[PATCH v3 2/4] rust: convert `Arc` to use `Refcount`

Gary Guo
2025-02-19T20:17:25Z

With `Refcount` type created, `Arc` can use `Refcount` instead of
calling into FFI directly.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
rust/kernel/sync/arc.rs | 65 +++++++++++++++++------------------------
1 file changed, 26 insertions(+), 39 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 3cefda7a4372..1f5fbc6b3742 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::{self, InPlaceInit, Init, PinInit},
+ sync::Refcount,
try_init,
- types::{ForeignOwnable, Opaque},
+ types::ForeignOwnable,
};
use core::{
alloc::Layout,
@@ -143,7 +143,7 @@ pub struct Arc<T: ?Sized> {
#[pin_data]
#[repr(C)]
struct ArcInner<T: ?Sized> {
- refcount: Opaque<bindings::refcount_t>,
+ refcount: Refcount,
data: T,
}

@@ -155,7 +155,7 @@ 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
@@ -207,8 +207,7 @@ 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,
};

@@ -290,7 +289,7 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
/// use kernel::sync::{Arc, UniqueArc};
///
/// let arc = Arc::new(42, GFP_KERNEL)?;
- /// let unique_arc = arc.into_unique_or_drop();
+ /// let unique_arc = Arc::into_unique_or_drop(arc);
///
/// // The above conversion should succeed since refcount of `arc` is 1.
/// assert!(unique_arc.is_some());
@@ -306,35 +305,30 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
/// let arc = Arc::new(42, GFP_KERNEL)?;
/// let another = arc.clone();
///
- /// let unique_arc = arc.into_unique_or_drop();
+ /// let unique_arc = Arc::into_unique_or_drop(arc);
///
/// // The above conversion should fail since refcount of `arc` is >1.
/// assert!(unique_arc.is_none());
///
/// # Ok::<(), Error>(())
/// ```
- pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
+ 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 me = ManuallyDrop::new(self);
+ let this = ManuallyDrop::new(this);
// SAFETY: We own a refcount, so the pointer is still valid.
- let refcount = unsafe { me.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
- // their values.
+ // INVARIANT: If the refcount failed to decrement because it is 1, then we have the
+ // exclusive ownership, so we may create a `UniqueArc`. We must pin the `UniqueArc`
+ // because the values was previously in an `Arc`, and they pin their values.
Some(Pin::from(UniqueArc {
- inner: ManuallyDrop::into_inner(me),
+ inner: ManuallyDrop::into_inner(this),
}))
} else {
None
@@ -396,14 +390,10 @@ 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) }
@@ -412,16 +402,14 @@ 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.
+ // NOTE: 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. However it is okay
+ // to have a transient reference to decrement the refcount, see
+ // https://github.com/rust-lang/rust/issues/55005.
+ let is_zero = unsafe { self.ptr.as_ref().refcount.dec_and_test() };
if is_zero {
// The count reached zero, we must free the memory.
//
@@ -673,8 +661,7 @@ 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 <- init::uninit::<T, AllocError>(),
}? AllocError),
flags,
--
2.47.2