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Date:   Thu, 30 Mar 2023 12:06:00 +0100
From:   Gary Guo <gary@garyguo.net>
To:     y86-dev@protonmail.com
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
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        Alice Ryhl <alice@ryhl.io>, rust-for-linux@vger.kernel.org,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH v3 08/13] rust: init: add `stack_pin_init!` macro
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On Wed, 29 Mar 2023 22:33:29 +0000
y86-dev@protonmail.com wrote:

> From: Benno Lossin <y86-dev@protonmail.com>
> 
> The `stack_pin_init!` macro allows pin-initializing a value on the
> stack. It accepts a `impl PinInit<T, E>` to initialize a `T`. It allows
> propagating any errors via `?` or handling it normally via `match`.
> 
> Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
> ---
>  rust/kernel/init.rs            | 70 +++++++++++++++++++++++++++++++---
>  rust/kernel/init/__internal.rs | 46 ++++++++++++++++++++++
>  2 files changed, 110 insertions(+), 6 deletions(-)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 428b5c2ac516..3358f14beffb 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -12,7 +12,8 @@
>  //!
>  //! To initialize a `struct` with an in-place constructor you will need two things:
>  //! - an in-place constructor,
> -//! - a memory location that can hold your `struct`.
> +//! - a memory location that can hold your `struct` (this can be the [stack], an [`Arc<T>`],
> +//!   [`UniqueArc<T>`], [`Box<T>`] or any other smart pointer that implements [`InPlaceInit`]).
>  //!
>  //! To get an in-place constructor there are generally three options:
>  //! - directly creating an in-place constructor using the [`pin_init!`] macro,
> @@ -180,6 +181,7 @@
>  //! [pinning]: https://doc.rust-lang.org/std/pin/index.html
>  //! [structurally pinned fields]:
>  //!     https://doc.rust-lang.org/std/pin/index.html#pinning-is-structural-for-field
> +//! [stack]: crate::stack_pin_init
>  //! [`Arc<T>`]: crate::sync::Arc
>  //! [`impl PinInit<Foo>`]: PinInit
>  //! [`impl PinInit<T, E>`]: PinInit
> @@ -199,6 +201,62 @@ pub mod __internal;
>  #[doc(hidden)]
>  pub mod macros;
> 
> +/// Initialize and pin a type directly on the stack.
> +///
> +/// # Examples
> +///
> +/// ```rust
> +/// # #![allow(clippy::disallowed_names, clippy::new_ret_no_self)]
> +/// # use kernel::{init, pin_init, stack_pin_init, init::*, sync::Mutex, new_mutex};
> +/// # use macros::pin_data;
> +/// # use core::pin::Pin;
> +/// #[pin_data]
> +/// struct Foo {
> +///     #[pin]
> +///     a: Mutex<usize>,
> +///     b: Bar,
> +/// }
> +///
> +/// #[pin_data]
> +/// struct Bar {
> +///     x: u32,
> +/// }
> +///
> +/// let a = new_mutex!(42, "Foo::a");
> +///
> +/// stack_pin_init!(let foo =? pin_init!(Foo {
> +///     a,
> +///     b: Bar {
> +///         x: 64,
> +///     },
> +/// }));
> +/// let foo: Pin<&mut Foo> = foo;
> +/// # Ok::<(), core::convert::Infallible>(())
> +/// ```
> +///
> +/// # Syntax
> +///
> +/// A normal `let` binding with optional type annotation. The expression is expected to implement
> +/// [`PinInit`]. Additionally a `?` can be put after the `=`, this will assign `Pin<&mut T>` to the
> +/// variable instead of `Result<Pin<&mut T>, E>`.
> +#[macro_export]
> +macro_rules! stack_pin_init {
> +    (let $var:ident $(: $t:ty)? = $val:expr) => {
> +        let mut $var = $crate::init::__internal::StackInit$(::<$t>)?::uninit();
> +        let mut $var = {
> +            let val = $val;
> +            unsafe { $crate::init::__internal::StackInit::init(&mut $var, val) }
> +        };
> +    };
> +    (let $var:ident $(: $t:ty)? =? $val:expr) => {
> +        let mut $var = $crate::init::__internal::StackInit$(::<$t>)?::uninit();
> +        let mut $var = {
> +            let val = $val;
