Re: [PATCH v2 2/2] rust: init: add `write_[pin_]init` functions

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jul 08, 2024 at 08:53:46PM +0000, Benno Lossin wrote:
> Sometimes it is necessary to split allocation and initialization into
> two steps. One such situation is when reusing existing allocations
> obtained via `Box::drop_contents`. See [1] for an example.
> In order to support this use case add `write_[pin_]init` functions to the
> pin-init API. These functions operate on already allocated smart
> pointers that wrap `MaybeUninit<T>`.
> 
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> Link: https://lore.kernel.org/rust-for-linux/f026532f-8594-4f18-9aa5-57ad3f5bc592@proton.me/ [1]
> ---
>  rust/kernel/init.rs    | 84 ++++++++++++++++++++++++++++++------------
>  rust/kernel/prelude.rs |  2 +-
>  2 files changed, 61 insertions(+), 25 deletions(-)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 9608f2bd2211..35c4d9b75a97 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1159,13 +1159,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>,
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = <Box<_> as BoxExt<_>>::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> -        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> -        // slot is valid and will not be moved, because we pin it later.
> -        unsafe { init.__pinned_init(slot)? };
> -        // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() }.into())
> +        <Box<_> as BoxExt<_>>::new_uninit(flags)?.write_pin_init(init)
>      }
>  
>      #[inline]
> @@ -1173,13 +1167,7 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = <Box<_> as BoxExt<_>>::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> -        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> -        // slot is valid.
> -        unsafe { init.__init(slot)? };
> -        // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() })
> +        <Box<_> as BoxExt<_>>::new_uninit(flags)?.write_init(init)
>      }
>  }
>  
> @@ -1189,13 +1177,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>,
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = UniqueArc::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> -        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> -        // slot is valid and will not be moved, because we pin it later.
> -        unsafe { init.__pinned_init(slot)? };
> -        // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() }.into())
> +        UniqueArc::new_uninit(flags)?.write_pin_init(init)
>      }
>  
>      #[inline]
> @@ -1203,13 +1185,67 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>      where
>          E: From<AllocError>,
>      {
> -        let mut this = UniqueArc::new_uninit(flags)?;
> -        let slot = this.as_mut_ptr();
> +        UniqueArc::new_uninit(flags)?.write_init(init)
> +    }
> +}
> +
> +/// Smart pointer containing uninitialized memory and that can write a value.
> +pub trait InPlaceWrite<T> {
> +    /// The type `Self` turns into when the contents are initialized.
> +    type Initialized;
> +
> +    /// Use the given initializer to write a value into `self`.
> +    ///
> +    /// Does not drop the current value and considers it as uninitialized memory.
> +    fn write_init<E>(self, init: impl Init<T, E>) -> Result<Self::Initialized, E>;
> +
> +    /// Use the given pin-initializer to write a value into `self`.
> +    ///
> +    /// Does not drop the current value and considers it as uninitialized memory.
> +    fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E>;
> +}
> +
> +impl<T> InPlaceWrite<T> for Box<MaybeUninit<T>> {
> +    type Initialized = Box<T>;
> +
> +    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
> +        let slot = self.as_mut_ptr();
>          // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
>          // slot is valid.
>          unsafe { init.__init(slot)? };
>          // SAFETY: All fields have been initialized.
> -        Ok(unsafe { this.assume_init() })
> +        Ok(unsafe { self.assume_init() })
> +    }
> +
> +    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
> +        let slot = self.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid and will not be moved, because we pin it later.
> +        unsafe { init.__pinned_init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { self.assume_init() }.into())
> +    }
> +}
> +
> +impl<T> InPlaceWrite<T> for UniqueArc<MaybeUninit<T>> {
> +    type Initialized = UniqueArc<T>;
> +
> +    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
> +        let slot = self.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid.
> +        unsafe { init.__init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { self.assume_init() })
> +    }
> +
> +    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
> +        let slot = self.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid and will not be moved, because we pin it later.
> +        unsafe { init.__pinned_init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { self.assume_init() }.into())
>      }
>  }
>  
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index b37a0b3180fb..4571daec0961 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -37,6 +37,6 @@
>  
>  pub use super::{str::CStr, ThisModule};
>  
> -pub use super::init::{InPlaceInit, Init, PinInit};
> +pub use super::init::{InPlaceInit, InPlaceWrite, Init, PinInit};
>  
>  pub use super::current;
> -- 
> 2.44.0
> 
>