Re: [PATCH v3 1/2] rust: irq: add support for request_irq()

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Jun 4, 2025 at 9:37 AM Benno Lossin <lossin@kernel.org> wrote:
>
> On Mon Jun 2, 2025 at 5:20 PM CEST, Alice Ryhl wrote:
> > On Wed, May 14, 2025 at 10:04:43PM +0200, Benno Lossin wrote:
> >> On Wed May 14, 2025 at 9:20 PM CEST, Daniel Almeida wrote:
> >> > +                )
> >> > +            });
> >> > +
> >> > +            if res.is_err() {
> >> > +                // SAFETY: We are returning an error, so we can destroy the slot.
> >> > +                unsafe { core::ptr::drop_in_place(addr_of_mut!((*slot).handler)) };
> >> > +            }
> >> > +
> >> > +            res
> >> > +        };
> >> > +
> >> > +        // SAFETY:
> >> > +        // - if this returns Ok, then every field of `slot` is fully
> >> > +        // initialized.
> >> > +        // - if this returns an error, then the slot does not need to remain
> >> > +        // valid.
> >> > +        unsafe { pin_init_from_closure(closure) }
> >>
> >> Please don't use `pin_init_from_closure`, instead do this:
> >>
> >>     pin_init!(Self {
> >>         irq,
> >>         handler,
> >>         _pin: PhantomPinned
> >>     })
> >>     .pin_chain(|this| {
> >>         // SAFETY: TODO: correct FFI safety requirements
> >>         to_result(unsafe {
> >>             bindings::request_irq(...)
> >>         })
> >>     })
> >>
> >> The `pin_chain` function is exactly for this use-case, doing some
> >> operation that might fail after initializing & it will drop the value
> >> when the closure fails.
> >
> > No, that doesn't work. Using pin_chain will call free_irq if the call to
> > request_irq fails, which is incorrect.
>
> Good catch. That's a bit annoying then... I wonder if there is a
> primitive missing in pin-init that could help with this... Any ideas?

I believe initializers for underscore fields would do it. We could
potentially abuse the _pin field, but frankly I think that's too
confusing to the reader.

Alice