linux-kernel - Re: [PATCH v10 8/8] rust: file: add abstraction for `poll

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Message-ID: <CAH5fLghE1RTGgnY=y=T0k62XfW6oqE-mQLSJEEc15YyZwdYU6Q@mail.gmail.com>
Date: Mon, 23 Sep 2024 11:10:19 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Gary Guo <gary@...yguo.net>
Cc: Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Christian Brauner <brauner@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Peter Zijlstra <peterz@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	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>, 
	Matthew Wilcox <willy@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Daniel Xu <dxu@...uu.xyz>, 
	Martin Rodriguez Reboredo <yakoyoku@...il.com>, Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v10 8/8] rust: file: add abstraction for `poll_table`

On Mon, Sep 16, 2024 at 12:24 AM Gary Guo <gary@...yguo.net> wrote:
>
> On Sun, 15 Sep 2024 14:31:34 +0000
> Alice Ryhl <aliceryhl@...gle.com> wrote:
> > +    /// 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 `file` and `self` need to be valid for the duration of this
> > +            // call to `qproc`, which they are because they are references.
> > +            //
> > +            // The `cv.wait_queue_head` pointer must be valid until an rcu grace period after the
> > +            // waiter is removed. The `PollCondVar` is pinned, so before `cv.wait_queue_head` can
> > +            // be destroyed, the destructor must run. That destructor first removes all waiters,
> > +            // and then waits for an rcu grace period. Therefore, `cv.wait_queue_head` is valid for
> > +            // long enough.
> > +            unsafe { qproc(file.as_ptr() as _, cv.wait_queue_head.get(), self.0.get()) };
> > +        }
>
> Should this be calling `poll_wait` instead?
>
> > +#[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_queue_head`.
> > +        unsafe { bindings::__wake_up_pollfree(self.inner.wait_queue_head.get()) };
>
> Should this use `wake_up_pollfree` (without the leading __)?

For both cases, that would require a Rust helper. But I suppose we could do it.

Alice