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

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Message-ID: <87y1oj9vmp.fsf@metaspace.dk>
Date:   Mon, 27 Feb 2023 12:48:34 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
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>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        asahi@...ts.linux.dev
Subject: Re: [PATCH 2/2] rust: sync: arc: Add
 UniqueArc<MaybeUninit<T>::assume_init()


Asahi Lina <lina@...hilina.net> writes:

> 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.
>
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>

>  rust/kernel/sync/arc.rs | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 752bd7c4699e..b8e9477fe865 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -512,6 +512,15 @@ 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() }
> +    }
> +
> +    /// Returns a UniqueArc<T>, assuming the MaybeUninit<T> has already been initialized.
> +    ///
> +    /// # Safety
> +    /// The contents of the UniqueArc must have already been fully 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