Re: [PATCH v6 14/15] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

[Andreas Hindborg <nmi@metaspace.dk>]

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

> `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>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Cc: Andreas Hindborg <a.hindborg@samsung.com>
> ---

Reviewed-by: 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 43a53fbe175d..d05caa723718 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/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;
> @@ -30,6 +31,7 @@ use core::{
>      pin::Pin,
>      ptr::NonNull,
>  };
> +use macros::pin_data;
>
>  /// A reference-counted pointer to an instance of `T`.
>  ///
> @@ -122,6 +124,7 @@ pub struct Arc<T: ?Sized> {
>      _p: PhantomData<ArcInner<T>>,
>  }
>
> +#[pin_data]
>  #[repr(C)]
>  struct ArcInner<T: ?Sized> {
>      refcount: Opaque<bindings::refcount_t>,
> @@ -502,9 +505,16 @@ 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()) },
>          })
>      }
>  }