Re: [PATCH v3 12/13] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

[Gary Guo]

On Wed, 29 Mar 2023 22:33:49 +0000
y86-dev@xxxxxxxxxxxxxx wrote:

> From: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
> 
> `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>

Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>

> ---
>  rust/kernel/lib.rs      |  1 -
>  rust/kernel/sync/arc.rs | 14 ++++++++++++--
>  2 files changed, 12 insertions(+), 3 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 77a3833cc265..4ed6329a5e5f 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>>> {
> -        Ok(UniqueArc::<MaybeUninit<T>> {
> +        // INVARIANT: The refcount is initialised to a non-zero value.
> +        let inner = Box::init(init!(ArcInner {
> +            // SAFETY: There are no safety requirements for this FFI call.
> +            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> +            data <- init::uninit(),
> +        }))?;
> +        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
> 
>