linux-kernel - Re: [PATCH v2 7/7] rust: file: add abstraction for `poll

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Date:   Fri, 08 Dec 2023 17:53:08 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
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 7/7] rust: file: add abstraction for `poll_table`

On 12/6/23 12:59, Alice Ryhl wrote:
> diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
> index 9bcbea04dac3..eeb291cc60db 100644
> --- a/rust/bindings/lib.rs
> +++ b/rust/bindings/lib.rs
> @@ -51,3 +51,4 @@ mod bindings_helper {
> 
>  pub const GFP_KERNEL: gfp_t = BINDINGS_GFP_KERNEL;
>  pub const __GFP_ZERO: gfp_t = BINDINGS___GFP_ZERO;
> +pub const POLLFREE: __poll_t = BINDINGS_POLLFREE;

You are no longer using this constant, should this still exist?

[...]

> +    fn get_qproc(&self) -> bindings::poll_queue_proc {
> +        let ptr = self.0.get();
> +        // SAFETY: The `ptr` is valid because it originates from a reference, and the `_qproc`
> +        // field is not modified concurrently with this call since we have an immutable reference.

This needs an invariant on `PollTable` (i.e. `self.0` is valid).

> +        unsafe { (*ptr)._qproc }
> +    }
> +
> +    /// Register this [`PollTable`] with the provided [`PollCondVar`], so that it can be notified
> +    /// using the condition variable.
> +    pub fn register_wait(&mut self, file: &File, cv: &PollCondVar) {
> +        if let Some(qproc) = self.get_qproc() {
> +            // SAFETY: The pointers to `self` and `file` are valid because they are references.

What about cv.wait_list...

> +            //
> +            // Before the wait list is destroyed, the destructor of `PollCondVar` will clear
> +            // everything in the wait list, so the wait list is not used after it is freed.
> +            unsafe { qproc(file.as_ptr() as _, cv.wait_list.get(), self.0.get()) };
> +        }
> +    }
> +}
> +
> +/// A wrapper around [`CondVar`] that makes it usable with [`PollTable`].
> +///
> +/// # Invariant
> +///
> +/// If `needs_synchronize_rcu` is false, then there is nothing registered with `register_wait`.

Not able to find `needs_synchronize_rcu` anywhere else, should this be
here?

> +///
> +/// [`CondVar`]: crate::sync::CondVar
> +#[pin_data(PinnedDrop)]
> +pub struct PollCondVar {
> +    #[pin]
> +    inner: CondVar,
> +}

[..]

> +#[pinned_drop]
> +impl PinnedDrop for PollCondVar {
> +    fn drop(self: Pin<&mut Self>) {
> +        // Clear anything registered using `register_wait`.
> +        //
> +        // SAFETY: The pointer points at a valid wait list.

I was a bit confused by "wait list", since the C type is named
`wait_queue_head`, maybe just use the type name?

-- 
Cheers,
Benno

> +        unsafe { bindings::__wake_up_pollfree(self.inner.wait_list.get()) };
> +
> +        // Wait for epoll items to be properly removed.
> +        //
> +        // SAFETY: Just an FFI call.
> +        unsafe { bindings::synchronize_rcu() };
> +    }
> +}