* [PATCH v4 1/2] rust: sync: add `ArcBorrow::from_raw`
2024-04-02 13:07 [PATCH v4 0/2] Arc methods for linked list Alice Ryhl
@ 2024-04-02 13:07 ` Alice Ryhl
2024-04-02 13:07 ` [PATCH v4 2/2] rust: sync: add `Arc::into_unique_or_drop` Alice Ryhl
2024-05-05 22:27 ` [PATCH v4 0/2] Arc methods for linked list Miguel Ojeda
2 siblings, 0 replies; 4+ messages in thread
From: Alice Ryhl @ 2024-04-02 13:07 UTC (permalink / raw)
To: Miguel Ojeda, Alex Gaynor, Wedson Almeida Filho, Boqun Feng,
Gary Guo, Björn Roy Baron, Benno Lossin, Andreas Hindborg
Cc: rust-for-linux, linux-kernel, Alice Ryhl
Allows access to a value in an `Arc` that is currently held as a raw
pointer due to use of `Arc::into_raw`, without destroying or otherwise
consuming that raw pointer.
This is a dependency of the linked list that Rust Binder uses. The
linked list uses this method when iterating over the linked list [1].
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240402-linked-list-v1-6-b1c59ba7ae3b@google.com [1]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/sync/arc.rs | 76 +++++++++++++++++++++++++++++++++++++------------
1 file changed, 58 insertions(+), 18 deletions(-)
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 7d4c4bf58388..53addb8876c2 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -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> {}
@@ -232,27 +265,13 @@ pub fn into_raw(self) -> *const 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`].
@@ -453,6 +472,27 @@ unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
_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> {
--
2.44.0.478.gd926399ef9-goog
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v4 2/2] rust: sync: add `Arc::into_unique_or_drop`
2024-04-02 13:07 [PATCH v4 0/2] Arc methods for linked list Alice Ryhl
2024-04-02 13:07 ` [PATCH v4 1/2] rust: sync: add `ArcBorrow::from_raw` Alice Ryhl
@ 2024-04-02 13:07 ` Alice Ryhl
2024-05-05 22:27 ` [PATCH v4 0/2] Arc methods for linked list Miguel Ojeda
2 siblings, 0 replies; 4+ messages in thread
From: Alice Ryhl @ 2024-04-02 13:07 UTC (permalink / raw)
To: Miguel Ojeda, Alex Gaynor, Wedson Almeida Filho, Boqun Feng,
Gary Guo, Björn Roy Baron, Benno Lossin, Andreas Hindborg
Cc: rust-for-linux, linux-kernel, Alice Ryhl
Decrement the refcount of an `Arc`, but handle the case where it hits
zero by taking ownership of the now-unique `Arc`, instead of destroying
and deallocating it.
This is a dependency of the linked list that Rust Binder uses. The
linked list uses this method as part of its `ListArc` abstraction [1].
Boqun Feng has authored the examples.
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240402-linked-list-v1-1-b1c59ba7ae3b@google.com [1]
Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/sync/arc.rs | 62 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 62 insertions(+)
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 53addb8876c2..30777241d8cf 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -290,6 +290,68 @@ pub fn as_arc_borrow(&self) -> ArcBorrow<'_, 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::try_new(42)?;
+ /// 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::try_new(42)?;
+ /// 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> {
--
2.44.0.478.gd926399ef9-goog
^ permalink raw reply related [flat|nested] 4+ messages in thread* Re: [PATCH v4 0/2] Arc methods for linked list
2024-04-02 13:07 [PATCH v4 0/2] Arc methods for linked list Alice Ryhl
2024-04-02 13:07 ` [PATCH v4 1/2] rust: sync: add `ArcBorrow::from_raw` Alice Ryhl
2024-04-02 13:07 ` [PATCH v4 2/2] rust: sync: add `Arc::into_unique_or_drop` Alice Ryhl
@ 2024-05-05 22:27 ` Miguel Ojeda
2 siblings, 0 replies; 4+ messages in thread
From: Miguel Ojeda @ 2024-05-05 22:27 UTC (permalink / raw)
To: Alice Ryhl
Cc: Miguel Ojeda, Alex Gaynor, Wedson Almeida Filho, Boqun Feng,
Gary Guo, Björn Roy Baron, Benno Lossin, Andreas Hindborg,
rust-for-linux, linux-kernel
On Tue, Apr 2, 2024 at 3:07 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> This patchset contains two useful methods for the Arc type. They will be
> used in my Rust linked list implementation [1], which Rust Binder uses. See
> the Rust Binder RFC [2] for more information.
>
> Link: https://lore.kernel.org/rust-for-linux/20240402-linked-list-v1-0-b1c59ba7ae3b@google.com/ [1]
> Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@google.com/ [2]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
[ Replace `try_new` with `new` in example since we now have the new
allocation APIs. - Miguel ]
Applied to `rust-next` -- thanks everyone!
Cheers,
Miguel
^ permalink raw reply [flat|nested] 4+ messages in thread