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

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Date:   Tue, 12 Dec 2023 17:05:09 +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>,
        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 12/12/23 10:51, Alice Ryhl wrote:
> 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.

Sounds good.

>>> +        };
>>> +
>>> +        // 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.

Also sounds good :)

-- 
Cheers,
Benno