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

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Message-ID: <aFmPZMLGngAE_IHJ@tardis.local>
Date: Mon, 23 Jun 2025 10:31:16 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Danilo Krummrich <dakr@...nel.org>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Krzysztof Wilczy´nski <kwilczynski@...nel.org>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v4 3/6] rust: irq: add support for non-threaded IRQs and
 handlers

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@...nel.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.

Regards,
Boqun

> ---
> Cheers,
> Benno