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

[Torvald Riegel]

On Tue, 2014-02-18 at 18:21 +0000, Peter Sewell wrote:
> On 18 February 2014 17:38, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, Feb 18, 2014 at 4:12 AM, Peter Sewell <Peter.Sewell@xxxxxxxxxxxx> wrote:
> >>
> >> For example, suppose we have, in one compilation unit:
> >>
> >>     void f(int ra, int*rb) {
> >>       if (ra==42)
> >>         *rb=42;
> >>       else
> >>         *rb=42;
> >>     }
> >
> > So this is a great example, and in general I really like your page at:
> >
> >> For more context, this example is taken from a summary of the thin-air
> >> problem by Mark Batty and myself,
> >> <www.cl.cam.ac.uk/~pes20/cpp/notes42.html>, and the problem with
> >> dependencies via other compilation units was AFAIK first pointed out
> >> by Hans Boehm.
> >
> > and the reason I like your page is that it really talks about the
> > problem by pointing to the "unoptimized" code, and what hardware would
> > do.
> 
> Thanks.  It's certainly necessary to separately understand what compiler
> optimisation and the hardware might do, to get anywhere here.   But...
> 
> > As mentioned, I think that's actually the *correct* way to think about
> > the problem space, because it allows the programmer to take hardware
> > characteristics into account, without having to try to "describe" them
> > at a source level.
> 
> ...to be clear, I am ultimately after a decent source-level description of what
> programmers can depend on, and we (Mark and I) view that page as
> identifying constraints on what that description can say.  There are too
> many compiler optimisations for people to reason directly in terms of
> the set of all transformations that they do, so we need some more
> concise and comprehensible envelope identifying what is allowed,
> as an interface between compiler writers and users.  AIUI that's basically
> what Torvald is arguing.

Yes, that's one reason.  Another one is that if a programmer would
actually want to use atomics in a machine-independent / portable way,
he/she does also not want to reason about how all those transformations
might interact with the machine's memory model.

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