linux-kernel - Re: [PATCH v3 1/9] rust: file: add Rust abstraction for `struct file`

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Date: Thu, 01 Feb 2024 09:30:46 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>
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>, Andreas Hindborg <a.hindborg@...sung.com>, Peter Zijlstra <peterz@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arve Hjønnevåg <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, Dan Williams <dan.j.williams@...el.com>, Kees Cook <keescook@...omium.org>, Matthew Wilcox <willy@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 1/9] rust: file: add Rust abstraction for `struct file`

On 29.01.24 17:34, Alice Ryhl wrote:
> On Fri, Jan 26, 2024 at 4:04 PM Benno Lossin <benno.lossin@...tonme> wrote:
>>> +///   closed.
>>> +/// * A light refcount must be dropped before returning to userspace.
>>> +#[repr(transparent)]
>>> +pub struct File(Opaque<bindings::file>);
>>> +
>>> +// SAFETY: By design, the only way to access a `File` is via an immutable reference or an `ARef`.
>>> +// This means that the only situation in which a `File` can be accessed mutably is when the
>>> +// refcount drops to zero and the destructor runs. It is safe for that to happen on any thread, so
>>> +// it is ok for this type to be `Send`.
>>
>> Technically, `drop` is never called for `File`, since it is only used
>> via `ARef<File>` which calls `dec_ref` instead. Also since it only contains
>> an `Opaque`, dropping it is a noop.
>> But what does `Send` mean for this type? Since it is used together with
>> `ARef`, being `Send` means that `File::dec_ref` can be called from any
>> thread. I think we are missing this as a safety requirement on
>> `AlwaysRefCounted`, do you agree?
>> I think the safety justification here could be (with the requirement added
>> to `AlwaysRefCounted`):
>>
>>       SAFETY:
>>       - `File::drop` can be called from any thread.
>>       - `File::dec_ref` can be called from any thread.
> 
> This wording was taken from rust/kernel/task.rs. I think it's out of
> scope to reword it.

Rewording the safety docs on `AlwaysRefCounted`, yes that is out of scope,
I was just checking if you agree that the current wording is incomplete.

> Besides, it says "destructor runs", not "drop runs". The destructor
> can be interpreted to mean the right thing for ARef.

To me "destructor runs" and "drop runs" are synonyms.

> The right safety comment would probably be that dec_ref can be called
> from any thread.

Yes and no, I would prefer if you could remove the "By design, ..."
part and only focus on `dec_ref` being callable from any thread and
it being ok to send a `File` to a different thread.

-- 
Cheers,
Benno