Date: Thu, 01 Feb 2024 09:38:13 +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 01.02.24 10:33, Alice Ryhl wrote:
> On Thu, Feb 1, 2024 at 10:31 AM Benno Lossin <benno.lossin@...tonme> wrote:
>>
>> On 29.01.24 17:34, Alice Ryhl wrote:
>>> On Fri, Jan 26, 2024 at 4:04 PM Benno Lossin <benno.lossin@...ton.me> 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.
> 
> That's not what I meant. The wording of this safety comment is
> identical to the wording in other existing safety comments in the
> kernel, such as e.g. the one for `impl Send for Task`.

Ah I see. But I still think changing it is better, since it would only get
shorter. The comment on `Task` can be fixed later.
Or do you want to keep consistency here? Because I would prefer to make
this right and then change `Task` later.

-- 
Cheers,
Benno

>>> 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.

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