Re: [PATCH 09/20] rust: types: implement `Unique<T>`

[Benno Lossin <benno.lossin@proton.me>]

On 04.07.24 19:06, Danilo Krummrich wrote:
> Implement the `Unique` type as a prerequisite for `KBox` and `Kvec`
> introduced in subsequent patches.
> 
> `Unique` serves as wrapper around a `NonNull`, but indicates that the
> possessor of this wrapper owns the referent.
> 
> This type already exists in Rust's core library, but, unfortunately, is
> exposed as unstable API and hence shouldn't be used in the kernel.

It's not really exposed, as the feature (ptr_internals) is an internal
unstable feature and thus probably perma-unstable.

(but your argument still holds, this just means that we *really* should
not be using it)

> This implementation of `Unique` is almost identical, but mostly stripped
> down to the functionality we need for `KBox` and `KVec`. Additionally,
> all unstable features are removed and / or replaced by stable ones.
> 
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  rust/kernel/types.rs | 176 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 176 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 2e7c9008621f..281327ea2932 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -409,3 +409,179 @@ pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `R`.
>      Right(R),
>  }
> +
> +/// A wrapper around a raw non-null `*mut T` that indicates that the possessor
> +/// of this wrapper owns the referent. Useful for building abstractions like
> +/// `Box<T>`, `Vec<T>`, `String`, and `HashMap<K, V>`.
> +///
> +/// Unlike `*mut T`, `Unique<T>` behaves "as if" it were an instance of `T`.
> +/// It implements `Send`/`Sync` if `T` is `Send`/`Sync`. It also implies
> +/// the kind of strong aliasing guarantees an instance of `T` can expect:
> +/// the referent of the pointer should not be modified without a unique path to
> +/// its owning Unique.
> +///
> +/// If you're uncertain of whether it's correct to use `Unique` for your purposes,
> +/// consider using `NonNull`, which has weaker semantics.
> +///
> +/// Unlike `*mut T`, the pointer must always be non-null, even if the pointer
> +/// is never dereferenced. This is so that enums may use this forbidden value
> +/// as a discriminant -- `Option<Unique<T>>` has the same size as `Unique<T>`.
> +/// However the pointer may still dangle if it isn't dereferenced.
> +///
> +/// Unlike `*mut T`, `Unique<T>` is covariant over `T`. This should always be correct
> +/// for any type which upholds Unique's aliasing requirements.

Since you copied this directly from the stdlib, should this be citing
it?

> +#[repr(transparent)]
> +pub struct Unique<T: ?Sized> {
> +    pointer: NonNull<T>,

Gary and I already mentioned this in the meeting, but I will put it here
for the record:

The `Unique` from std is special, in the sense that the Rust compiler
will emit the `noalias` LLVM attribute.

This gives std's `Box` type a possible performance advantage (IIRC Gary
had a compiler explorer example that showed different instruction
count).

I thought that we could mimic this behavior if we would change the type
of `pointer` to `ManuallyDrop<Box<T>>`. But that only works if the
pointer is always valid, since otherwise we can't call `Box::from_raw`.

So currently I don't see a workaround that would give us the noalias
attribute back.

It would be a good idea to get some benchmarks on how this affects
performance, Andreas or Alice do you think you can give a bit of compute
time, once this patchset is more mature?

> +    // NOTE: this marker has no consequences for variance, but is necessary
> +    // for dropck to understand that we logically own a `T`.
> +    //
> +    // For details, see:
> +    // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
> +    _marker: PhantomData<T>,
> +}
> +
> +/// `Unique` pointers are `Send` if `T` is `Send` because the data they
> +/// reference is unaliased. Note that this aliasing invariant is
> +/// unenforced by the type system; the abstraction using the
> +/// `Unique` must enforce it.
> +unsafe impl<T: Send + ?Sized> Send for Unique<T> {}
> +
> +/// `Unique` pointers are `Sync` if `T` is `Sync` because the data they
> +/// reference is unaliased. Note that this aliasing invariant is
> +/// unenforced by the type system; the abstraction using the
> +/// `Unique` must enforce it.
> +unsafe impl<T: Sync + ?Sized> Sync for Unique<T> {}
> +
> +impl<T: Sized> Unique<T> {
> +    /// Creates a new `Unique` that is dangling, but well-aligned.
> +    ///
> +    /// This is useful for initializing types which lazily allocate, like
> +    /// `Vec::new` does.
> +    ///
> +    /// Note that the pointer value may potentially represent a valid pointer to
> +    /// a `T`, which means this must not be used as a "not yet initialized"
> +    /// sentinel value. Types that lazily allocate must track initialization by
> +    /// some other means.
> +    #[must_use]
> +    #[inline]
> +    pub const fn dangling() -> Self {
> +        Unique {
> +            pointer: NonNull::dangling(),
> +            _marker: PhantomData,
> +        }
> +    }
> +}
> +
> +impl<T: ?Sized> Unique<T> {
> +    /// Creates a new `Unique`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must be non-null.
> +    #[inline]
> +    pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
> +        // SAFETY: the caller must guarantee that `ptr` is non-null.
> +        unsafe {
> +            Unique {
> +                pointer: NonNull::new_unchecked(ptr),
> +                _marker: PhantomData,
> +            }
> +        }
> +    }
> +
> +    /// Creates a new `Unique` if `ptr` is non-null.
> +    #[allow(clippy::manual_map)]
> +    #[inline]
> +    pub fn new(ptr: *mut T) -> Option<Self> {

I think we should give `Unique` the invariant that it is unique, making
this function `unsafe`, since the caller must ensure that only this
unique has access to the pointee.

---
Cheers,
Benno

> +        if let Some(pointer) = NonNull::new(ptr) {
> +            Some(Unique {
> +                pointer,
> +                _marker: PhantomData,
> +            })
> +        } else {
> +            None
> +        }
> +    }