Message-Id: <DAU5SDZWTB21.2S8F08BVX1ZE1@kernel.org>
Date: Mon, 23 Jun 2025 21:27:43 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
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 9:25 PM CEST, Benno Lossin wrote:
> 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@...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.
>
> 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.

Ah oops, already had the devres improvements in mind, this version uses
the non-pinned devres, which when not dropped will leak an `Arc` in the
`DevresInner`... Which also isn't desired.

---
Cheers,
Benno

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