Messages in this thread | | | From | "Andreas Hindborg (Samsung)" <> | Subject | Re: [PATCH v2 3/8] rust: sync: add `Arc::{from_raw, into_raw}` | Date | Mon, 05 Jun 2023 16:49:53 +0200 |
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Gary Guo <gary@garyguo.net> writes:
> 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`.
Even better 👍
BR Andreas
> >> 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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