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    // Allow `unreachable_pub` warnings when sync is not enabled
    // due to the usage of `Notify` within the `rt` feature set.
    // When this module is compiled with `sync` enabled we will warn on
    // this lint. When `rt` is enabled we use `pub(crate)` which
    // triggers this warning but it is safe to ignore in this case.
    #![cfg_attr(not(feature = "sync"), allow(unreachable_pub, dead_code))]

    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::Mutex;
    use crate::util::linked_list::{self, LinkedList};

    use std::cell::UnsafeCell;
    use std::future::Future;
    use std::marker::PhantomPinned;
    use std::pin::Pin;
    use std::ptr::NonNull;
    use std::sync::atomic::Ordering::SeqCst;
    use std::task::{Context, Poll, Waker};

    type WaitList = LinkedList<Waiter, <Waiter as linked_list::Link>::Target>;

    /// Notify a single task to wake up.
    ///
    /// `Notify` provides a basic mechanism to notify a single task of an event.
    /// `Notify` itself does not carry any data. Instead, it is to be used to signal
    /// another task to perform an operation.
    ///
    /// `Notify` can be thought of as a [`Semaphore`] starting with 0 permits.
    /// [`notified().await`] waits for a permit to become available, and [`notify_one()`]
    /// sets a permit **if there currently are no available permits**.
    ///
    /// The synchronization details of `Notify` are similar to
    /// [`thread::park`][park] and [`Thread::unpark`][unpark] from std. A [`Notify`]
    /// value contains a single permit. [`notified().await`] waits for the permit to
    /// be made available, consumes the permit, and resumes.  [`notify_one()`] sets the
    /// permit, waking a pending task if there is one.
    ///
    /// If `notify_one()` is called **before** `notified().await`, then the next call to
    /// `notified().await` will complete immediately, consuming the permit. Any
    /// subsequent calls to `notified().await` will wait for a new permit.
    ///
    /// If `notify_one()` is called **multiple** times before `notified().await`, only a
    /// **single** permit is stored. The next call to `notified().await` will
    /// complete immediately, but the one after will wait for a new permit.
    ///
    /// # Examples
    ///
    /// Basic usage.
    ///
    /// ```
    /// use tokio::sync::Notify;
    /// use std::sync::Arc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let notify = Arc::new(Notify::new());
    ///     let notify2 = notify.clone();
    ///
    ///     tokio::spawn(async move {
    ///         notify2.notified().await;
    ///         println!("received notification");
    ///     });
    ///
    ///     println!("sending notification");
    ///     notify.notify_one();
    /// }
    /// ```
    ///
    /// Unbound mpsc channel.
    ///
    /// ```
    /// use tokio::sync::Notify;
    ///
    /// use std::collections::VecDeque;
    /// use std::sync::Mutex;
    ///
    /// struct Channel<T> {
    ///     values: Mutex<VecDeque<T>>,
    ///     notify: Notify,
    /// }
    ///
    /// impl<T> Channel<T> {
    ///     pub fn send(&self, value: T) {
    ///         self.values.lock().unwrap()
    ///             .push_back(value);
    ///
    ///         // Notify the consumer a value is available
    ///         self.notify.notify_one();
    ///     }
    ///
    ///     pub async fn recv(&self) -> T {
    ///         loop {
    ///             // Drain values
    ///             if let Some(value) = self.values.lock().unwrap().pop_front() {
    ///                 return value;
    ///             }
    ///
    ///             // Wait for values to be available
    ///             self.notify.notified().await;
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// [park]: std::thread::park
    /// [unpark]: std::thread::Thread::unpark
    /// [`notified().await`]: Notify::notified()
    /// [`notify_one()`]: Notify::notify_one()
    /// [`Semaphore`]: crate::sync::Semaphore
    #[derive(Debug)]
    pub struct Notify {
        // This uses 2 bits to store one of `EMPTY`,
        // `WAITING` or `NOTIFIED`. The rest of the bits
        // are used to store the number of times `notify_waiters`
        // was called.
        state: AtomicUsize,
        waiters: Mutex<WaitList>,
    }

    #[derive(Debug, Clone, Copy)]
    enum NotificationType {
        // Notification triggered by calling `notify_waiters`
        AllWaiters,
        // Notification triggered by calling `notify_one`
        OneWaiter,
    }

    #[derive(Debug)]
    struct Waiter {
        /// Intrusive linked-list pointers
        pointers: linked_list::Pointers<Waiter>,

