Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-04T18:55:30Z

On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote:
> On Fri, Jun 04, 2021 at 06:10:55PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 04, 2021 at 10:35:18AM -0500, Segher Boessenkool wrote:
> > > On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote:
> > > > On naming (sorry Paul for forgetting that in the initial mail); while I
> > > > think using the volatile qualifier for the language feature (can we haz
> > > > plz, kthxbai) makes perfect sense, Paul felt that we might use a
> > > > 'better' name for the kernel use, ctrl_dep_if() was proposed.
> > >
> > > In standard C statements do not have qualifiers. Unless you can
> > > convince the ISO C committee to have them on "if", you will have a very
> > > hard time convincing any serious compiler to do this.
> >
> > While some people like talking to the Committee, I would much rather
> > explore language extensions with the compiler communities. Such
> > extensions can then make their way into the Committee once they show
> > their usefulness.
>
> My point is that you ask compiler developers to paint themselves into a
> corner if you ask them to change such fundamental C syntax.

Once we have some experience with a language extension, the official
syntax for a standardized version of that extension can be bikeshedded.
Committees being what they are, what we use in the meantime will
definitely not be what is chosen, so there is not a whole lot of point
in worrying about the exact syntax in the meantime. ;-)

> > If you have another proposal on how to express this; one you'd rather
> > see implemented, I'm all ears.
>
> I would love to see something that meshes well with the rest of C. But
> there is no 1-1 translation from C code to machine code (not in either
> direction), so anything that more or less depends on that will always
> be awkward. If you can actually express the dependency in your source
> code that will get us 95% to where we want to be.
>
> > Data dependencies, control dependencies and address dependencies, C
> > doesn't really like them, we rely on them. It would be awesome if we can
> > fix this.
>
> Yes. The problem is that C is a high-level language. All C semantics
> are expressed on a an "as-if" level, never as "do this, then that" --
> well, of course that *is* what it says, it's an imperative language just
> like most, but that is just how you *think* about things on a conceptual
> level, there is nothing that says the machine code has to do the same
> thing in the same order as you wrote!

Which is exactly why these conversations are often difficult. There is
a tension between pushing the as-if rule as far as possible within the
compiler on the one hand and allowing developers to write code that does
what is needed on the other. ;-)

Thanx, Paul