diff mbox series

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

Message ID 20230601134946.3887870-4-aliceryhl@google.com
State New
Headers
Series rust: workqueue: add bindings for the workqueue |

Commit Message

Alice Ryhl June 1, 2023, 1:49 p.m. UTC 
  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(-)

Comments

Gary Guo June 1, 2023, 5:26 p.m. UTC | #1 
On Thu,  1 Jun 2023 13:49:41 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> 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>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/lib.rs      |  1 +
>  rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 42 insertions(+), 1 deletion(-)
Andreas Hindborg June 2, 2023, 10:51 a.m. UTC | #2 
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)
        };
        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.

Best regards,
Andreas

> +        let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> +        let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> +
> +        // 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
Gary Guo June 5, 2023, 2:31 p.m. UTC | #3 
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
Andreas Hindborg June 5, 2023, 2:49 p.m. UTC | #4 
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
Boqun Feng June 5, 2023, 3 p.m. UTC | #5 
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
Boqun Feng June 5, 2023, 3:20 p.m. UTC | #6 
On Mon, Jun 05, 2023 at 08:00:57AM -0700, Boqun Feng wrote:
> 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.
> 

(Forget the link to the function)
[1]: https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_byte_offset

> Regards,
> Boqun
> 
> > Best,
> > Gary
Andreas Hindborg June 5, 2023, 6:34 p.m. UTC | #7 
Boqun Feng <boqun.feng@gmail.com> writes:

> 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`.

I like `transmute_copy()` better for being more explicit.

>
> 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.

Idk, it is sort of similar to the original approach. I think it is a
good idea to be a bit explicit about what we are doing here.

BR Andreas
Benno Lossin June 11, 2023, 3:48 p.m. UTC | #8 
On 01.06.23 15:49, Alice Ryhl wrote:
> 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>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
diff mbox series

Patch

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>);
+        let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
+        let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
+
+        // 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