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    // Copyright 2016 Amanieu d'Antras
    //
    // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
    // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
    // http://opensource.org/licenses/MIT>, at your option. This file may not be
    // copied, modified, or distributed except according to those terms.
    use crate::thread_parker::{ThreadParker, ThreadParkerT, UnparkHandleT};
    use crate::util::UncheckedOptionExt;
    use crate::word_lock::WordLock;
    use core::{
        cell::{Cell, UnsafeCell},
        ptr,
        sync::atomic::{AtomicPtr, AtomicUsize, Ordering},
    };
    use instant::Instant;
    use smallvec::SmallVec;
    use std::time::Duration;

    static NUM_THREADS: AtomicUsize = AtomicUsize::new(0);

    /// Holds the pointer to the currently active `HashTable`.
    ///
    /// # Safety
    ///
    /// Except for the initial value of null, it must always point to a valid `HashTable` instance.
    /// Any `HashTable` this global static has ever pointed to must never be freed.
    static HASHTABLE: AtomicPtr<HashTable> = AtomicPtr::new(ptr::null_mut());

    // Even with 3x more buckets than threads, the memory overhead per thread is
    // still only a few hundred bytes per thread.
    const LOAD_FACTOR: usize = 3;

    struct HashTable {
        // Hash buckets for the table
        entries: Box<[Bucket]>,

        // Number of bits used for the hash function
        hash_bits: u32,

        // Previous table. This is only kept to keep leak detectors happy.
        _prev: *const HashTable,
    }

    impl HashTable {
        #[inline]
        fn new(num_threads: usize, prev: *const HashTable) -> Box<HashTable> {
            let new_size = (num_threads * LOAD_FACTOR).next_power_of_two();
            let hash_bits = 0usize.leading_zeros() - new_size.leading_zeros() - 1;

            let now = Instant::now();
            let mut entries = Vec::with_capacity(new_size);
            for i in 0..new_size {
                // We must ensure the seed is not zero
                entries.push(Bucket::new(now, i as u32 + 1));
            }

            Box::new(HashTable {
                entries: entries.into_boxed_slice(),
                hash_bits,
                _prev: prev,
            })
        }
    }

    #[repr(align(64))]
    struct Bucket {
        // Lock protecting the queue
        mutex: WordLock,

        // Linked list of threads waiting on this bucket
        queue_head: Cell<*const ThreadData>,
        queue_tail: Cell<*const ThreadData>,

        // Next time at which point be_fair should be set
        fair_timeout: UnsafeCell<FairTimeout>,
    }

    impl Bucket {
        #[inline]
        pub fn new(timeout: Instant, seed: u32) -> Self {
            Self {
                mutex: WordLock::new(),
                queue_head: Cell::new(ptr::null()),
                queue_tail: Cell::new(ptr::null()),
                fair_timeout: UnsafeCell::new(FairTimeout::new(timeout, seed)),
            }
        }
    }

    struct FairTimeout {
        // Next time at which point be_fair should be set
        timeout: Instant,

        // the PRNG state for calculating the next timeout
        seed: u32,
    }

    impl FairTimeout {
        #[inline]
        fn new(timeout: Instant, seed: u32) -> FairTimeout {
            FairTimeout { timeout, seed }
        }

        // Determine whether we should force a fair unlock, and update the timeout
        #[inline]
        fn should_timeout(&mut self) -> bool {
            let now = Instant::now();
            if now > self.timeout {
                // Time between 0 and 1ms.
                let nanos = self.gen_u32() % 1_000_000;
                self.timeout = now + Duration::new(0, nanos);
                true
            } else {
                false
            }
        }

        // Pseudorandom number generator from the "Xorshift RNGs" paper by George Marsaglia.
        fn gen_u32(&mut self) -> u32 {
            self.seed ^= self.seed << 13;
            self.seed ^= self.seed >> 17;
            self.seed ^= self.seed << 5;
            self.seed
        }
    }

    struct ThreadData {
        parker: ThreadParker,

        // Key that this thread is sleeping on. This may change if the thread is
        // requeued to a different key.
        key: AtomicUsize,

