Messages in this thread | | | From | Andreas Hindborg <> | Subject | Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` | Date | Wed, 24 May 2023 12:20:36 +0200 |
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Alice Ryhl <aliceryhl@google.com> writes:
> 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> > --- > rust/kernel/lib.rs | 1 + > rust/kernel/sync/arc.rs | 44 +++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 45 insertions(+) > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index cdf9fe999328..82854c86e65d 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -22,6 +22,7 @@ > #![feature(generic_associated_types)] > #![feature(new_uninit)] > #![feature(pin_macro)] > +#![feature(ptr_metadata)] > #![feature(receiver_trait)] > #![feature(unsize)] > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs > index e6d206242465..7c55a9178dfb 100644 > --- a/rust/kernel/sync/arc.rs > +++ b/rust/kernel/sync/arc.rs > @@ -210,6 +210,50 @@ 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`]. > + /// > + /// This code relies on the `repr(C)` layout of structs as described in > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>. > + /// > + /// # 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`]. > + pub unsafe fn from_raw(ptr: *const T) -> Self { > + // SAFETY: The safety requirement ensures that the pointer is valid. > + let val_align = core::mem::align_of_val(unsafe { &*ptr }); > + let refcount_size = core::mem::size_of::<Opaque<bindings::refcount_t>>(); > + > + // Use the `repr(C)` algorithm to compute the offset of `data` in `ArcInner`. > + // > + // Pseudo-code for the `#[repr(C)]` algorithm can be found here: > + // <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs> > + let mut val_offset = refcount_size; > + let val_misalign = val_offset % val_align; > + if val_misalign > 0 { > + val_offset += val_align - val_misalign; > + } > + > + // This preserves the metadata in the pointer, if any. > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
I can't follow this. How does this work? `ptr` was for field `inner.data: T`, but we are casting to `ArcInner<T>`.
> + let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut (); > + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
Metadata was obtained from a pointer pointing to `inner.data`, we then move it back to beginning of `ArcInner<T>` and then reconstruct the potentially fat pointer with metadata from the pointer to `T`? How can this be right?
BR Andreas
> + > + // 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
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