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

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Date: Thu, 1 Feb 2024 10:33:17 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
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 Thu, Feb 1, 2024 at 10:31 AM Benno Lossin <benno.lossin@...ton.me> 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`.

> > 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.