        /// Waiting task's waker
        waker: Option<Waker>,

        /// `true` if the notification has been assigned to this waiter.
        notified: Option<NotificationType>,

        /// Should not be `Unpin`.
        _p: PhantomPinned,
    }

    /// Future returned from `notified()`
    #[derive(Debug)]
    pub struct Notified<'a> {
        /// The `Notify` being received on.
        notify: &'a Notify,

        /// The current state of the receiving process.
        state: State,

        /// Entry in the waiter `LinkedList`.
        waiter: UnsafeCell<Waiter>,
    }

    unsafe impl<'a> Send for Notified<'a> {}
    unsafe impl<'a> Sync for Notified<'a> {}

    #[derive(Debug)]
    enum State {
        Init(usize),
        Waiting,
        Done,
    }

    const NOTIFY_WAITERS_SHIFT: usize = 2;
    const STATE_MASK: usize = (1 << NOTIFY_WAITERS_SHIFT) - 1;
    const NOTIFY_WAITERS_CALLS_MASK: usize = !STATE_MASK;

    /// Initial "idle" state
    const EMPTY: usize = 0;

    /// One or more threads are currently waiting to be notified.
    const WAITING: usize = 1;

    /// Pending notification
    const NOTIFIED: usize = 2;

    fn set_state(data: usize, state: usize) -> usize {
        (data & NOTIFY_WAITERS_CALLS_MASK) | (state & STATE_MASK)
    }

    fn get_state(data: usize) -> usize {
        data & STATE_MASK
    }

    fn get_num_notify_waiters_calls(data: usize) -> usize {
        (data & NOTIFY_WAITERS_CALLS_MASK) >> NOTIFY_WAITERS_SHIFT
    }

    fn inc_num_notify_waiters_calls(data: usize) -> usize {
        data + (1 << NOTIFY_WAITERS_SHIFT)
    }

    impl Notify {
        /// Create a new `Notify`, initialized without a permit.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Notify;
        ///
        /// let notify = Notify::new();
        /// ```
        pub fn new() -> Notify {
            Notify {
                state: AtomicUsize::new(0),
                waiters: Mutex::new(LinkedList::new()),
            }
        }

        /// Create a new `Notify`, initialized without a permit.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Notify;
        ///
        /// static NOTIFY: Notify = Notify::const_new();
        /// ```
        #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
        #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
        pub const fn const_new() -> Notify {
            Notify {
                state: AtomicUsize::new(0),
                waiters: Mutex::const_new(LinkedList::new()),
            }
        }

        /// Wait for a notification.
        ///
        /// Equivalent to:
        ///
        /// ```ignore
        /// async fn notified(&self);
        /// ```
        ///
        /// Each `Notify` value holds a single permit. If a permit is available from
        /// an earlier call to [`notify_one()`], then `notified().await` will complete
        /// immediately, consuming that permit. Otherwise, `notified().await` waits
        /// for a permit to be made available by the next call to `notify_one()`.
        ///
        /// [`notify_one()`]: Notify::notify_one
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Notify;
        /// use std::sync::Arc;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let notify = Arc::new(Notify::new());
        ///     let notify2 = notify.clone();
        ///
        ///     tokio::spawn(async move {
        ///         notify2.notified().await;
        ///         println!("received notification");
        ///     });
        ///
        ///     println!("sending notification");
        ///     notify.notify_one();
        /// }
        /// ```
        pub fn notified(&self) -> Notified<'_> {
            // we load the number of times notify_waiters
            // was called and store that in our initial state
            let state = self.state.load(SeqCst);
            Notified {
                notify: self,
                state: State::Init(state >> NOTIFY_WAITERS_SHIFT),
                waiter: UnsafeCell::new(Waiter {
                    pointers: linked_list::Pointers::new(),
                    waker: None,
                    notified: None,
                    _p: PhantomPinned,
                }),
            }
        }

        /// Notifies a waiting task
        ///
        /// If a task is currently waiting, that task is notified. Otherwise, a
        /// permit is stored in this `Notify` value and the **next** call to
        /// [`notified().await`] will complete immediately consuming the permit made
        /// available by this call to `notify_one()`.
        ///
        /// At most one permit may be stored by `Notify`. Many sequential calls to
        /// `notify_one` will result in a single permit being stored. The next call to
        /// `notified().await` will complete immediately, but the one after that
        /// will wait.
        ///
        /// [`notified().await`]: Notify::notified()
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Notify;
        /// use std::sync::Arc;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let notify = Arc::new(Notify::new());
        ///     let notify2 = notify.clone();
        ///
        ///     tokio::spawn(async move {
        ///         notify2.notified().await;
        ///         println!("received notification");
        ///     });
        ///
        ///     println!("sending notification");
        ///     notify.notify_one();
        /// }
        /// ```
        // Alias for old name in 0.x
        #[cfg_attr(docsrs, doc(alias = "notify"))]
        pub fn notify_one(&self) {
            // Load the current state
            let mut curr = self.state.load(SeqCst);

