rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

`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 <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-15-y86-dev@protonmail.comSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

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;
 

rust/kernel/sync/arc.rs

@@ -18,7 +18,8 @@
 use crate::{
     bindings,
     error::{self, Error},
-    init::{InPlaceInit, Init, PinInit},
+    init::{self, InPlaceInit, Init, PinInit},
+    try_init,
     types::{ForeignOwnable, Opaque},
 };
 use alloc::boxed::Box;
@@ -31,6 +32,7 @@ use core::{
     pin::Pin,
     ptr::NonNull,
 };
+use macros::pin_data;
 
 mod std_vendor;
 
@@ -125,6 +127,7 @@ pub struct Arc<T: ?Sized> {
     _p: PhantomData<ArcInner<T>>,
 }
 
+#[pin_data]
 #[repr(C)]
 struct ArcInner<T: ?Sized> {
     refcount: Opaque<bindings::refcount_t>,
@@ -505,9 +508,16 @@ 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()) },
         })
     }
 }