linux-kernel - Re: [PATCH v2 2/2] rust: sync: arc: Add UniqueArc<MaybeUninit<T>::assume

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Date:   Mon, 3 Apr 2023 16:26:09 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Asahi Lina <lina@...hilina.net>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>,
        Martin Rodriguez Reboredo <yakoyoku@...il.com>,
        Benno Lossin <y86-dev@...tonmail.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        asahi@...ts.linux.dev
Subject: Re: [PATCH v2 2/2] rust: sync: arc: Add
 UniqueArc<MaybeUninit<T>::assume_init()

On Mon, 03 Apr 2023 19:01:12 +0900
Asahi Lina <lina@...hilina.net> wrote:

> We can already create `UniqueArc<MaybeUninit<T>>` instances with
> `UniqueArc::try_new_uninit()` and write to them with `write()`. Add
> the missing unsafe `assume_init()` function to promote it to
> `UniqueArc<T>`, so users can do piece-wise initialization of the
> contents instead of doing it all at once as long as they keep the
> invariants (the same requirements as `MaybeUninit::assume_init()`).
> 
> This mirrors the std `Arc::assume_init()` function. In the kernel,
> since we have `UniqueArc`, arguably this only belongs there since most
> use cases will initialize it immediately after creating it, before
> demoting it to `Arc` to share it.
> 
> Co-authored-by: Benno Lossin <y86-dev@...tonmail.com>
> Signed-off-by: Benno Lossin <y86-dev@...tonmail.com>
> Signed-off-by: Asahi Lina <lina@...hilina.net>

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

> ---
>  rust/kernel/sync/arc.rs | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index b904ed898d3a..b664b41e473d 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -489,6 +489,17 @@ impl<T> UniqueArc<MaybeUninit<T>> {
>      /// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
>      pub fn write(mut self, value: T) -> UniqueArc<T> {
>          self.deref_mut().write(value);
> +        // SAFETY: We have just written the contents fully.
> +        unsafe { self.assume_init() }
> +    }
> +
> +    /// Unsafely assume that `self` is initialized.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller guarantees that the value behind this pointer has been initialized. It is
> +    /// *immediate* UB to call this when the value is not initialized.
> +    pub unsafe fn assume_init(self) -> UniqueArc<T> {
>          let inner = ManuallyDrop::new(self).inner.ptr;
>          UniqueArc {
>              // SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be
>