linux-kernel - Re: [PATCH 12/13] rust: sync: introduce `CondVar`

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Date:   Thu, 30 Mar 2023 14:59:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 12/13] rust: sync: introduce `CondVar`

On Thu, Mar 30, 2023 at 01:39:53AM -0300, Wedson Almeida Filho wrote:

> +impl CondVar {
> +    /// Constructs a new condvar initialiser.
> +    #[allow(clippy::new_ret_no_self)]
> +    pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
> +        pin_init!(Self {
> +            _pin: PhantomPinned,
> +            // SAFETY: `__init_waitqueue_head` initialises the waitqueue head, and both `name` and
> +            // `key` have static lifetimes so they live indefinitely.
> +            wait_list <- unsafe {
> +                Opaque::ffi_init2(
> +                    bindings::__init_waitqueue_head,
> +                    name.as_char_ptr(),
> +                    key.as_ptr(),
> +                )
> +            },
> +        })
> +    }
> +
> +    fn wait_internal<T: ?Sized, B: Backend>(&self, wait_state: u32, guard: &mut Guard<'_, T, B>) {
> +        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 _)
> +        };

I can't read this rust gunk, but where is the condition test gone?

Also, where is the loop gone to?

> +
> +        // SAFETY: No arguments, switches to another thread.
> +        guard.do_unlocked(|| unsafe { bindings::schedule() });
> +
> +        // SAFETY: Both `wait` and `wait_list` point to valid memory.
> +        unsafe { bindings::finish_wait(self.wait_list.get(), wait.get()) };
> +    }
> +
> +    /// 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, reacquiring the lock on wake up. It wakes up when notified by
> +    /// [`CondVar::notify_one`] or [`CondVar::notify_all`], or when the thread receives a signal.
> +    /// It may also wake up spuriously.
> +    ///
> +    /// Returns whether there is a signal pending.
> +    #[must_use = "wait returns if a signal is pending, so the caller must check the return value"]
> +    pub fn wait<T: ?Sized, B: Backend>(&self, guard: &mut Guard<'_, T, B>) -> bool {
> +        self.wait_internal(bindings::TASK_INTERRUPTIBLE, guard);
> +        Task::current().signal_pending()
> +    }
> +
> +    /// Releases the lock and waits for a notification in uninterruptible mode.
> +    ///
> +    /// Similar to [`CondVar::wait`], except that the wait is not interruptible. That is, the
> +    /// thread won't wake up due to signals. It may, however, wake up supirously.
> +    pub fn wait_uninterruptible<T: ?Sized, B: Backend>(&self, guard: &mut Guard<'_, T, B>) {
> +        self.wait_internal(bindings::TASK_UNINTERRUPTIBLE, guard)
> +    }
> +
> +    /// Calls the kernel function to notify the appropriate number of threads with the given flags.
> +    fn notify(&self, count: i32, flags: u32) {
> +        // SAFETY: `wait_list` points to valid memory.
> +        unsafe {
> +            bindings::__wake_up(
> +                self.wait_list.get(),
> +                bindings::TASK_NORMAL,
> +                count,
> +                flags as _,
> +            )
> +        };
> +    }
> +
> +    /// Wakes a single waiter up, if any.
> +    ///
> +    /// This is not 'sticky' in the sense that if no thread is waiting, the notification is lost
> +    /// completely (as opposed to automatically waking up the next waiter).
> +    pub fn notify_one(&self) {
> +        self.notify(1, 0);
> +    }
> +
> +    /// Wakes all waiters up, if any.
> +    ///
> +    /// This is not 'sticky' in the sense that if no thread is waiting, the notification is lost
> +    /// completely (as opposed to automatically waking up the next waiter).
> +    pub fn notify_all(&self) {
> +        self.notify(0, 0);
> +    }
> +}