        // Linked list of parked threads in a bucket
        next_in_queue: Cell<*const ThreadData>,

        // UnparkToken passed to this thread when it is unparked
        unpark_token: Cell<UnparkToken>,

        // ParkToken value set by the thread when it was parked
        park_token: Cell<ParkToken>,

        // Is the thread parked with a timeout?
        parked_with_timeout: Cell<bool>,

        // Extra data for deadlock detection
        #[cfg(feature = "deadlock_detection")]
        deadlock_data: deadlock::DeadlockData,
    }

    impl ThreadData {
        fn new() -> ThreadData {
            // Keep track of the total number of live ThreadData objects and resize
            // the hash table accordingly.
            let num_threads = NUM_THREADS.fetch_add(1, Ordering::Relaxed) + 1;
            grow_hashtable(num_threads);

            ThreadData {
                parker: ThreadParker::new(),
                key: AtomicUsize::new(0),
                next_in_queue: Cell::new(ptr::null()),
                unpark_token: Cell::new(DEFAULT_UNPARK_TOKEN),
                park_token: Cell::new(DEFAULT_PARK_TOKEN),
                parked_with_timeout: Cell::new(false),
                #[cfg(feature = "deadlock_detection")]
                deadlock_data: deadlock::DeadlockData::new(),
            }
        }
    }

    // Invokes the given closure with a reference to the current thread `ThreadData`.
    #[inline(always)]
    fn with_thread_data<T>(f: impl FnOnce(&ThreadData) -> T) -> T {
        // Unlike word_lock::ThreadData, parking_lot::ThreadData is always expensive
        // to construct. Try to use a thread-local version if possible. Otherwise just
        // create a ThreadData on the stack
        let mut thread_data_storage = None;
        thread_local!(static THREAD_DATA: ThreadData = ThreadData::new());
        let thread_data_ptr = THREAD_DATA
            .try_with(|x| x as *const ThreadData)
            .unwrap_or_else(|_| thread_data_storage.get_or_insert_with(ThreadData::new));

        f(unsafe { &*thread_data_ptr })
    }

    impl Drop for ThreadData {
        fn drop(&mut self) {
            NUM_THREADS.fetch_sub(1, Ordering::Relaxed);
        }
    }

    /// Returns a reference to the latest hash table, creating one if it doesn't exist yet.
    /// The reference is valid forever. However, the `HashTable` it references might become stale
    /// at any point. Meaning it still exists, but it is not the instance in active use.
    #[inline]
    fn get_hashtable() -> &'static HashTable {
        let table = HASHTABLE.load(Ordering::Acquire);

        // If there is no table, create one
        if table.is_null() {
            create_hashtable()
        } else {
            // SAFETY: when not null, `HASHTABLE` always points to a `HashTable` that is never freed.
            unsafe { &*table }
        }
    }

    /// Returns a reference to the latest hash table, creating one if it doesn't exist yet.
    /// The reference is valid forever. However, the `HashTable` it references might become stale
    /// at any point. Meaning it still exists, but it is not the instance in active use.
    #[cold]
    fn create_hashtable() -> &'static HashTable {
        let new_table = Box::into_raw(HashTable::new(LOAD_FACTOR, ptr::null()));

        // If this fails then it means some other thread created the hash table first.
        let table = match HASHTABLE.compare_exchange(
            ptr::null_mut(),
            new_table,
            Ordering::AcqRel,
            Ordering::Acquire,
        ) {
            Ok(_) => new_table,
            Err(old_table) => {
                // Free the table we created
                // SAFETY: `new_table` is created from `Box::into_raw` above and only freed here.
                unsafe {
                    Box::from_raw(new_table);
                }
                old_table
            }
        };
        // SAFETY: The `HashTable` behind `table` is never freed. It is either the table pointer we
        // created here, or it is one loaded from `HASHTABLE`.
        unsafe { &*table }
    }

    // Grow the hash table so that it is big enough for the given number of threads.
    // This isn't performance-critical since it is only done when a ThreadData is
    // created, which only happens once per thread.
    fn grow_hashtable(num_threads: usize) {
        // Lock all buckets in the existing table and get a reference to it
        let old_table = loop {
            let table = get_hashtable();

