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-rw-r--r--rust/kernel/sync/arc.rs189
1 files changed, 147 insertions, 42 deletions
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 7d4c4bf58388..3673496c2363 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -16,7 +16,7 @@
//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
use crate::{
- bindings,
+ alloc::{box_ext::BoxExt, AllocError, Flags},
error::{self, Error},
init::{self, InPlaceInit, Init, PinInit},
try_init,
@@ -24,7 +24,7 @@ use crate::{
};
use alloc::boxed::Box;
use core::{
- alloc::{AllocError, Layout},
+ alloc::Layout,
fmt,
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
@@ -57,7 +57,7 @@ mod std_vendor;
/// }
///
/// // Create a refcounted instance of `Example`.
-/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
+/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
///
/// // Get a new pointer to `obj` and increment the refcount.
/// let cloned = obj.clone();
@@ -96,7 +96,7 @@ mod std_vendor;
/// }
/// }
///
-/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
+/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
/// obj.use_reference();
/// obj.take_over();
/// # Ok::<(), Error>(())
@@ -119,7 +119,7 @@ mod std_vendor;
/// impl MyTrait for Example {}
///
/// // `obj` has type `Arc<Example>`.
-/// let obj: Arc<Example> = Arc::try_new(Example)?;
+/// let obj: Arc<Example> = Arc::new(Example, GFP_KERNEL)?;
///
/// // `coerced` has type `Arc<dyn MyTrait>`.
/// let coerced: Arc<dyn MyTrait> = obj;
@@ -137,6 +137,39 @@ struct ArcInner<T: ?Sized> {
data: T,
}
+impl<T: ?Sized> ArcInner<T> {
+ /// Converts a pointer to the contents of an [`Arc`] into a pointer to the [`ArcInner`].
+ ///
+ /// # Safety
+ ///
+ /// `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>();
+ // 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
+ // binary, so its layout is not so large that it can trigger arithmetic overflow.
+ let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
+
+ // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
+ // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
+ //
+ // This is documented at:
+ // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
+ let ptr = ptr as *const ArcInner<T>;
+
+ // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
+ // pointer, since it originates from a previous call to `Arc::into_raw` on an `Arc` that is
+ // still valid.
+ let ptr = unsafe { ptr.byte_sub(val_offset) };
+
+ // SAFETY: The pointer can't be null since you can't have an `ArcInner<T>` value at the null
+ // address.
+ unsafe { NonNull::new_unchecked(ptr.cast_mut()) }
+ }
+}
+
// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
@@ -162,7 +195,7 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
impl<T> Arc<T> {
/// Constructs a new reference counted instance of `T`.
- pub fn try_new(contents: T) -> Result<Self, AllocError> {
+ 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.
@@ -170,7 +203,7 @@ impl<T> Arc<T> {
data: contents,
};
- let inner = Box::try_new(value)?;
+ let inner = <Box<_> as BoxExt<_>>::new(value, flags)?;
// SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
// `Arc` object.
@@ -181,22 +214,22 @@ impl<T> Arc<T> {
///
/// If `T: !Unpin` it will not be able to move afterwards.
#[inline]
- pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
+ pub fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Self>
where
Error: From<E>,
{
- UniqueArc::pin_init(init).map(|u| u.into())
+ UniqueArc::pin_init(init, flags).map(|u| u.into())
}
/// Use the given initializer to in-place initialize a `T`.
///
/// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
#[inline]
- pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
+ pub fn init<E>(init: impl Init<T, E>, flags: Flags) -> error::Result<Self>
where
Error: From<E>,
{
- UniqueArc::init(init).map(|u| u.into())
+ UniqueArc::init(init, flags).map(|u| u.into())
}
}
@@ -232,27 +265,13 @@ impl<T: ?Sized> Arc<T> {
/// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
/// must not be called more than once for each previous call to [`Arc::into_raw`].
pub unsafe fn from_raw(ptr: *const T) -> Self {
- let refcount_layout = Layout::new::<bindings::refcount_t>();
- // 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
- // binary, so its layout is not so large that it can trigger arithmetic overflow.
- let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
-
- // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
- // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
- //
- // This is documented at:
- // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
- let ptr = ptr as *const ArcInner<T>;
-
- // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
- // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
- let ptr = unsafe { ptr.byte_sub(val_offset) };
+ // SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
+ // `Arc` that is still valid.
+ let ptr = unsafe { ArcInner::container_of(ptr) };
// SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
// reference count held then will be owned by the new `Arc` object.
- unsafe { Self::from_inner(NonNull::new_unchecked(ptr.cast_mut())) }
+ unsafe { Self::from_inner(ptr) }
}
/// Returns an [`ArcBorrow`] from the given [`Arc`].
@@ -271,6 +290,68 @@ impl<T: ?Sized> Arc<T> {
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_ptr())
}
+
+ /// Converts this [`Arc`] into a [`UniqueArc`], or destroys it if it is not unique.