            // If the state is `EMPTY`, transition to `NOTIFIED` and return.
            while let EMPTY | NOTIFIED = get_state(curr) {
                // The compare-exchange from `NOTIFIED` -> `NOTIFIED` is intended. A
                // happens-before synchronization must happen between this atomic
                // operation and a task calling `notified().await`.
                let new = set_state(curr, NOTIFIED);
                let res = self.state.compare_exchange(curr, new, SeqCst, SeqCst);

                match res {
                    // No waiters, no further work to do
                    Ok(_) => return,
                    Err(actual) => {
                        curr = actual;
                    }
                }
            }

            // There are waiters, the lock must be acquired to notify.
            let mut waiters = self.waiters.lock();

            // The state must be reloaded while the lock is held. The state may only
            // transition out of WAITING while the lock is held.
            curr = self.state.load(SeqCst);

            if let Some(waker) = notify_locked(&mut waiters, &self.state, curr) {
                drop(waiters);
                waker.wake();
            }
        }

        /// Notifies all waiting tasks
        ///
        /// If a task is currently waiting, that task is notified. Unlike with
        /// `notify_one()`, no permit is stored to be used by the next call to
        /// `notified().await`. The purpose of this method is to notify all
        /// already registered waiters. Registering for notification is done by
        /// acquiring an instance of the `Notified` future via calling `notified()`.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Notify;
        /// use std::sync::Arc;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let notify = Arc::new(Notify::new());
        ///     let notify2 = notify.clone();
        ///
        ///     let notified1 = notify.notified();
        ///     let notified2 = notify.notified();
        ///
        ///     let handle = tokio::spawn(async move {
        ///         println!("sending notifications");
        ///         notify2.notify_waiters();
        ///     });
        ///
        ///     notified1.await;
        ///     notified2.await;
        ///     println!("received notifications");
        /// }
        /// ```
        pub fn notify_waiters(&self) {
            const NUM_WAKERS: usize = 32;

            let mut wakers: [Option<Waker>; NUM_WAKERS] = Default::default();
            let mut curr_waker = 0;

            // There are waiters, the lock must be acquired to notify.
            let mut waiters = self.waiters.lock();

            // The state must be reloaded while the lock is held. The state may only
            // transition out of WAITING while the lock is held.
            let curr = self.state.load(SeqCst);

            if let EMPTY | NOTIFIED = get_state(curr) {
                // There are no waiting tasks. In this case, no synchronization is
                // established between `notify` and `notified().await`.
                // All we need to do is increment the number of times this
                // method was called.
                self.state.store(inc_num_notify_waiters_calls(curr), SeqCst);
                return;
            }

            // At this point, it is guaranteed that the state will not
            // concurrently change, as holding the lock is required to
            // transition **out** of `WAITING`.
            'outer: loop {
                while curr_waker < NUM_WAKERS {
                    match waiters.pop_back() {
                        Some(mut waiter) => {
                            // Safety: `waiters` lock is still held.
                            let waiter = unsafe { waiter.as_mut() };

                            assert!(waiter.notified.is_none());

                            waiter.notified = Some(NotificationType::AllWaiters);

                            if let Some(waker) = waiter.waker.take() {
                                wakers[curr_waker] = Some(waker);
                                curr_waker += 1;
                            }
                        }
                        None => {
                            break 'outer;
                        }
                    }
                }

                drop(waiters);

                for waker in wakers.iter_mut().take(curr_waker) {
                    waker.take().unwrap().wake();
                }

                curr_waker = 0;

                // Acquire the lock again.
                waiters = self.waiters.lock();
            }

            // All waiters will be notified, the state must be transitioned to
            // `EMPTY`. As transitioning **from** `WAITING` requires the lock to be
            // held, a `store` is sufficient.
            let new = set_state(inc_num_notify_waiters_calls(curr), EMPTY);
            self.state.store(new, SeqCst);

