Messages in this thread | | | Date | Sat, 15 Jul 2023 09:42:33 +0000 | From | Benno Lossin <> | Subject | Re: [PATCH v3 2/9] rust: sync: add `Arc::{from_raw, into_raw}` |
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> From: Wedson Almeida Filho <walmeida@microsoft.com> > > 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>
LGTM, two small documentation nits below, with them fixed you can add:
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> --- > v2 -> v3: > * Use `transmute_copy` to convert metadata. > * Use `add` instead of `wrapping_add`. > * Dropped Reviewed-by from Martin and Benno due to changes. > > rust/kernel/lib.rs | 1 + > rust/kernel/sync/arc.rs | 49 +++++++++++++++++++++++++++++++++++++++-- > 2 files changed, 48 insertions(+), 2 deletions(-) > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index 788914fd935f..6963d11092c4 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -17,6 +17,7 @@ > #![feature(const_refs_to_cell)] > #![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 a89843cacaad..4046b534e53b 100644 > --- a/rust/kernel/sync/arc.rs > +++ b/rust/kernel/sync/arc.rs > @@ -24,13 +24,13 @@ > }; > 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; > > @@ -212,6 +212,51 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self { > } > } > > + /// 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`]. > + /// > + /// This code relies on the `repr(C)` layout of structs as described in > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
Why is this in the documentation? I feel like it should be a normal code comment at the very start of the function.
> + /// > + /// # Safety > + /// > + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it > + /// can only be called once for each previous call to [`Arc::into_raw`].
"it can only" -> "it must only"
-- Cheers, Benno
> + 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 > -- > 2.41.0.255.g8b1d071c50-goog >
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