Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`

[Christian Brauner <brauner@kernel.org>]

On Thu, Nov 30, 2023 at 09:17:56AM +0000, Alice Ryhl wrote:
> Christian Brauner <brauner@kernel.org> writes:
> >>>> +    /// Prevent values of this type from being moved to a different task.
> >>>> +    ///
> >>>> +    /// This is necessary because the C FFI calls assume that `current` is set to the task that
> >>>> +    /// owns the fd in question.
> >>>> +    _not_send_sync: PhantomData<*mut ()>,
> >>> 
> >>> I don't fully understand this. Can you explain in a little more detail
> >>> what you mean by this and how this works?
> >> 
> >> Yeah, so, this has to do with the Rust trait `Send` that controls
> >> whether it's okay for a value to get moved from one thread to another.
> >> In this case, we don't want it to be `Send` so that it can't be moved to
> >> another thread, since current might be different there.
> >> 
> >> The `Send` trait is automatically applied to structs whenever *all*
> >> fields of the struct are `Send`. So to ensure that a struct is not
> >> `Send`, you add a field that is not `Send`.
> >> 
> >> The `PhantomData` type used here is a special zero-sized type.
> >> Basically, it says "pretend this struct has a field of type `*mut ()`,
> >> but don't actually add the field". So for the purposes of `Send`, it has
> >> a non-Send field, but since its wrapped in `PhantomData`, the field is
> >> not there at runtime.
> > 
> > This probably a stupid suggestion, question. But while PhantomData gives
> > the right hint of what is happening I wouldn't mind if that was very
> > explicitly called NoSendTrait or just add the explanatory comment. Yes,
> > that's a lot of verbiage but you'd help us a lot.
> 
> I suppose we could add a typedef:
> 
> type NoSendTrait = PhantomData<*mut ()>;
> 
> and use that as the field type. The way I did it here is the "standard"
> way of doing it, and if you look at code outside the kernel, you will
> also find them using `PhantomData` like this. However, I don't mind
> adding the typedef if you think it is helpful.

I'm fine with just a comment as well. I just need to be able to read
this a bit faster. I'm basically losing half a day just dealing with
this patchset and that's not realistic if I want to keep up with other
patches that get sent.

And if you resend and someone else review you might have to answer the
same question again.