> +            unsafe { $crate::init::__internal::StackInit::init(&mut $var, val)? }
> +        };
> +    };
> +}
> +
>  /// Construct an in-place, pinned initializer for `struct`s.
>  ///
>  /// This macro defaults the error to [`Infallible`]. If you need [`Error`], then use
> @@ -916,8 +974,8 @@ macro_rules! try_init {
>  /// A pinned initializer for `T`.
>  ///
>  /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
> -/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`]. Use the [`InPlaceInit::pin_init`] function of a
> -/// smart pointer like [`Arc<T>`] on this.
> +/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`] or even the stack (see [`stack_pin_init!`]). Use the
> +/// [`InPlaceInit::pin_init`] function of a smart pointer like [`Arc<T>`] on this.
>  ///
>  /// Also see the [module description](self).
>  ///
> @@ -952,9 +1010,9 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
>  /// An initializer for `T`.
>  ///
>  /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
> -/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`]. Use the [`InPlaceInit::init`] function of a smart
> -/// pointer like [`Arc<T>`] on this. Because [`PinInit<T, E>`] is a super trait, you can
> -/// use every function that takes it as well.
> +/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`] or even the stack (see [`stack_pin_init!`]). Use the
> +/// [`InPlaceInit::init`] function of a smart pointer like [`Arc<T>`] on this. Because
> +/// [`PinInit<T, E>`] is a super trait, you can use every function that takes it as well.
>  ///
>  /// Also see the [module description](self).
>  ///
> diff --git a/rust/kernel/init/__internal.rs b/rust/kernel/init/__internal.rs
> index 4a3c7bf27a06..bf33c8e96e6d 100644
> --- a/rust/kernel/init/__internal.rs
> +++ b/rust/kernel/init/__internal.rs
> @@ -89,6 +89,52 @@ unsafe impl<T: ?Sized> HasInitData for T {
>      }
>  }
> 
> +/// Stack initializer helper type. Use [`stack_pin_init`] instead of this primitive.
> +///
> +/// # Invariants
> +///
> +/// If `self.1` is true, then `self.0` is initialized.
> +///
> +/// [`stack_pin_init`]: kernel::stack_pin_init
> +pub struct StackInit<T>(MaybeUninit<T>, bool);
> +
> +impl<T> Drop for StackInit<T> {
> +    #[inline]
> +    fn drop(&mut self) {
> +        if self.1 {
> +            // SAFETY: As we are being dropped, we only call this once. And since `self.1 == true`,
> +            // `self.0` has to be initialized.
> +            unsafe { self.0.assume_init_drop() };
> +        }
> +    }
> +}
> +
> +impl<T> StackInit<T> {
> +    /// Creates a new [`StackInit<T>`] that is uninitialized. Use [`stack_pin_init`] instead of this
> +    /// primitive.
> +    ///
> +    /// [`stack_pin_init`]: kernel::stack_pin_init
> +    #[inline]
> +    pub fn uninit() -> Self {
> +        Self(MaybeUninit::uninit(), false)
> +    }
> +
> +    /// Initializes the contents and returns the result.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller ensures that `self` is on the stack and not accessible in any other way, if this
> +    /// function returns `Ok`.
> +    #[inline]
> +    pub unsafe fn init<E>(&mut self, init: impl PinInit<T, E>) -> Result<Pin<&mut T>, E> {

Could this be made safe if the signature takes `self: Pin<&mut Self>`
instead?

The std `pin!` macro is stable in
1.68 so we can just `core::pin::pin!(StackInit::uninit())` and then
call `init` on it.

Best,
Gary

> +        // SAFETY: The memory slot is valid and this type ensures that it will stay pinned.
> +        unsafe { init.__pinned_init(self.0.as_mut_ptr())? };
> +        self.1 = true;
> +        // SAFETY: The slot is now pinned, since we will never give access to `&mut T`.
> +        Ok(unsafe { Pin::new_unchecked(self.0.assume_init_mut()) })
> +    }
> +}
> +
>  /// When a value of this type is dropped, it drops a `T`.
>  ///
>  /// Can be forgotton to prevent the drop.
> --
> 2.39.2
> 
>