Re: [PATCH v4 06/35] cleanup: Basic compatibility with context analysis

[Marco Elver <elver@google.com>]

On Fri, 12 Dec 2025 at 10:43, Peter Zijlstra <peterz@infradead.org> wrote:
[..]
> > Correct. We're trading false negatives over false positives at this
> > point, just to get things to compile cleanly.
>
> Right, and this all 'works' right up to the point someone sticks a
> must_not_hold somewhere.
>
> > > > Better support for Linux's scoped guard design could be added in
> > > > future if deemed critical.
> > >
> > > I would think so, per the above I don't think this is 'right'.
> >
> > It's not sound, but we'll avoid false positives for the time being.
> > Maybe we can wrangle the jigsaw of macros to let it correctly acquire
> > and then release (via a 2nd cleanup function), it might be as simple
> > as marking the 'constructor' with the right __acquires(..), and then
> > have a 2nd __attribute__((cleanup)) variable that just does a no-op
> > release via __release(..) so we get the already supported pattern
> > above.
>
> Right, like I mentioned in my previous email; it would be lovely if at
> the very least __always_inline would get a *very* early pass such that
> the above could be resolved without inter-procedural bits. I really
> don't consider an __always_inline as another procedure.
>
> Because as I already noted yesterday, cleanup is now all
> __always_inline, and as such *should* all end up in the one function.
>
> But yes, if we can get a magical mash-up of __cleanup and __release (let
> it be knows as __release_on_cleanup ?) that might also work I suppose.
> But I vastly prefer __always_inline actually 'working' ;-)

The truth is that __always_inline working in this way is currently
infeasible. Clang and LLVM's architecture simply disallow this today:
the semantic analysis that -Wthread-safety does happens over the AST,
whereas always_inline is processed by early passes in the middle-end
already within LLVM's pipeline, well after semantic analysis. There's
a complexity budget limit for semantic analysis (type checking,
warnings, assorted other errors), and path-sensitive &
intra-procedural analysis over the plain AST is outside that budget.
Which is why tools like clang-analyzer exist (symbolic execution),
where it's possible to afford that complexity since that's not
something that runs for a normal compile.

I think I've pushed the current version of Clang's -Wthread-safety
already far beyond what folks were thinking is possible (a variant of
alias analysis), but even my healthy disregard for the impossible
tells me that making path-sensitive intra-procedural analysis even if
just for __always_inline functions is quite possibly a fool's errand.

So either we get it to work with what we have, or give up.

Thanks,
-- Marco