Date:   Sat, 25 Feb 2023 00:41:03 +0000
From:   Gary Guo <gary@garyguo.net>
To:     Asahi Lina <lina@asahilina.net>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        asahi@lists.linux.dev
Subject: Re: [PATCH 2/2] rust: sync: arc: Add
 UniqueArc<MaybeUninit<T>::assume_init()
Message-ID: <20230225004103.5f50c10a.gary@garyguo.net>
In-Reply-To: <20230224-rust-arc-v1-2-568eea613a41@asahilina.net>
References: <20230224-rust-arc-v1-0-568eea613a41@asahilina.net>
        <20230224-rust-arc-v1-2-568eea613a41@asahilina.net>
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On Fri, 24 Feb 2023 16:59:34 +0900
Asahi Lina <lina@asahilina.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.
> 
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
>  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.

The types in doc comments should be surrounded by backticks.

Best,
Gary