linux-kernel - Re: [RFC PATCH v1 3/5] rust: file: add `FileDescriptorReservation`

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Date:   Wed, 9 Aug 2023 01:02:20 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [RFC PATCH v1 3/5] rust: file: add `FileDescriptorReservation`

On 7/20/23 12:28, Alice Ryhl wrote:
> From: Wedson Almeida Filho <walmeida@...rosoft.com>
> 
> This allows the creation of a file descriptor in two steps: first, we
> reserve a slot for it, then we commit or drop the reservation. The first
> step may fail (e.g., the current process ran out of available slots),
> but commit and drop never fail (and are mutually exclusive).
> 
> Co-Developed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> [...]
> +/// A file descriptor reservation.
> +///
> +/// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
> +/// then we commit or drop the reservation. The first step may fail (e.g., the current process ran
> +/// out of available slots), but commit and drop never fail (and are mutually exclusive).

This "drop" suggests to me that there was a method that it does said
action, and indeed it is `Drop::drop`. But if I look at the doc comment
of `commit` then it doesn't look that these two are mutex.

     /// Commits the reservation.
     ///
     /// The previously reserved file descriptor is bound to `file`.

I'd put a mention that `FileDescriptorReservation` gets forgotten when
`commit` is called so then it clears up that it's mutex with drop.

> +///
> +/// # Invariants
> +///
> +/// The fd stored in this struct must correspond to a reserved file descriptor of the current task.
> +pub struct FileDescriptorReservation {
> [...]
> +}
> [...]