Messages in this thread | | | Date | Mon, 5 Jun 2023 15:31:42 +0100 | From | Gary Guo <> | Subject | Re: [PATCH v2 3/8] rust: sync: add `Arc::{from_raw, into_raw}` |
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On Fri, 02 Jun 2023 12:51:08 +0200 "Andreas Hindborg (Samsung)" <nmi@metaspace.dk> wrote:
> 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> > > Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> > > --- > > rust/kernel/lib.rs | 1 + > > rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++- > > 2 files changed, 42 insertions(+), 1 deletion(-) > > > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > > index 7ea777b731e6..ad9142928fb1 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..684be9f73aca 100644 > > --- a/rust/kernel/sync/arc.rs > > +++ b/rust/kernel/sync/arc.rs > > @@ -24,7 +24,7 @@ > > }; > > use alloc::boxed::Box; > > use core::{ > > - alloc::AllocError, > > + alloc::{AllocError, Layout}, > > fmt, > > marker::{PhantomData, Unsize}, > > mem::{ManuallyDrop, MaybeUninit}, > > @@ -212,6 +212,46 @@ 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>. > > + /// > > + /// # 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 { > > + let refcount_layout = Layout::new::<bindings::refcount_t>(); > > + // SAFETY: The caller guarantees that the pointer is valid. > > + let val_layout = unsafe { Layout::for_value(&*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 }; > > + > > + // This preserves the metadata in the pointer, if any. > > + // > > + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at > > + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>. > > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>); > > Thanks for updating the comment with the link. I looked into this and I > find that what we are doing here, even though it works, does not feel > right at all. We should be able to do this: > > let metadata = core::ptr::metadata(ptr); > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut (); > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata); > > but the way `Pointee::Metadata` is defined will not allow this, even > though we know it is valid. I would suggest the following instead: > > let metadata = core::ptr::metadata(ptr); > // Convert <T as Pointee>::Metadata to <ArcInner<T> as > // Pointee>::Metadata. We know they have identical representation and thus this is OK. > let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe { > &*((&metadata as *const <T as Pointee>::Metadata as *const ()) > as *const <ArcInner<T> as Pointee>::Metadata) > };
This could just be a `transmute_copy`.
> let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut (); > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata); > > Even though it is a bit more complex, it captures what we are trying to > do better.
I agree this captures the semantics better.
Best, Gary
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