linux-kernel - Re: [PATCH 2/2] rust: sync: add `CondVar::wait

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Date:   Tue, 12 Dec 2023 10:51:22 +0100
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     Benno Lossin <benno.lossin@...ton.me>
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>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: sync: add `CondVar::wait_timeout`

On Fri, Dec 8, 2023 at 8:04 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On 12/6/23 11:09, Alice Ryhl wrote:
> > +    /// Atomically releases the given lock (whose ownership is proven by the guard) and puts the
> > +    /// thread to sleep. It wakes up when notified by [`CondVar::notify_one`] or
> > +    /// [`CondVar::notify_all`], or when the thread receives a signal.
> > +    ///
> > +    /// Returns whether there is a signal pending.
> > +    fn wait_internal_timeout<T, B>(
> > +        &self,
> > +        wait_state: u32,
> > +        guard: &mut Guard<'_, T, B>,
> > +        timeout: u64,
> > +    ) -> u64
> > +    where
> > +        T: ?Sized,
> > +        B: Backend,
> > +    {
> > +        let wait = Opaque::<bindings::wait_queue_entry>::uninit();
> > +
> > +        // SAFETY: `wait` points to valid memory.
> > +        unsafe { bindings::init_wait(wait.get()) };
> > +
> > +        // SAFETY: Both `wait` and `wait_list` point to valid memory.
> > +        unsafe {
> > +            bindings::prepare_to_wait_exclusive(self.wait_list.get(), wait.get(), wait_state as _)
>
> Does `.into()` work here? If for some reason the type here changes, we
> probably want to know about it.

I think we may be able to eliminate this cast by using c_int for the
integer type.

> > +        };
> > +
> > +        // SAFETY: Switches to another thread.
> > +        let timeout =
> > +            guard.do_unlocked(|| unsafe { bindings::schedule_timeout(timeout as _) as _ });
>
> Ditto.

Here, we're casting u64->long and then long->u64. How about this?

u64->long - Use timeout.try_into().unwrap_or(MAX_SCHEDULE_TIMEOUT),
since MAX_SCHEDULE_TIMEOUT is LONG_MAX.

long->u64 - This value is guaranteed to be less than the argument
passed to schedule_timeout. Use .into() for infallible cast.

Alice