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SubjectRe: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency
On Thu, Oct 14, 2021 at 05:58:16PM +0200, Florian Weimer wrote:
> * Paul E. McKenney:
>
> > On Sun, Oct 10, 2021 at 04:02:02PM +0200, Florian Weimer wrote:
> >> * Linus Torvalds:
> >>
> >> > On Fri, Oct 1, 2021 at 9:26 AM Florian Weimer <fweimer@redhat.com> wrote:
> >> >>
> >> >> Will any conditional branch do, or is it necessary that it depends in
> >> >> some way on the data read?
> >> >
> >> > The condition needs to be dependent on the read.
> >> >
> >> > (Easy way to see it: if the read isn't related to the conditional or
> >> > write data/address, the read could just be delayed to after the
> >> > condition and the store had been done).
> >>
> >> That entirely depends on how the hardware is specified to work. And
> >> the hardware could recognize certain patterns as always producing the
> >> same condition codes, e.g., AND with zero. Do such tests still count?
> >> It depends on what the specification says.
> >>
> >> What I really dislike about this: Operators like & and < now have side
> >> effects, and is no longer possible to reason about arithmetic
> >> expressions in isolation.
> >
> > Is there a reasonable syntax that might help with these issues?
>
> Is this really a problem of syntax?

No, but we seem to need some way to communicate the control-dependency's
ordering intent to the compiler. ;-)

> > Yes, I know, we for sure have conflicting constraints on "reasonable"
> > on copy on this email. What else is new? ;-)
> >
> > I could imagine a tag of some sort on the load and store, linking the
> > operations that needed to be ordered. You would also want that same
> > tag on any conditional operators along the way? Or would the presence
> > of the tags on the load and store suffice?
>
> If the load is assigned to a local variable whose address is not taken
> and which is only assigned this once, it could be used to label the
> store. Then the compiler checks if all paths from the load to the
> store feature a condition that depends on the local variable (where
> qualifying conditions probably depend on the architecture). If it
> can't prove that is the case, it emits a fake no-op condition that
> triggers the hardware barrier. This formulation has the advantage
> that it does not add side effects to operators like <. It even
> generalizes to different barrier-implying instructions besides
> conditional branches.

So something like this?

tagvar = READ_ONCE(a);
if (tagvar)
WRITE_ONCE_COND(b, 1, tagvar);

(This seems to me to be an eminently reasonable syntax.)

Or did I miss a turn in there somewhere?

> But I'm not sure if all this complexity will be a tangible improvement
> over just using that no-op condition all the time (whether implied by
> READ_ONCE, or in a separate ctrl_dep macro).

That is an excellent question. I have no idea what the answer is. ;-)

Thanx, Paul

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