            // Check if we need to resize the existing table
            if table.entries.len() >= LOAD_FACTOR * num_threads {
                return;
            }

            // Lock all buckets in the old table
            for bucket in &table.entries[..] {
                bucket.mutex.lock();
            }

            // Now check if our table is still the latest one. Another thread could
            // have grown the hash table between us reading HASHTABLE and locking
            // the buckets.
            if HASHTABLE.load(Ordering::Relaxed) == table as *const _ as *mut _ {
                break table;
            }

            // Unlock buckets and try again
            for bucket in &table.entries[..] {
                // SAFETY: We hold the lock here, as required
                unsafe { bucket.mutex.unlock() };
            }
        };

        // Create the new table
        let mut new_table = HashTable::new(num_threads, old_table);

        // Move the entries from the old table to the new one
        for bucket in &old_table.entries[..] {
            // SAFETY: The park, unpark* and check_wait_graph_fast functions create only correct linked
            // lists. All `ThreadData` instances in these lists will remain valid as long as they are
            // present in the lists, meaning as long as their threads are parked.
            unsafe { rehash_bucket_into(bucket, &mut new_table) };
        }

        // Publish the new table. No races are possible at this point because
        // any other thread trying to grow the hash table is blocked on the bucket
        // locks in the old table.
        HASHTABLE.store(Box::into_raw(new_table), Ordering::Release);

        // Unlock all buckets in the old table
        for bucket in &old_table.entries[..] {
            // SAFETY: We hold the lock here, as required
            unsafe { bucket.mutex.unlock() };
        }
    }

    /// Iterate through all `ThreadData` objects in the bucket and insert them into the given table
    /// in the bucket their key correspond to for this table.
    ///
    /// # Safety
    ///
    /// The given `bucket` must have a correctly constructed linked list under `queue_head`, containing
    /// `ThreadData` instances that must stay valid at least as long as the given `table` is in use.
    ///
    /// The given `table` must only contain buckets with correctly constructed linked lists.
    unsafe fn rehash_bucket_into(bucket: &'static Bucket, table: &mut HashTable) {
        let mut current: *const ThreadData = bucket.queue_head.get();
        while !current.is_null() {
            let next = (*current).next_in_queue.get();
            let hash = hash((*current).key.load(Ordering::Relaxed), table.hash_bits);
            if table.entries[hash].queue_tail.get().is_null() {
                table.entries[hash].queue_head.set(current);
            } else {
                (*table.entries[hash].queue_tail.get())
                    .next_in_queue
                    .set(current);
            }
            table.entries[hash].queue_tail.set(current);
            (*current).next_in_queue.set(ptr::null());
            current = next;
        }
    }

    // Hash function for addresses
    #[cfg(target_pointer_width = "32")]
    #[inline]
    fn hash(key: usize, bits: u32) -> usize {
        key.wrapping_mul(0x9E3779B9) >> (32 - bits)
    }
    #[cfg(target_pointer_width = "64")]
    #[inline]
    fn hash(key: usize, bits: u32) -> usize {
        key.wrapping_mul(0x9E3779B97F4A7C15) >> (64 - bits)
    }

    /// Locks the bucket for the given key and returns a reference to it.
    /// The returned bucket must be unlocked again in order to not cause deadlocks.
    #[inline]
    fn lock_bucket(key: usize) -> &'static Bucket {
        loop {
            let hashtable = get_hashtable();

            let hash = hash(key, hashtable.hash_bits);
            let bucket = &hashtable.entries[hash];

            // Lock the bucket
            bucket.mutex.lock();

            // If no other thread has rehashed the table before we grabbed the lock
            // then we are good to go! The lock we grabbed prevents any rehashes.
            if HASHTABLE.load(Ordering::Relaxed) == hashtable as *const _ as *mut _ {
                return bucket;
            }

            // Unlock the bucket and try again
            // SAFETY: We hold the lock here, as required
            unsafe { bucket.mutex.unlock() };
        }
    }

