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

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Message-ID: <20231211153435.4162296-1-aliceryhl@google.com>
Date:   Mon, 11 Dec 2023 15:34:35 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     benno.lossin@...ton.me
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.com, arve@...roid.com, bjorn3_gh@...tonmail.com,
        boqun.feng@...il.com, brauner@...nel.org, cmllamas@...gle.com,
        dan.j.williams@...el.com, dxu@...uu.xyz, gary@...yguo.net,
        gregkh@...uxfoundation.org, joel@...lfernandes.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, maco@...roid.com, ojeda@...nel.org,
        peterz@...radead.org, rust-for-linux@...r.kernel.org,
        surenb@...gle.com, tglx@...utronix.de, tkjos@...roid.com,
        viro@...iv.linux.org.uk, wedsonaf@...il.com, willy@...radead.org
Subject: Re: [PATCH v2 4/7] rust: file: add `FileDescriptorReservation`

Benno Lossin <benno.lossin@...ton.me> writes:
> On 12/6/23 12:59, Alice Ryhl wrote:
> > +    /// Commits the reservation.
> > +    ///
> > +    /// The previously reserved file descriptor is bound to `file`. This method consumes the
> > +    /// [`FileDescriptorReservation`], so it will not be usable after this call.
> > +    pub fn fd_install(self, file: ARef<File>) {
> > +        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`, and `file.ptr` is
> > +        // guaranteed to have an owned ref count by its type invariants.
> 
> There is no mention of the requirement that `current` has not changed
> (you do mention it on the `_not_send` field, but I think it should also
> be in the safety comment here).
> 
> [...]
> > +impl Drop for FileDescriptorReservation {
> > +    fn drop(&mut self) {
> > +        // SAFETY: `self.fd` was returned by a previous call to `get_unused_fd_flags`.
> 
> Ditto.

I'll update this.

> > +/// 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.
> 
> Then you can also change the usages of `PhantomData<*mut ()>` in
> `Guard` and `TaskRef`.
> 
> It would be nice to use `NotThreadSafe` as the value instead of
> `PhantomData`, since it is a bit confusing... 
> I think we might be able to also have a constant with the same name
> that is just `pub const NotThreadSafe: NotThreadSafe = PhantomData;`.

I was able to get this to work with a `const`, so I will use that.

Alice