Re: [PATCH v5 9/9] rust: file: add abstraction for `poll_table`

[Trevor Gross]

On Fri, Feb 9, 2024 at 5:22 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> The existing `CondVar` abstraction is a wrapper around
> `wait_queue_head`, but it does not support all use-cases of the C
> `wait_queue_head` type. To be specific, a `CondVar` cannot be registered
> with a `struct poll_table`. This limitation has the advantage that you
> do not need to call `synchronize_rcu` when destroying a `CondVar`.
>
> However, we need the ability to register a `poll_table` with a
> `wait_queue_head` in Rust Binder. To enable this, introduce a type
> called `PollCondVar`, which is like `CondVar` except that you can
> register a `poll_table`. We also introduce `PollTable`, which is a safe
> wrapper around `poll_table` that is intended to be used with
> `PollCondVar`.
>
> The destructor of `PollCondVar` unconditionally calls `synchronize_rcu`
> to ensure that the removal of epoll waiters has fully completed before
> the `wait_queue_head` is destroyed.
>
> That said, `synchronize_rcu` is rather expensive and is not needed in
> all cases: If we have never registered a `poll_table` with the
> `wait_queue_head`, then we don't need to call `synchronize_rcu`. (And
> this is a common case in Binder - not all processes use Binder with
> epoll.) The current implementation does not account for this, but if we
> find that it is necessary to improve this, a future patch could store a
> boolean next to the `wait_queue_head` to keep track of whether a
> `poll_table` has ever been registered.
>
> Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

One nit below

Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>

> +/// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class.
> +#[macro_export]
> +macro_rules! new_poll_condvar {
> +    ($($name:literal)?) => {
> +        $crate::sync::poll::PollCondVar::new($crate::optional_name!($($name)?), $crate::static_lock_class!())
> +    };
> +}

Length > 100, this could wrap:

    macro_rules! new_poll_condvar {
        ($($name:literal)?) => {
            $crate::sync::poll::PollCondVar::new(
                $crate::optional_name!($($name)?), $crate::static_lock_class!()
            )
        };
    }