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Message-ID: <20250622125802.3224264-4-gary@kernel.org>
Date: Sun, 22 Jun 2025 13:57:29 +0100
From: Gary Guo <gary@...nel.org>
To: Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>,
Boqun Feng <boqun.feng@...il.com>,
Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <lossin@...nel.org>,
Andreas Hindborg <a.hindborg@...nel.org>,
Alice Ryhl <aliceryhl@...gle.com>,
Trevor Gross <tmgross@...ch.edu>,
Danilo Krummrich <dakr@...nel.org>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Mark Rutland <mark.rutland@....com>,
Tamir Duberstein <tamird@...il.com>,
Xiangfei Ding <dingxiangfei2009@...il.com>,
Alex Mantel <alexmantel93@...lbox.org>
Cc: rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v4 3/5] rust: convert `Arc` to use `Refcount`
From: Gary Guo <gary@...yguo.net>
With `Refcount` type created, `Arc` can use `Refcount` instead of
calling into FFI directly.
Signed-off-by: Gary Guo <gary@...yguo.net>
---
rust/kernel/sync/arc.rs | 45 +++++++++++-------------------------
rust/kernel/sync/refcount.rs | 8 +++++++
2 files changed, 22 insertions(+), 31 deletions(-)
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 09a471fea4da..fc63ded00f6e 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -8,7 +8,7 @@
//! threads.
//!
//! It is different from the standard library's [`Arc`] in a few ways:
-//! 1. It is backed by the kernel's `refcount_t` type.
+//! 1. It is backed by the kernel's [`Refcount`] type.
//! 2. It does not support weak references, which allows it to be half the size.
//! 3. It saturates the reference count instead of aborting when it goes over a threshold.
//! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
@@ -18,10 +18,10 @@
use crate::{
alloc::{AllocError, Flags, KBox},
- bindings,
init::InPlaceInit,
+ sync::Refcount,
try_init,
- types::{ForeignOwnable, Opaque},
+ types::ForeignOwnable,
};
use core::{
alloc::Layout,
@@ -144,7 +144,7 @@ pub struct Arc<T: ?Sized> {
#[pin_data]
#[repr(C)]
pub struct ArcInner<T: ?Sized> {
- refcount: Opaque<bindings::refcount_t>,
+ refcount: Refcount,
data: T,
}
@@ -156,7 +156,7 @@ impl<T: ?Sized> ArcInner<T> {
/// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must
/// not yet have been destroyed.
unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
- let refcount_layout = Layout::new::<bindings::refcount_t>();
+ let refcount_layout = Layout::new::<Refcount>();
// SAFETY: The caller guarantees that the pointer is valid.
let val_layout = Layout::for_value(unsafe { &*ptr });
// SAFETY: We're computing the layout of a real struct that existed when compiling this
@@ -228,8 +228,7 @@ impl<T> Arc<T> {
pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
// INVARIANT: The refcount is initialised to a non-zero value.
let value = ArcInner {
- // SAFETY: There are no safety requirements for this FFI call.
- refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+ refcount: Refcount::new(1),
data: contents,
};
@@ -347,18 +346,13 @@ pub fn into_unique_or_drop(this: Self) -> Option<Pin<UniqueArc<T>>> {
// We will manually manage the refcount in this method, so we disable the destructor.
let this = ManuallyDrop::new(this);
// SAFETY: We own a refcount, so the pointer is still valid.
- let refcount = unsafe { this.ptr.as_ref() }.refcount.get();
+ let refcount = unsafe { &this.ptr.as_ref().refcount };
// If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
// return without further touching the `Arc`. If the refcount reaches zero, then there are
// no other arcs, and we can create a `UniqueArc`.
- //
- // SAFETY: We own a refcount, so the pointer is not dangling.
- let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
- if is_zero {
- // SAFETY: We have exclusive access to the arc, so we can perform unsynchronized
- // accesses to the refcount.
- unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) };
+ if refcount.dec_and_test() {
+ refcount.set(1);
// INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We
// must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin
@@ -428,14 +422,10 @@ fn as_ref(&self) -> &T {
impl<T: ?Sized> Clone for Arc<T> {
fn clone(&self) -> Self {
- // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
- // safe to dereference it.
- let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
-
- // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
+ // INVARIANT: `Refcount` saturates the refcount, so it cannot overflow to zero.
// SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
// safe to increment the refcount.
- unsafe { bindings::refcount_inc(refcount) };
+ unsafe { self.ptr.as_ref().refcount.inc() };
// SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
unsafe { Self::from_inner(self.ptr) }
@@ -444,16 +434,10 @@ fn clone(&self) -> Self {
impl<T: ?Sized> Drop for Arc<T> {
fn drop(&mut self) {
- // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
- // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
- // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
- // freed/invalid memory as long as it is never dereferenced.
- let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
-
// INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
// this instance is being dropped, so the broken invariant is not observable.
- // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
- let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
+ // SAFETY: By the type invariant, there is necessarily a reference to the object.
+ let is_zero = unsafe { self.ptr.as_ref().refcount.dec_and_test() };
if is_zero {
// The count reached zero, we must free the memory.
//
@@ -747,8 +731,7 @@ pub fn new_uninit(flags: Flags) -> Result<UniqueArc<MaybeUninit<T>>, AllocError>
// INVARIANT: The refcount is initialised to a non-zero value.
let inner = KBox::try_init::<AllocError>(
try_init!(ArcInner {
- // SAFETY: There are no safety requirements for this FFI call.
- refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+ refcount: Refcount::new(1),
data <- pin_init::uninit::<T, AllocError>(),
}? AllocError),
flags,
diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
index a0fc22f6d645..8e7b9b0c1979 100644
--- a/rust/kernel/sync/refcount.rs
+++ b/rust/kernel/sync/refcount.rs
@@ -71,6 +71,14 @@ pub fn dec(&self) {
/// must come after.
///
/// Returns true if the resulting refcount is 0, false otherwise.
+ ///
+ /// # Notes
+ ///
+ /// A common pattern of using `Refcount` is to free memory when the reference count reaches
+ /// zero. This means that the reference to `Refcount` could become invalid after calling this
+ /// function. This is fine as long as the reference to `Refcount` is no longer used when this
+ /// function returns `false`. It is not necessary to use raw pointers in this scenario, see
+ /// https://github.com/rust-lang/rust/issues/55005.
#[inline]
#[must_use = "use `dec` instead if you do not need to test if it is 0"]
pub fn dec_and_test(&self) -> bool {
--
2.49.0
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