Re: [PATCH v5 14/15] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

Gary Guo
2023-04-04T13:25:44Z

On Mon, 03 Apr 2023 16:05:56 +0000
Benno Lossin <y86-dev@protonmail.com> wrote:

> `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>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
> 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()) },
> })
> }
> }
> --
> 2.39.2
>
>