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Date:   Thu, 30 Nov 2023 11:54:51 +0000
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Subject: Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`
From:   Alice Ryhl <aliceryhl@google.com>
To:     brauner@kernel.org
Cc:     a.hindborg@samsung.com, alex.gaynor@gmail.com,
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Christian Brauner <brauner@kernel.org> writes:
> 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.

What do you think about this wording?

/// Prevent values of this type from being moved to a different task.
/// 
/// This field has the type `PhantomData<*mut ()>`, which does not
/// implement the Send trait. By adding a field with this property, we
/// ensure that the `FileDescriptorReservation` struct will not
/// implement the Send trait either. This has the consequence that the
/// compiler will prevent you from moving values of type
/// `FileDescriptorReservation` into a different task, which we want
/// because other tasks might have a different value of `current`. We
/// want to avoid that because `fd_install` assumes that the value of
/// `current` is unchanged since the call to `get_unused_fd_flags`.
/// 
/// The `PhantomData` type has size zero, so the field does not exist at
/// runtime.

Alice