    /// Locks the bucket for the given key and returns a reference to it. But checks that the key
    /// hasn't been changed in the meantime due to a requeue.
    /// The returned bucket must be unlocked again in order to not cause deadlocks.
    #[inline]
    fn lock_bucket_checked(key: &AtomicUsize) -> (usize, &'static Bucket) {
        loop {
            let hashtable = get_hashtable();
            let current_key = key.load(Ordering::Relaxed);

            let hash = hash(current_key, hashtable.hash_bits);
            let bucket = &hashtable.entries[hash];

            // Lock the bucket
            bucket.mutex.lock();

            // Check that both the hash table and key are correct while the bucket
            // is locked. Note that the key can't change once we locked the proper
            // bucket for it, so we just keep trying until we have the correct key.
            if HASHTABLE.load(Ordering::Relaxed) == hashtable as *const _ as *mut _
                && key.load(Ordering::Relaxed) == current_key
            {
                return (current_key, bucket);
            }

            // Unlock the bucket and try again
            // SAFETY: We hold the lock here, as required
            unsafe { bucket.mutex.unlock() };
        }
    }

    /// Locks the two buckets for the given pair of keys and returns references to them.
    /// The returned buckets must be unlocked again in order to not cause deadlocks.
    ///
    /// If both keys hash to the same value, both returned references will be to the same bucket. Be
    /// careful to only unlock it once in this case, always use `unlock_bucket_pair`.
    #[inline]
    fn lock_bucket_pair(key1: usize, key2: usize) -> (&'static Bucket, &'static Bucket) {
        loop {
            let hashtable = get_hashtable();

            let hash1 = hash(key1, hashtable.hash_bits);
            let hash2 = hash(key2, hashtable.hash_bits);

            // Get the bucket at the lowest hash/index first
            let bucket1 = if hash1 <= hash2 {
                &hashtable.entries[hash1]
            } else {
                &hashtable.entries[hash2]
            };

            // Lock the first bucket
            bucket1.mutex.lock();

            // If no other thread has rehashed the table before we grabbed the lock
            // then we are good to go! The lock we grabbed prevents any rehashes.
            if HASHTABLE.load(Ordering::Relaxed) == hashtable as *const _ as *mut _ {
                // Now lock the second bucket and return the two buckets
                if hash1 == hash2 {
                    return (bucket1, bucket1);
                } else if hash1 < hash2 {
                    let bucket2 = &hashtable.entries[hash2];
                    bucket2.mutex.lock();
                    return (bucket1, bucket2);
                } else {
                    let bucket2 = &hashtable.entries[hash1];
                    bucket2.mutex.lock();
                    return (bucket2, bucket1);
                }
            }

            // Unlock the bucket and try again
            // SAFETY: We hold the lock here, as required
            unsafe { bucket1.mutex.unlock() };
        }
    }

    /// Unlock a pair of buckets
    ///
    /// # Safety
    ///
    /// Both buckets must be locked
    #[inline]
    unsafe fn unlock_bucket_pair(bucket1: &Bucket, bucket2: &Bucket) {
        bucket1.mutex.unlock();
        if !ptr::eq(bucket1, bucket2) {
            bucket2.mutex.unlock();
        }
    }

    /// Result of a park operation.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub enum ParkResult {
        /// We were unparked by another thread with the given token.
        Unparked(UnparkToken),

        /// The validation callback returned false.
        Invalid,

        /// The timeout expired.
        TimedOut,
    }

    impl ParkResult {
        /// Returns true if we were unparked by another thread.
        #[inline]
        pub fn is_unparked(self) -> bool {
            if let ParkResult::Unparked(_) = self {
                true
            } else {
                false
            }
        }
    }

    /// Result of an unpark operation.
    #[derive(Copy, Clone, Default, Eq, PartialEq, Debug)]
    pub struct UnparkResult {
        /// The number of threads that were unparked.
        pub unparked_threads: usize,

        /// The number of threads that were requeued.
        pub requeued_threads: usize,

        /// Whether there are any threads remaining in the queue. This only returns
        /// true if a thread was unparked.
        pub have_more_threads: bool,