+ ///
+ /// When this destroys the `Arc`, it does so while properly avoiding races. This means that
+ /// this method will never call the destructor of the value.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::{Arc, UniqueArc};
+ ///
+ /// let arc = Arc::new(42, GFP_KERNEL)?;
+ /// let unique_arc = arc.into_unique_or_drop();
+ ///
+ /// // The above conversion should succeed since refcount of `arc` is 1.
+ /// assert!(unique_arc.is_some());
+ ///
+ /// assert_eq!(*(unique_arc.unwrap()), 42);
+ ///
+ /// # Ok::<(), Error>(())
+ /// ```
+ ///
+ /// ```
+ /// use kernel::sync::{Arc, UniqueArc};
+ ///
+ /// let arc = Arc::new(42, GFP_KERNEL)?;
+ /// let another = arc.clone();
+ ///
+ /// let unique_arc = arc.into_unique_or_drop();
+ ///
+ /// // 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>>> {
+ // We will manually manage the refcount in this method, so we disable the destructor.
+ let me = ManuallyDrop::new(self);
+ // SAFETY: We own a refcount, so the pointer is still valid.
+ let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
+
+ // 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)) };
+
+ // 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.
+ Some(Pin::from(UniqueArc {
+ inner: ManuallyDrop::into_inner(me),
+ }))
+ } else {
+ None
+ }
+ }
}
impl<T: 'static> ForeignOwnable for Arc<T> {
@@ -387,7 +468,7 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
/// e.into()
/// }
///
-/// let obj = Arc::try_new(Example)?;
+/// let obj = Arc::new(Example, GFP_KERNEL)?;
/// let cloned = do_something(obj.as_arc_borrow());
///
/// // Assert that both `obj` and `cloned` point to the same underlying object.
@@ -411,7 +492,7 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
/// }
/// }
///
-/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
+/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
/// obj.as_arc_borrow().use_reference();
/// # Ok::<(), Error>(())
/// ```
@@ -453,6 +534,27 @@ impl<T: ?Sized> ArcBorrow<'_, T> {
_p: PhantomData,
}
}
+
+ /// Creates an [`ArcBorrow`] to an [`Arc`] that has previously been deconstructed with
+ /// [`Arc::into_raw`].
+ ///
+ /// # Safety
+ ///
+ /// * The provided pointer must originate from a call to [`Arc::into_raw`].
+ /// * For the duration of the lifetime annotated on this `ArcBorrow`, the reference count must
+ /// not hit zero.
+ /// * For the duration of the lifetime annotated on this `ArcBorrow`, there must not be a
+ /// [`UniqueArc`] reference to this value.
+ pub unsafe fn from_raw(ptr: *const T) -> Self {
+ // SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
+ // `Arc` that is still valid.
+ let ptr = unsafe { ArcInner::container_of(ptr) };
+
+ // SAFETY: The caller promises that the value remains valid since the reference count must
+ // not hit zero, and no mutable reference will be created since that would involve a
+ // `UniqueArc`.
+ unsafe { Self::new(ptr) }
+ }
}
impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
@@ -499,7 +601,7 @@ impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
/// }
///
/// fn test() -> Result<Arc<Example>> {
-/// let mut x = UniqueArc::try_new(Example { a: 10, b: 20 })?;
+/// let mut x = UniqueArc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
/// x.a += 1;
/// x.b += 1;
/// Ok(x.into())
@@ -522,7 +624,7 @@ impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
/// }
///
/// fn test() -> Result<Arc<Example>> {
-/// let x = UniqueArc::try_new_uninit()?;
+/// let x = UniqueArc::new_uninit(GFP_KERNEL)?;
/// Ok(x.write(Example { a: 10, b: 20 }).into())
/// }
///
@@ -542,7 +644,7 @@ impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
/// }
///
/// fn test() -> Result<Arc<Example>> {
-/// let mut pinned = Pin::from(UniqueArc::try_new(Example { a: 10, b: 20 })?);
+/// let mut pinned = Pin::from(UniqueArc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?);
/// // We can modify `pinned` because it is `Unpin`.
/// pinned.as_mut().a += 1;
/// Ok(pinned.into())
@@ -556,21 +658,24 @@ pub struct UniqueArc<T: ?Sized> {
impl<T> UniqueArc<T> {
/// Tries to allocate a new [`UniqueArc`] instance.
- pub fn try_new(value: T) -> Result<Self, AllocError> {
+ pub fn new(value: T, flags: Flags) -> Result<Self, AllocError> {
Ok(Self {
// INVARIANT: The newly-created object has a refcount of 1.
- inner: Arc::try_new(value)?,
+ inner: Arc::new(value, flags)?,
})
}
/// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
- pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
+ pub fn new_uninit(flags: Flags) -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
// 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))?;
+ 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),
+ flags,
+ )?;
Ok(UniqueArc {
// INVARIANT: The newly-created object has a refcount of 1.
// SAFETY: The pointer from the `Box` is valid.
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