rust: init: add write_[pin_]init functions

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>`. Link: https://lore.kernel.org/rust-for-linux/f026532f-8594-4f18-9aa5-57ad3f5bc592@proton.me/ [1] Signed-off-by: Benno Lossin <benno.lossin@proton.me> Reviewed-by: Boqun Feng <boqun.feng@gmail.com> Reviewed-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Gary Guo <gary@garyguo.net> Link: https://lore.kernel.org/r/20240819112415.99810-2-benno.lossin@proton.me Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: BennoLossin

rust/kernel/init.rs

@@ -1183,27 +1183,15 @@ impl<T> InPlaceInit<T> for Box<T> {
  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]
  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)
  }
 }
 
@@ -1215,27 +1203,75 @@ impl<T> InPlaceInit<T> for UniqueArc<T> {
  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]
  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())
  }
 }
 

rust/kernel/prelude.rs

@@ -37,6 +37,6 @@ pub use super::error::{code::*, Error, Result};
 
 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;