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

Benno Lossin
2023-04-03T16:06:32Z

`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 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