linux-kernel - Re: [PATCH v2 4/7] rust: file: add `FileDescriptorReservation`

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Message-ID: <CAH5fLghg7=eF38fNeUTEFe9zH95H-Que_911kJ+g_rsNrODHsw@mail.gmail.com>
Date:   Fri, 8 Dec 2023 08:43:25 +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 v2 4/7] rust: file: add `FileDescriptorReservation`

On Fri, Dec 8, 2023 at 8:37 AM Benno Lossin <benno.lossin@...ton.me> wrote:
> > +/// Zero-sized type to mark types not [`Send`].
> > +///
> > +/// Add this type as a field to your struct if your type should not be sent to a different task.
> > +/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
> > +/// whole type is `!Send`.
> > +///
> > +/// If a type is `!Send` it is impossible to give control over an instance of the type to another
> > +/// task. This is useful when a type stores task-local information for example file descriptors.
> > +pub type NotThreadSafe = PhantomData<*mut ()>;
>
> This should be in its own commit.

Will do.

> Then you can also change the usages of `PhantomData<*mut ()>` in
> `Guard` and `TaskRef`.

Will do.

> It would be nice to use `NotThreadSafe` as the value instead of
> `PhantomData`, since it is a bit confusing...

That doesn't compile, unfortunately.

> I think we might be able to also have a constant with the same name
> that is just `pub const NotThreadSafe: NotThreadSafe = PhantomData;`.

I can try to get this to work, but I worry that they will shadow each other.

Alice