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Message-ID: <20230329223239.138757-13-y86-dev@protonmail.com>
Date: Wed, 29 Mar 2023 22:33:49 +0000
From: y86-dev@...tonmail.com
To: Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>,
Wedson Almeida Filho <wedsonaf@...il.com>,
Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Alice Ryhl <alice@...l.io>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
patches@...ts.linux.dev
Subject: [PATCH v3 12/13] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`
From: Benno Lossin <y86-dev@...tonmail.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@...tonmail.com>
---
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
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