Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

[Linus Torvalds]

On Tue, Sep 28, 2021 at 2:15 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> Introduce the ctrl_dep macro in the generic headers, and use it
> everywhere it appears relevant.

The control dependency is so subtle - just see our discussions - that
I really think every single use of it needs to have a comment about
why it's needed.

Right now, that patch seems to just sprinkle them more or less
randomly. That's absolutely not what I want. It will just mean that
other people start sprinkling them randomly even more, and nobody will
dare remove them.

So I'd literally want a comment about "this needs a control
dependency, because otherwise the compiler could merge the two
identical stores X and Y".

When you have a READ_ONCE() in the conditional, and a WRITE_ONCE() in
the statement protected by the conditional, there is *no* need to
randomly sprinkle noise that doesn't matter.

And if there *is* need ("look, we have that same store in both the if-
and the else-statement" or whatever), then say so, and state that
thing.

               Linus