel@triegel.csb>
Date:	Mon, 17 Feb 2014 22:05:58 +0100
From:	Torvald Riegel <triegel@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 12:23 -0800, Paul E. McKenney wrote:
> On Mon, Feb 17, 2014 at 08:55:47PM +0100, Torvald Riegel wrote:
> > 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.
> 
> I suggest that I go through the Linux kernel's requirements for atomics
> and memory barriers and see how they map to C11 atomics.  With that done,
> we would have very specific examples to go over.  Without that done, the
> discussion won't converge very well.
> 
> Seem reasonable?

Sounds good!

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