Messages in this thread Patch in this message | | | Date | Fri, 31 Mar 2023 21:54:41 +0000 | From | y86-dev@protonma ... | Subject | [PATCH v4 14/15] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit` |
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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
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