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Date: Thu, 25 Apr 2024 21:34:44 +0000
To: Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, Alice Ryhl <aliceryhl@google.com>
From: Benno Lossin <benno.lossin@proton.me>
Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 2/2] rust: init: add re-initialization functions
Message-ID: <20240425213419.3904105-2-benno.lossin@proton.me>
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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 `re_[pin_]init` functions to the
pin-init API. These functions operate on already allocated smart
pointers that contain `MaybeUninit<T>`.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/rust-for-linux/f026532f-8594-4f18-9aa5-57ad3f=
5bc592@proton.me/ [1]
---
 rust/kernel/init.rs    | 88 ++++++++++++++++++++++++++++++------------
 rust/kernel/prelude.rs |  2 +-
 2 files changed, 65 insertions(+), 25 deletions(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 9608f2bd2211..b37b23f07bf7 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1159,13 +1159,8 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: =
Flags) -> Result<Pin<Self>,
     where
         E: From<AllocError>,
     {
-        let mut this =3D <Box<_> as BoxExt<_>>::new_uninit(flags)?;
-        let slot =3D 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())
+        let this =3D <Box<_> as BoxExt<_>>::new_uninit(flags)?;
+        this.re_pin_init(init)
     }
=20
     #[inline]
@@ -1173,13 +1168,8 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) =
-> Result<Self, E>
     where
         E: From<AllocError>,
     {
-        let mut this =3D <Box<_> as BoxExt<_>>::new_uninit(flags)?;
-        let slot =3D 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() })
+        let this =3D <Box<_> as BoxExt<_>>::new_uninit(flags)?;
+        this.re_init(init)
     }
 }
=20
@@ -1189,13 +1179,8 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: =
Flags) -> Result<Pin<Self>,
     where
         E: From<AllocError>,
     {
-        let mut this =3D UniqueArc::new_uninit(flags)?;
-        let slot =3D 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())
+        let this =3D UniqueArc::new_uninit(flags)?;
+        this.re_pin_init(init)
     }
=20
     #[inline]
@@ -1203,13 +1188,68 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags)=
 -> Result<Self, E>
     where
         E: From<AllocError>,
     {
-        let mut this =3D UniqueArc::new_uninit(flags)?;
-        let slot =3D this.as_mut_ptr();
+        let this =3D UniqueArc::new_uninit(flags)?;
+        this.re_init(init)
+    }
+}
+
+/// Smart pointer that can re-initialize its content.
+pub trait InPlaceReInit<T> {
+    /// The type `Self` turns into when re-initialized.
+    type Initialized;
+
+    /// Re-initializes `self` with the given initializer.
+    ///
+    /// Does not drop the current value and considers it as uninitialized =
memory.
+    fn re_init<E>(self, init: impl Init<T, E>) -> Result<Self::Initialized=
, E>;
+
+    /// Re-initializes `self` with the given initializer.
+    ///
+    /// Does not drop the current value and considers it as uninitialized =
memory.
+    fn re_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::=
Initialized>, E>;
+}
+
+impl<T> InPlaceReInit<T> for Box<MaybeUninit<T>> {
+    type Initialized =3D Box<T>;
+
+    fn re_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initial=
ized, E> {
+        let slot =3D 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 re_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Se=
lf::Initialized>, E> {
+        let slot =3D 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> InPlaceReInit<T> for UniqueArc<MaybeUninit<T>> {
+    type Initialized =3D UniqueArc<T>;
+
+    fn re_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initial=
ized, E> {
+        let slot =3D 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 re_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Se=
lf::Initialized>, E> {
+        let slot =3D 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())
     }
 }
=20
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index b37a0b3180fb..078b2b1d84ae 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -37,6 +37,6 @@
=20
 pub use super::{str::CStr, ThisModule};
=20
-pub use super::init::{InPlaceInit, Init, PinInit};
+pub use super::init::{InPlaceInit, InPlaceReInit, Init, PinInit};
=20
 pub use super::current;
--=20
2.44.0