Re: mmotm 2009-02-02-17-12 uploaded (x86/nopmd etc.)

Ingo Molnar
2009-02-04T21:29:44Z

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 4 Feb 2009 21:08:46 +0100
> Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > * Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > > > > This is getting painful.
> > > >
> > > > the include file spaghetti is ... interesting there, and it's historic.
> > > >
> > > > I could blame it on highmem, PAE or paravirt - but i'll only blame it on
> > > > paravirt for now because those developers are still around! ;-)
> > > >
> > > > Jeremy, any ideas how to reduce the historic dependency mess in that area?
> > > > I think we should go on three routes at once:
> > > >
> > > > - agressive splitup and separation of type definitions from method
> > > > declaration (+ inline definitions). The spinlock_types.h / spinlock.h
> > > > splitup was really nice in solving such dependency problems.
> > >
> > > I like this one. The mixing up of declare-something with use-something
> > > is often the source of our woes.
> >
> > yes. I mapped this problem area once and this is how the include file
> > spaghetti gets generated in practice:
> >
> > - type A gets declared
> > - type A gets _used_ in the same file in an inline method, BUT,
> >
> > that usage also brings in instantiated use of type X1, X2 and X3.
> >
> > if all types are declared like that everywhere, it can be seen (and it's a
> > mathematical certainty) that the only conflict-free way of doing this is to:
> >
> > - initially add random #include lines to bring in type X1, X2 and X3.
> > Which brings in recursive dependencies from those X1 X2 and X3 files.
> >
> > - when the stuff hits the fan then folks are in a big mess already and
> > only a deep restructuring could gets them out of it - which they rarely
> > do in an iterative environment. So they work it around iteratively:
> > instead of new nice inline methods [which we really prefer] they delay
> > all the 'usage' instantiation to .c file via the use of CPP macros
> > [which we hate because they hide bugs and cause bugs].
>
> None of which would happen if we didn't also have an inlining fetish.

inlining is a nice and convenient tool that helps us do better code in many
cases. It has this long-term dependency-deteriorating effect though.

Ingo