            // Release the lock before notifying
            drop(waiters);

            for waker in wakers.iter_mut().take(curr_waker) {
                waker.take().unwrap().wake();
            }
        }
    }

    impl Default for Notify {
        fn default() -> Notify {
            Notify::new()
        }
    }

    fn notify_locked(waiters: &mut WaitList, state: &AtomicUsize, curr: usize) -> Option<Waker> {
        loop {
            match get_state(curr) {
                EMPTY | NOTIFIED => {
                    let res = state.compare_exchange(curr, set_state(curr, NOTIFIED), SeqCst, SeqCst);

                    match res {
                        Ok(_) => return None,
                        Err(actual) => {
                            let actual_state = get_state(actual);
                            assert!(actual_state == EMPTY || actual_state == NOTIFIED);
                            state.store(set_state(actual, NOTIFIED), SeqCst);
                            return None;
                        }
                    }
                }
                WAITING => {
                    // At this point, it is guaranteed that the state will not
                    // concurrently change as holding the lock is required to
                    // transition **out** of `WAITING`.
                    //
                    // Get a pending waiter
                    let mut waiter = waiters.pop_back().unwrap();

                    // Safety: `waiters` lock is still held.
                    let waiter = unsafe { waiter.as_mut() };

                    assert!(waiter.notified.is_none());

                    waiter.notified = Some(NotificationType::OneWaiter);
                    let waker = waiter.waker.take();

                    if waiters.is_empty() {
                        // As this the **final** waiter in the list, the state
                        // must be transitioned to `EMPTY`. As transitioning
                        // **from** `WAITING` requires the lock to be held, a
                        // `store` is sufficient.
                        state.store(set_state(curr, EMPTY), SeqCst);
                    }

                    return waker;
                }
                _ => unreachable!(),
            }
        }
    }

    // ===== impl Notified =====

    impl Notified<'_> {
        /// A custom `project` implementation is used in place of `pin-project-lite`
        /// as a custom drop implementation is needed.
        fn project(self: Pin<&mut Self>) -> (&Notify, &mut State, &UnsafeCell<Waiter>) {
            unsafe {
                // Safety: both `notify` and `state` are `Unpin`.

                is_unpin::<&Notify>();
                is_unpin::<AtomicUsize>();

                let me = self.get_unchecked_mut();
                (&me.notify, &mut me.state, &me.waiter)
            }
        }
    }

    impl Future for Notified<'_> {
        type Output = ();

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            use State::*;

            let (notify, state, waiter) = self.project();

            loop {
                match *state {
                    Init(initial_notify_waiters_calls) => {
                        let curr = notify.state.load(SeqCst);

                        // Optimistically try acquiring a pending notification
                        let res = notify.state.compare_exchange(
                            set_state(curr, NOTIFIED),
                            set_state(curr, EMPTY),
                            SeqCst,
                            SeqCst,
                        );

                        if res.is_ok() {
                            // Acquired the notification
                            *state = Done;
                            return Poll::Ready(());
                        }

                        // Acquire the lock and attempt to transition to the waiting
                        // state.
                        let mut waiters = notify.waiters.lock();

                        // Reload the state with the lock held
                        let mut curr = notify.state.load(SeqCst);

                        // if notify_waiters has been called after the future
                        // was created, then we are done
                        if get_num_notify_waiters_calls(curr) != initial_notify_waiters_calls {
                            *state = Done;
                            return Poll::Ready(());
                        }

                        // Transition the state to WAITING.
                        loop {
                            match get_state(curr) {
                                EMPTY => {
                                    // Transition to WAITING
                                    let res = notify.state.compare_exchange(
                                        set_state(curr, EMPTY),
                                        set_state(curr, WAITING),
                                        SeqCst,
                                        SeqCst,
                                    );

                                    if let Err(actual) = res {
                                        assert_eq!(get_state(actual), NOTIFIED);
                                        curr = actual;
                                    } else {
                                        break;
                                    }
                                }
                                WAITING => break,
                                NOTIFIED => {
                                    // Try consuming the notification
                                    let res = notify.state.compare_exchange(
                                        set_state(curr, NOTIFIED),
                                        set_state(curr, EMPTY),
                                        SeqCst,
                                        SeqCst,
                                    );

                                    match res {
                                        Ok(_) => {
                                            // Acquired the notification
                                            *state = Done;
                                            return Poll::Ready(());
                                        }
                                        Err(actual) => {
                                            assert_eq!(get_state(actual), EMPTY);
                                            curr = actual;
                                        }
                                    }
                                }
                                _ => unreachable!(),
                            }
                        }

