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authorWedson Almeida Filho <walmeida@microsoft.com>2023-08-28 10:48:01 +0000
committerTejun Heo <tj@kernel.org>2023-09-25 09:46:41 -1000
commita8321776ca0b13ec4d4fc817144fe1b3f6ba4625 (patch)
treeae3da57a8aa504d6cea518e6306b8b42333f86cd
parent6465e260f48790807eef06b583b38ca9789b6072 (diff)
rust: sync: add `Arc::{from_raw, into_raw}`
These methods can be used to turn an `Arc` into a raw pointer and back, in a way that preserves the metadata for fat pointers. This is done using the unstable ptr_metadata feature [1]. However, it could also be done using the unstable pointer_byte_offsets feature [2], which is likely to have a shorter path to stabilization than ptr_metadata. Link: https://github.com/rust-lang/rust/issues/81513 [1] Link: https://github.com/rust-lang/rust/issues/96283 [2] Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com> Co-developed-by: Alice Ryhl <aliceryhl@google.com> Signed-off-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Benno Lossin <benno.lossin@proton.me> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Tejun Heo <tj@kernel.org>
-rw-r--r--rust/kernel/lib.rs1
-rw-r--r--rust/kernel/sync/arc.rs46
2 files changed, 45 insertions, 2 deletions
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index e8811700239a..9f75d6f3d07b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -16,6 +16,7 @@
#![feature(coerce_unsized)]
#![feature(dispatch_from_dyn)]
#![feature(new_uninit)]
+#![feature(ptr_metadata)]
#![feature(receiver_trait)]
#![feature(unsize)]
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 3d496391a9bd..76c36b5467e0 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -24,13 +24,13 @@ use crate::{
};
use alloc::boxed::Box;
use core::{
- alloc::AllocError,
+ alloc::{AllocError, Layout},
fmt,
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
ops::{Deref, DerefMut},
pin::Pin,
- ptr::NonNull,
+ ptr::{NonNull, Pointee},
};
use macros::pin_data;
@@ -215,6 +215,48 @@ impl<T: ?Sized> Arc<T> {
}
}
+ /// Convert the [`Arc`] into a raw pointer.
+ ///
+ /// The raw pointer has ownership of the refcount that this Arc object owned.
+ pub fn into_raw(self) -> *const T {
+ let ptr = self.ptr.as_ptr();
+ core::mem::forget(self);
+ // SAFETY: The pointer is valid.
+ unsafe { core::ptr::addr_of!((*ptr).data) }
+ }
+
+ /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
+ ///
+ /// # Safety
+ ///
+ /// `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 };
+
+ let metadata: <T as Pointee>::Metadata = core::ptr::metadata(ptr);
+ // SAFETY: The metadata of `T` and `ArcInner<T>` is the same because `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 metadata: <ArcInner<T> as Pointee>::Metadata =
+ unsafe { core::mem::transmute_copy(&metadata) };
+ // 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 as *mut u8).sub(val_offset) as *mut () };
+ let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
+
+ // 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)) }
+ }
+
/// Returns an [`ArcBorrow`] from the given [`Arc`].
///
/// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method