Message-Id: <20240727042442.682109-1-alexmantel93@mailbox.org>
Date: Fri, 26 Jul 2024 21:24:42 -0700
From: Alex Mantel <alexmantel93@...lbox.org>
To: ojeda@...nel.org,
	alex.gaynor@...il.com,
	wedsonaf@...il.com,
	boqun.feng@...il.com,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me,
	a.hindborg@...sung.com,
	aliceryhl@...gle.com,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Alex Mantel <alexmantel93@...lbox.org>
Subject: [PATCH v3] rust: Implement the smart pointer `InPlaceInit` for `Arc`

For pinned and unpinned initialization of structs, a trait named
`InPlaceInit` exists for uniform access. `Arc` did not implement
`InPlaceInit` yet, although the functions already existed. The main
reason for that, was that the trait itself returned a `Pin<Self>`. The
`Arc` implementation of the kernel is already implicitly pinned.

To enable `Arc` to implement `InPlaceInit` and to have uniform access,
for in-place and pinned in-place initialization, an associated type is
introduced for `InPlaceInit`. The new implementation of `InPlaceInit`
for `Arc` sets `Arc` as the associated type. Older implementations use
an explicit `Pin<T>` as the associated type. The implemented methods for
`Arc` are mostly moved from a direct implementation on `Arc`. There
should be no user impact. The implementation for `ListArc` is omitted,
because it is not merged yet.

Link: https://github.com/Rust-for-Linux/linux/issues/1079
Signed-off-by: Alex Mantel <alexmantel93@...lbox.org>
---
Hello again!

This is the 3rd version of my very first patch. I applied the
suggestions on the 2nd submission. Thank you again for your feedback,
looking for more!

v1: 
  * https://lore.kernel.org/rust-for-linux/20240717034801.262343-2-alexmantel93@mailbox.org/

v2: 
  * remove the `From:` from the patch.
  * add the prefix `rust: ` to the subject.
  * Remove the empty line between `Link` and `Signed-off-by`.
  * https://lore.kernel.org/all/20240719192234.330341-1-alexmantel93@mailbox.org/

v3:
  * Rename PinnedResult to PinnedSelf
  * Adjust documentation for PinnedSelf

 rust/kernel/init.rs     | 39 +++++++++++++++++++++++++++++++++++----
 rust/kernel/sync/arc.rs | 25 ++-----------------------
 2 files changed, 37 insertions(+), 27 deletions(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 68605b633..fa5f182fe 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -213,6 +213,7 @@
 use crate::{
     alloc::{box_ext::BoxExt, AllocError, Flags},
     error::{self, Error},
+    sync::Arc,
     sync::UniqueArc,
     types::{Opaque, ScopeGuard},
 };
@@ -1112,11 +1113,17 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
 
 /// Smart pointer that can initialize memory in-place.
 pub trait InPlaceInit<T>: Sized {
+    /// Pinned version of Rusts `Self`.
+    ///
+    /// If a type already implicitly pins its pointee, `Pin<Self>` is unnecessary. In this case use
+    /// `Self`, otherwise just use `Pin<Self>`.
+    type PinnedSelf;
+
     /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this
     /// type.
     ///
     /// If `T: !Unpin` it will not be able to move afterwards.
-    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>, E>
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
     where
         E: From<AllocError>;
 
@@ -1124,7 +1131,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>,
     /// type.
     ///
     /// If `T: !Unpin` it will not be able to move afterwards.
-    fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Pin<Self>>
+    fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Self::PinnedSelf>
     where
         Error: From<E>,
     {
@@ -1153,9 +1160,31 @@ fn init<E>(init: impl Init<T, E>, flags: Flags) -> error::Result<Self>
     }
 }
 
+impl<T> InPlaceInit<T> for Arc<T> {
+    type PinnedSelf = Self;
+
+    #[inline]
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
+    where
+        E: From<AllocError>,
+    {
+        UniqueArc::try_pin_init(init, flags).map(|u| u.into())
+    }
+
+    #[inline]
+    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
+    where
+        E: From<AllocError>,
+    {
+        UniqueArc::try_init(init, flags).map(|u| u.into())
+    }
+}
+
 impl<T> InPlaceInit<T> for Box<T> {
+    type PinnedSelf = Pin<Self>;
+
     #[inline]
-    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>, E>
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
     where
         E: From<AllocError>,
     {
@@ -1184,8 +1213,10 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
 }
 
 impl<T> InPlaceInit<T> for UniqueArc<T> {
+    type PinnedSelf = Pin<Self>;
+
     #[inline]
-    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>, E>
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
     where
         E: From<AllocError>,
     {
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 3673496c2..3021f30fd 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -12,12 +12,13 @@
 //! 2. It does not support weak references, which allows it to be half the size.
 //! 3. It saturates the reference count instead of aborting when it goes over a threshold.
 //! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
+//! 5. The object in [`Arc`] is pinned implicitly.
 //!
 //! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
 
 use crate::{
     alloc::{box_ext::BoxExt, AllocError, Flags},
-    error::{self, Error},
+    bindings,
     init::{self, InPlaceInit, Init, PinInit},
     try_init,
     types::{ForeignOwnable, Opaque},
@@ -209,28 +210,6 @@ pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
         // `Arc` object.
         Ok(unsafe { Self::from_inner(Box::leak(inner).into()) })
     }
-
-    /// Use the given initializer to in-place initialize a `T`.
-    ///
-    /// If `T: !Unpin` it will not be able to move afterwards.
-    #[inline]
-    pub fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Self>
-    where
-        Error: From<E>,
-    {
-        UniqueArc::pin_init(init, flags).map(|u| u.into())
-    }
-
-    /// Use the given initializer to in-place initialize a `T`.
-    ///
-    /// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
-    #[inline]
-    pub fn init<E>(init: impl Init<T, E>, flags: Flags) -> error::Result<Self>
-    where
-        Error: From<E>,
-    {
-        UniqueArc::init(init, flags).map(|u| u.into())
-    }
 }
 
 impl<T: ?Sized> Arc<T> {
-- 
2.39.2

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