Re: [PATCH v2 3/8] rust: sync: add `Arc::{from_raw, into_raw}`

["Andreas Hindborg (Samsung)" <nmi@metaspace.dk>]

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