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From:   Alice Ryhl <aliceryhl@google.com>
Date:   Tue, 12 Dec 2023 10:59:32 +0100
Message-ID: <CAH5fLgiQ-7gbwP2RLoVDfDqoA+nXPboBW6eTKiv45Yam_Vjv_A@mail.gmail.com>
Subject: Re: [PATCH v2 7/7] rust: file: add abstraction for `poll_table`
To:     Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Carlos Llamas <cmllamas@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org
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On Fri, Dec 8, 2023 at 6:53=E2=80=AFPM Benno Lossin <benno.lossin@proton.me=
> wrote:
>
> 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 =3D BINDINGS_GFP_KERNEL;
> >  pub const __GFP_ZERO: gfp_t =3D BINDINGS___GFP_ZERO;
> > +pub const POLLFREE: __poll_t =3D BINDINGS_POLLFREE;
>
> You are no longer using this constant, should this still exist?

Nice catch, thanks!

> > +    fn get_qproc(&self) -> bindings::poll_queue_proc {
> > +        let ptr =3D self.0.get();
> > +        // SAFETY: The `ptr` is valid because it originates from a ref=
erence, 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).

How would you phrase it?

> > +        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) =3D self.get_qproc() {
> > +            // SAFETY: The pointers to `self` and `file` are valid bec=
ause they are references.
>
> What about cv.wait_list...

I can add it to the list of things that are valid due to references.

> > +            //
> > +            // Before the wait list is destroyed, the destructor of `P=
ollCondVar` will clear
> > +            // everything in the wait list, so the wait list is not us=
ed after it is freed.
> > +            unsafe { qproc(file.as_ptr() as _, cv.wait_list.get(), sel=
f.0.get()) };
> > +        }
> > +    }
> > +}
> > +
> > +/// A wrapper around [`CondVar`] that makes it usable with [`PollTable=
`].
> > +///
> > +/// # Invariant
> > +///
> > +/// If `needs_synchronize_rcu` is false, then there is nothing registe=
red with `register_wait`.
>
> Not able to find `needs_synchronize_rcu` anywhere else, should this be
> here?

Sorry, this shouldn't be there. It was something I experimented with,
but gave up on.

> > +#[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?

I will update all instances of "wait list" to "wait_queue_head". It's
because I incorrectly remembered the C type name to be "wait_list".

Alice