[PATCH v1 3/3] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

[y86-dev]

`UniqueArc::try_new_uninit` calls `Arc::try_new(MaybeUninit::uninit())`.
This results in the uninitialized memory being placed on the stack,
which may be arbitrarily large due to the generic `T` and thus could
cause a stack overflow for large types.

Change the implementation to use the pin-init API which enables in-place
initialization. In particular it avoids having to first construct and
then move the uninitialized memory from the stack into the final location.

Signed-off-by: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
---
 rust/kernel/lib.rs      |  1 -
 rust/kernel/sync/arc.rs | 12 +++++++++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 3e2777d26ff5..d9df77132fa2 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -27,7 +27,6 @@
 #[cfg(not(CONFIG_RUST))]
 compile_error!("Missing kernel configuration for conditional compilation");
 
-#[allow(unused_extern_crates)]
 // Allow proc-macros to refer to `::kernel` inside the `kernel` crate (this crate).
 extern crate self as kernel;
 
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 5b4f64dd3ac8..6782d66039e1 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -18,6 +18,7 @@
 use crate::{
     bindings,
     error::{Error, Result},
+    init,
     init::{InPlaceInit, Init, PinInit},
     types::{ForeignOwnable, Opaque},
 };
@@ -29,6 +30,7 @@ use core::{
     pin::Pin,
     ptr::NonNull,
 };
+use macros::pin_data;
 
 /// A reference-counted pointer to an instance of `T`.
 ///
@@ -121,6 +123,7 @@ pub struct Arc<T: ?Sized> {
     _p: PhantomData<ArcInner<T>>,
 }
 
+#[pin_data]
 #[repr(C)]
 struct ArcInner<T: ?Sized> {
     refcount: Opaque<bindings::refcount_t>,
@@ -501,9 +504,16 @@ impl<T> UniqueArc<T> {
 
     /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
     pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>> {
+        // INVARIANT: The refcount is initialised to a non-zero value.
+        let inner = Box::init(init!(ArcInner<MaybeUninit<T>> {
+            // SAFETY: There are no safety requirements for this FFI call.
+            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+            data <- init::uninit(),
+        }))?;
         Ok(UniqueArc::<MaybeUninit<T>> {
             // INVARIANT: The newly-created object has a ref-count of 1.
-            inner: Arc::try_new(MaybeUninit::uninit())?,
+            // SAFETY: The pointer from the `Box` is valid.
+            inner: unsafe { Arc::from_inner(Box::leak(inner).into()) },
         })
     }
 }
-- 
2.39.2