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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jun 05, 2023 at 03:31:42PM +0100, Gary Guo wrote:
> 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`.
> 

Or just `transmute`:

	let metadata = unsafe {
		core::mem::transmute<_, <ArcInner<T> as
		Pointee>>::Metadata>(metadata)
	};

? Since `Pointee::Metadata` is `Copy`.

Regards,
Boqun

> >         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.
> 

I actually wish that we could use `wrapping_byte_offset`[1], and just

	// `*const T` and `*const ArcInner<T>` should have the same
	// metdata, so convert the pointer type first.
	let ptr = ptr as *const ArcInner<T>;

	// .. and then adjust the byte offset.
	let ptr = ptr.wrapping_byte_offset(-val_offset);

This may be the opposite direction as Andreas proposed ;-), but the
result is less code.

Regards,
Boqun

> Best,
> Gary