                        // Safety: called while locked.
                        unsafe {
                            (*waiter.get()).waker = Some(cx.waker().clone());
                        }

                        // Insert the waiter into the linked list
                        //
                        // safety: pointers from `UnsafeCell` are never null.
                        waiters.push_front(unsafe { NonNull::new_unchecked(waiter.get()) });

                        *state = Waiting;

                        return Poll::Pending;
                    }
                    Waiting => {
                        // Currently in the "Waiting" state, implying the caller has
                        // a waiter stored in the waiter list (guarded by
                        // `notify.waiters`). In order to access the waker fields,
                        // we must hold the lock.

                        let waiters = notify.waiters.lock();

                        // Safety: called while locked
                        let w = unsafe { &mut *waiter.get() };

                        if w.notified.is_some() {
                            // Our waker has been notified. Reset the fields and
                            // remove it from the list.
                            w.waker = None;
                            w.notified = None;

                            *state = Done;
                        } else {
                            // Update the waker, if necessary.
                            if !w.waker.as_ref().unwrap().will_wake(cx.waker()) {
                                w.waker = Some(cx.waker().clone());
                            }

                            return Poll::Pending;
                        }

                        // Explicit drop of the lock to indicate the scope that the
                        // lock is held. Because holding the lock is required to
                        // ensure safe access to fields not held within the lock, it
                        // is helpful to visualize the scope of the critical
                        // section.
                        drop(waiters);
                    }
                    Done => {
                        return Poll::Ready(());
                    }
                }
            }
        }
    }

    impl Drop for Notified<'_> {
        fn drop(&mut self) {
            use State::*;

            // Safety: The type only transitions to a "Waiting" state when pinned.
            let (notify, state, waiter) = unsafe { Pin::new_unchecked(self).project() };

            // This is where we ensure safety. The `Notified` value is being
            // dropped, which means we must ensure that the waiter entry is no
            // longer stored in the linked list.
            if let Waiting = *state {
                let mut waiters = notify.waiters.lock();
                let mut notify_state = notify.state.load(SeqCst);

                // `Notify.state` may be in any of the three states (Empty, Waiting,
                // Notified). It doesn't actually matter what the atomic is set to
                // at this point. We hold the lock and will ensure the atomic is in
                // the correct state once the lock is dropped.
                //
                // Because the atomic state is not checked, at first glance, it may
                // seem like this routine does not handle the case where the
                // receiver is notified but has not yet observed the notification.
                // If this happens, no matter how many notifications happen between
                // this receiver being notified and the receive future dropping, all
                // we need to do is ensure that one notification is returned back to
                // the `Notify`. This is done by calling `notify_locked` if `self`
                // has the `notified` flag set.

                // remove the entry from the list
                //
                // safety: the waiter is only added to `waiters` by virtue of it
                // being the only `LinkedList` available to the type.
                unsafe { waiters.remove(NonNull::new_unchecked(waiter.get())) };

                if waiters.is_empty() {
                    notify_state = set_state(notify_state, EMPTY);
                    // If the state *should* be `NOTIFIED`, the call to
                    // `notify_locked` below will end up doing the
                    // `store(NOTIFIED)`. If a concurrent receiver races and
                    // observes the incorrect `EMPTY` state, it will then obtain the
                    // lock and block until `notify.state` is in the correct final
                    // state.
                    notify.state.store(notify_state, SeqCst);
                }

                // See if the node was notified but not received. In this case, if
                // the notification was triggered via `notify_one`, it must be sent
                // to the next waiter.
                //
                // Safety: with the entry removed from the linked list, there can be
                // no concurrent access to the entry
                if let Some(NotificationType::OneWaiter) = unsafe { (*waiter.get()).notified } {
                    if let Some(waker) = notify_locked(&mut waiters, &notify.state, notify_state) {
                        drop(waiters);
                        waker.wake();
                    }
                }
            }
        }
    }

    /// # Safety
    ///
    /// `Waiter` is forced to be !Unpin.
    unsafe impl linked_list::Link for Waiter {
        type Handle = NonNull<Waiter>;
        type Target = Waiter;

        fn as_raw(handle: &NonNull<Waiter>) -> NonNull<Waiter> {
            *handle
        }

        unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
            ptr
        }

        unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
            NonNull::from(&mut target.as_mut().pointers)
        }
    }

    fn is_unpin<T: Unpin>() {}