        /// This is set to true on average once every 0.5ms for any given key. It
        /// should be used to switch to a fair unlocking mechanism for a particular
        /// unlock.
        pub be_fair: bool,

        /// Private field so new fields can be added without breakage.
        _sealed: (),
    }

    /// Operation that `unpark_requeue` should perform.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub enum RequeueOp {
        /// Abort the operation without doing anything.
        Abort,

        /// Unpark one thread and requeue the rest onto the target queue.
        UnparkOneRequeueRest,

        /// Requeue all threads onto the target queue.
        RequeueAll,

        /// Unpark one thread and leave the rest parked. No requeuing is done.
        UnparkOne,

        /// Requeue one thread and leave the rest parked on the original queue.
        RequeueOne,
    }

    /// Operation that `unpark_filter` should perform for each thread.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub enum FilterOp {
        /// Unpark the thread and continue scanning the list of parked threads.
        Unpark,

        /// Don't unpark the thread and continue scanning the list of parked threads.
        Skip,

        /// Don't unpark the thread and stop scanning the list of parked threads.
        Stop,
    }

    /// A value which is passed from an unparker to a parked thread.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub struct UnparkToken(pub usize);

    /// A value associated with a parked thread which can be used by `unpark_filter`.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub struct ParkToken(pub usize);

    /// A default unpark token to use.
    pub const DEFAULT_UNPARK_TOKEN: UnparkToken = UnparkToken(0);

    /// A default park token to use.
    pub const DEFAULT_PARK_TOKEN: ParkToken = ParkToken(0);

    /// Parks the current thread in the queue associated with the given key.
    ///
    /// The `validate` function is called while the queue is locked and can abort
    /// the operation by returning false. If `validate` returns true then the
    /// current thread is appended to the queue and the queue is unlocked.
    ///
    /// The `before_sleep` function is called after the queue is unlocked but before
    /// the thread is put to sleep. The thread will then sleep until it is unparked
    /// or the given timeout is reached.
    ///
    /// The `timed_out` function is also called while the queue is locked, but only
    /// if the timeout was reached. It is passed the key of the queue it was in when
    /// it timed out, which may be different from the original key if
    /// `unpark_requeue` was called. It is also passed a bool which indicates
    /// whether it was the last thread in the queue.
    ///
    /// # Safety
    ///
    /// You should only call this function with an address that you control, since
    /// you could otherwise interfere with the operation of other synchronization
    /// primitives.
    ///
    /// The `validate` and `timed_out` functions are called while the queue is
    /// locked and must not panic or call into any function in `parking_lot`.
    ///
    /// The `before_sleep` function is called outside the queue lock and is allowed
    /// to call `unpark_one`, `unpark_all`, `unpark_requeue` or `unpark_filter`, but
    /// it is not allowed to call `park` or panic.
    #[inline]
    pub unsafe fn park(
        key: usize,
        validate: impl FnOnce() -> bool,
        before_sleep: impl FnOnce(),
        timed_out: impl FnOnce(usize, bool),
        park_token: ParkToken,
        timeout: Option<Instant>,
    ) -> ParkResult {
        // Grab our thread data, this also ensures that the hash table exists
        with_thread_data(|thread_data| {
            // Lock the bucket for the given key
            let bucket = lock_bucket(key);

            // If the validation function fails, just return
            if !validate() {
                // SAFETY: We hold the lock here, as required
                bucket.mutex.unlock();
                return ParkResult::Invalid;
            }

            // Append our thread data to the queue and unlock the bucket
            thread_data.parked_with_timeout.set(timeout.is_some());
            thread_data.next_in_queue.set(ptr::null());
            thread_data.key.store(key, Ordering::Relaxed);
            thread_data.park_token.set(park_token);
            thread_data.parker.prepare_park();
            if !bucket.queue_head.get().is_null() {
                (*bucket.queue_tail.get()).next_in_queue.set(thread_data);
            } else {
                bucket.queue_head.set(thread_data);
            }
            bucket.queue_tail.set(thread_data);
            // SAFETY: We hold the lock here, as required
            bucket.mutex.unlock();

            // Invoke the pre-sleep callback
            before_sleep();

