Messages in this thread | | | From | Trevor Gross <> | Date | Mon, 5 Feb 2024 15:02:46 -0600 | Subject | Re: [PATCH] rust: stop using ptr_metadata feature |
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On Mon, Feb 5, 2024 at 3:19 AM Alice Ryhl <aliceryhl@google.com> wrote: > > The `byte_sub` method was stabilized in Rust 1.75.0. By using that > method, we no longer need the unstable `ptr_metadata` feature for > implementing `Arc::from_raw`. > > This brings us one step closer towards not using unstable compiler > features. > > Signed-off-by: Alice Ryhl <aliceryhl@google.com> > --- > This patch is based on rust-next because it depends on the patch [1] > that upgrades to Rust 1.75.0. > > [1]: https://lore.kernel.org/all/20231224172128.271447-1-ojeda@kernel.org/ > > rust/kernel/lib.rs | 1 - > rust/kernel/sync/arc.rs | 10 ++++------ > 2 files changed, 4 insertions(+), 7 deletions(-) > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > [...] > @@ -239,18 +239,16 @@ pub unsafe fn from_raw(ptr: *const T) -> Self { > // 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>.
The comment should be reworded, no more metadata and no unsafe block so it doesn't have to be SAFETY.
> - let metadata: <ArcInner<T> as Pointee>::Metadata = > - unsafe { core::mem::transmute_copy(&metadata) }; > + let ptr = ptr as *mut ArcInner<T>;
Nit: this could be `.cast::<ArcInner<T>>().cast_mut()` to make the intentional mutability change clear.
> // 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); > + let ptr = unsafe { ptr.byte_sub(val_offset) }; > > // 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. > > base-commit: f090f0d0eea9666a96702b29bc9a64cbabee85c5 > -- > 2.43.0.594.gd9cf4e227d-goog > >
Worth noting that the same change has been in upstream for a while, since https://github.com/rust-lang/rust/pull/99113.
With the above changed or otherwise clarified:
Reviewed-by: Trevor Gross <tmgross@umich.edu>
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