[PATCH v4 14/15] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

[y86-dev@protonmail.com]

From: Benno Lossin <y86-dev@protonmail.com>

`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@protonmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andreas Hindborg <a.hindborg@samsung.com>
---
 rust/kernel/lib.rs      |  1 -
 rust/kernel/sync/arc.rs | 16 +++++++++++++---
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 821bd067151c..2d7606135ef6 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -28,7 +28,6 @@
 #[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 24dc96603090..5f64affd8f33 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -17,7 +17,8 @@

 use crate::{
     bindings,
-    init::{InPlaceInit, Init, PinInit},
+    init::{self, InPlaceInit, Init, PinInit},
+    try_init,
     types::{ForeignOwnable, Opaque},
 };
 use alloc::boxed::Box;
@@ -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>>, AllocError> {
-        Ok(UniqueArc::<MaybeUninit<T>> {
+        // INVARIANT: The refcount is initialised to a non-zero value.
+        let inner = Box::try_init::<AllocError>(try_init!(ArcInner {
+            // SAFETY: There are no safety requirements for this FFI call.
+            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+            data <- init::uninit::<T, AllocError>(),
+        }? AllocError))?;
+        Ok(UniqueArc {
             // 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