[PATCH v3 2/2] rust: init: add `write_[pin_]init` functions

Benno Lossin
2024-08-03T14:17:12Z

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>
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 495c09ebe3a3..1c3cfad89165 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1154,13 +1154,7 @@ 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]
@@ -1168,13 +1162,7 @@ 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)
}
}

@@ -1184,13 +1172,7 @@ 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]
@@ -1198,13 +1180,67 @@ 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;
--
2.45.2