Re: [PATCH v4 3/6] rust: irq: add support for non-threaded IRQs and handlers

["Benno Lossin" <lossin@kernel.org>]

On Mon Jun 23, 2025 at 7:31 PM CEST, Boqun Feng wrote:
> On Mon, Jun 23, 2025 at 05:26:14PM +0200, Benno Lossin wrote:
>> On Mon Jun 23, 2025 at 5:10 PM CEST, Alice Ryhl wrote:
>> > On Mon, Jun 9, 2025 at 12:47 PM Danilo Krummrich <dakr@kernel.org> wrote:
>> >> On Sun, Jun 08, 2025 at 07:51:08PM -0300, Daniel Almeida wrote:
>> >> > +        dev: &'a Device<Bound>,
>> >> > +        irq: u32,
>> >> > +        flags: Flags,
>> >> > +        name: &'static CStr,
>> >> > +        handler: T,
>> >> > +    ) -> impl PinInit<Self, Error> + 'a {
>> >> > +        let closure = move |slot: *mut Self| {
>> >> > +            // SAFETY: The slot passed to pin initializer is valid for writing.
>> >> > +            unsafe {
>> >> > +                slot.write(Self {
>> >> > +                    inner: Devres::new(
>> >> > +                        dev,
>> >> > +                        RegistrationInner {
>> >> > +                            irq,
>> >> > +                            cookie: slot.cast(),
>> >> > +                        },
>> >> > +                        GFP_KERNEL,
>> >> > +                    )?,
>> >> > +                    handler,
>> >> > +                    _pin: PhantomPinned,
>> >> > +                })
>> >> > +            };
>> >> > +
>> >> > +            // SAFETY:
>> >> > +            // - The callbacks are valid for use with request_irq.
>> >> > +            // - If this succeeds, the slot is guaranteed to be valid until the
>> >> > +            // destructor of Self runs, which will deregister the callbacks
>> >> > +            // before the memory location becomes invalid.
>> >> > +            let res = to_result(unsafe {
>> >> > +                bindings::request_irq(
>> >> > +                    irq,
>> >> > +                    Some(handle_irq_callback::<T>),
>> >> > +                    flags.into_inner() as usize,
>> >> > +                    name.as_char_ptr(),
>> >> > +                    slot.cast(),
>> >> > +                )
>> >> > +            });
>> >> > +
>> >> > +            if res.is_err() {
>> >> > +                // SAFETY: We are returning an error, so we can destroy the slot.
>> >> > +                unsafe { core::ptr::drop_in_place(&raw 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) }
>> >>
>> >> Can't we use try_pin_init!() instead, move request_irq() into the initializer of
>> >> RegistrationInner and initialize inner last?
>> >
>> > We need a pointer to the entire struct when calling
>> > bindings::request_irq. I'm not sure this allows you to easily get one?
>> > I don't think using container_of! here is worth it.
>> 
>> There is the `&this in` syntax (`this` is of type `NonNull<Self>`):
>> 
>>     try_pin_init!(&this in Self {
>>         inner: Devres::new(
>>             dev,
>>             RegistrationInner {
>>                 irq,
>>                 cookie: this.as_ptr().cast(),
>>             },
>>             GFP_KERNEL,
>>         )?,
>>         handler,
>>         _pin: {
>>             to_result(unsafe {
>>                 bindings::request_irq(
>>                     irq,
>>                     Some(handle_irq_callback::<T>),
>>                     flags.into_inner() as usize,
>>                     name.as_char_ptr(),
>>                     slot.as_ptr().cast(),
>
> this is "this" instead of "slot", right?
>
>>                 )
>>             })?;
>>             PhantomPinned
>>         },
>>     })
>> 
>> Last time around, I also asked this question and you replied with that
>> we need to abort the initializer when `request_irq` returns false and
>> avoid running `Self::drop` (thus we can't do it using `pin_chain`).
>> 
>> I asked what we could do instead and you mentioned the `_: {}`
>> initializers and those would indeed solve it, but we can abuse the
>> `_pin` field for that :)
>> 
>
> Hmm.. but if request_irq() fails, aren't we going to call `drop` on
> `inner`, which drops the `Devres` which will eventually call
> `RegistrationInner::drop()`? And that's a `free_irq()` without
> `request_irq()` succeeded.

That is indeed correct :(

But hold on, we aren't allowed to forget the `Devres`, it's a pinned
type and thus the pin guarantee is that it must be dropped before the
underlying memory is freed. So the current version is unsound.

---
Cheers,
Benno