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

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Message-ID: <1392803164.18779.8498.camel@triegel.csb>
Date:	Wed, 19 Feb 2014 10:46:04 +0100
From:	Torvald Riegel <triegel@...hat.com>
To:	Peter.Sewell@...cam.ac.uk
Cc:	"mark.batty@...cam.ac.uk" <Mark.Batty@...cam.ac.uk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	"ramana.radhakrishnan" <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Tue, 2014-02-18 at 23:48 +0000, Peter Sewell wrote:
> On 18 February 2014 20:43, Torvald Riegel <triegel@...hat.com> wrote:
> > On Tue, 2014-02-18 at 12:12 +0000, Peter Sewell wrote:
> >> Several of you have said that the standard and compiler should not
> >> permit speculative writes of atomics, or (effectively) that the
> >> compiler should preserve dependencies.  In simple examples it's easy
> >> to see what that means, but in general it's not so clear what the
> >> language should guarantee, because dependencies may go via non-atomic
> >> code in other compilation units, and we have to consider the extent to
> >> which it's desirable to limit optimisation there.
> >
> > [...]
> >
> >> 2) otherwise, the language definition should prohibit it but the
> >>    compiler would have to preserve dependencies even in compilation
> >>    units that have no mention of atomics.  It's unclear what the
> >>    (runtime and compiler development) cost of that would be in
> >>    practice - perhaps Torvald could comment?
> >
> > If I'm reading the standard correctly, it requires that data
> > dependencies are preserved through loads and stores, including nonatomic
> > ones.  That sounds convenient because it allows programmers to use
> > temporary storage.
> 
> The standard only needs this for consume chains,

That's right, and the runtime cost / implementation problems of
mo_consume was what I was making statements about.  Sorry if that wasn't
clear.

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