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

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Message-ID: <1dd1a3e8-ef9a-4e89-891f-b49d82acc5f8@gmail.com>
Date:   Wed, 6 Dec 2023 17:05:21 +0000
From:   Tiago Lam <tiagolam@...il.com>
To:     Alice Ryhl <aliceryhl@...gle.com>, 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>,
        Benno Lossin <benno.lossin@...ton.me>,
        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>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: sync: add `CondVar::wait_timeout`


On 06/12/2023 10:09, Alice Ryhl wrote:
[...]
> diff --git a/rust/kernel/sync/condvar.rs b/rust/kernel/sync/condvar.rs
> index 9861c6749ad0..a6a6b6ab0c39 100644
> --- a/rust/kernel/sync/condvar.rs
> +++ b/rust/kernel/sync/condvar.rs
> @@ -120,6 +120,63 @@ fn wait_internal<T: ?Sized, B: Backend>(&self, wait_state: u32, guard: &mut Guar
>           unsafe { bindings::finish_wait(self.wait_list.get(), wait.get()) };
>       }
>   
> +    /// 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 _)
> +        };
> +
> +        // SAFETY: Switches to another thread.
> +        let timeout =
> +            guard.do_unlocked(|| unsafe { bindings::schedule_timeout(timeout as _) as _ });

It looks like `schedule_timeout()` simply calls `schedule()` when the 
timeout passed is `MAX_SCHEDULE_TIMEOUT`, so `wait_internal_timeout()` 
could be merged together with the already existing `wait_internal()`, 
where `wait_internal()` would always call `schedule_timeout()`? I may be 
missing something, so just wondering why you decided to introduce 
another method.

> +
> +        // SAFETY: Both `wait` and `wait_list` point to valid memory.
> +        unsafe { bindings::finish_wait(self.wait_list.get(), wait.get()) };
> +
> +        timeout
> +    }
> +
> +    /// Releases the lock and waits for a notification in interruptible mode.
> +    ///
> +    /// 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 a timeout occurs, or when the thread receives a signal.
> +    ///
> +    /// Returns whether there is a signal pending.
> +    #[must_use = "wait_timeout returns if a signal is pending, so the caller must check the return value"]
> +    pub fn wait_timeout<T: ?Sized, B: Backend>(
> +        &self,
> +        guard: &mut Guard<'_, T, B>,
> +        jiffies: u64,
> +    ) -> CondVarTimeoutResult {

Should this be called `wait_timeout_interruptable` instead, so that if 
we need to add one using the `TASK_INTERRUPTIBLE` state later we don't 
need to modfy it again? It also matches the 
`schedule_timeout_interruptible` one in the kernel (although that's not 
a reason to change it just in itself).

> +        let res = self.wait_internal_timeout(bindings::TASK_INTERRUPTIBLE, guard, jiffies);
> +
> +        match (res as _, crate::current!().signal_pending()) {
> +            (jiffies, true) => CondVarTimeoutResult::Signal { jiffies },
> +            (0, false) => CondVarTimeoutResult::Timeout,
> +            (jiffies, false) => CondVarTimeoutResult::Woken { jiffies },
> +        }
> +    }
> +
>       /// Releases the lock and waits for a notification in interruptible mode.
>       ///
>       /// Atomically releases the given lock (whose ownership is proven by the guard) and puts the
> @@ -177,3 +234,19 @@ pub fn notify_all(&self) {
>           self.notify(0, 0);
>       }
>   }
> +
> +/// The return type of `wait_timeout`.
> +pub enum CondVarTimeoutResult {
> +    /// The timeout was reached.
> +    Timeout,
> +    /// Somebody woke us up.
> +    Woken {
> +        /// Remaining sleep duration.
> +        jiffies: u64,
> +    },
> +    /// A signal occurred.
> +    Signal {
> +        /// Remaining sleep duration.
> +        jiffies: u64,
> +    },
> +}


Is `Signal` and `Woken` only going to hold a single value? Would it be 
best represented as a tuple struct instead, like so?

     pub enum CondVarTimeoutResult {
         /// The timeout was reached.
         Timeout,
         /// Somebody woke us up.
         Woken (u64),
         /// A signal occurred.
         Signal (u64),
     }

Regard,
Tiago.