            // Park our thread and determine whether we were woken up by an unpark
            // or by our timeout. Note that this isn't precise: we can still be
            // unparked since we are still in the queue.
            let unparked = match timeout {
                Some(timeout) => thread_data.parker.park_until(timeout),
                None => {
                    thread_data.parker.park();
                    // call deadlock detection on_unpark hook
                    deadlock::on_unpark(thread_data);
                    true
                }
            };

            // If we were unparked, return now
            if unparked {
                return ParkResult::Unparked(thread_data.unpark_token.get());
            }

            // Lock our bucket again. Note that the hashtable may have been rehashed in
            // the meantime. Our key may also have changed if we were requeued.
            let (key, bucket) = lock_bucket_checked(&thread_data.key);

            // Now we need to check again if we were unparked or timed out. Unlike the
            // last check this is precise because we hold the bucket lock.
            if !thread_data.parker.timed_out() {
                // SAFETY: We hold the lock here, as required
                bucket.mutex.unlock();
                return ParkResult::Unparked(thread_data.unpark_token.get());
            }

            // We timed out, so we now need to remove our thread from the queue
            let mut link = &bucket.queue_head;
            let mut current = bucket.queue_head.get();
            let mut previous = ptr::null();
            let mut was_last_thread = true;
            while !current.is_null() {
                if current == thread_data {
                    let next = (*current).next_in_queue.get();
                    link.set(next);
                    if bucket.queue_tail.get() == current {
                        bucket.queue_tail.set(previous);
                    } else {
                        // Scan the rest of the queue to see if there are any other
                        // entries with the given key.
                        let mut scan = next;
                        while !scan.is_null() {
                            if (*scan).key.load(Ordering::Relaxed) == key {
                                was_last_thread = false;
                                break;
                            }
                            scan = (*scan).next_in_queue.get();
                        }
                    }

                    // Callback to indicate that we timed out, and whether we were the
                    // last thread on the queue.
                    timed_out(key, was_last_thread);
                    break;
                } else {
                    if (*current).key.load(Ordering::Relaxed) == key {
                        was_last_thread = false;
                    }
                    link = &(*current).next_in_queue;
                    previous = current;
                    current = link.get();
                }
            }

            // There should be no way for our thread to have been removed from the queue
            // if we timed out.
            debug_assert!(!current.is_null());

            // Unlock the bucket, we are done
            // SAFETY: We hold the lock here, as required
            bucket.mutex.unlock();
            ParkResult::TimedOut
        })
    }

    /// Unparks one thread from the queue associated with the given key.
    ///
    /// The `callback` function is called while the queue is locked and before the
    /// target thread is woken up. The `UnparkResult` argument to the function
    /// indicates whether a thread was found in the queue and whether this was the
    /// last thread in the queue. This value is also returned by `unpark_one`.
    ///
    /// The `callback` function should return an `UnparkToken` value which will be
    /// passed to the thread that is unparked. If no thread is unparked then the
    /// returned value is ignored.
    ///
    /// # Safety
    ///
    /// You should only call this function with an address that you control, since
    /// you could otherwise interfere with the operation of other synchronization
    /// primitives.
    ///
    /// The `callback` function is called while the queue is locked and must not
    /// panic or call into any function in `parking_lot`.
    #[inline]
    pub unsafe fn unpark_one(
        key: usize,
        callback: impl FnOnce(UnparkResult) -> UnparkToken,
    ) -> UnparkResult {
        // Lock the bucket for the given key
        let bucket = lock_bucket(key);

        // Find a thread with a matching key and remove it from the queue
        let mut link = &bucket.queue_head;
        let mut current = bucket.queue_head.get();
        let mut previous = ptr::null();
        let mut result = UnparkResult::default();
        while !current.is_null() {
            if (*current).key.load(Ordering::Relaxed) == key {
                // Remove the thread from the queue
                let next = (*current).next_in_queue.get();
                link.set(next);
                if bucket.queue_tail.get() == current {
                    bucket.queue_tail.set(previous);
                } else {
                    // Scan the rest of the queue to see if there are any other
                    // entries with the given key.
                    let mut scan = next;
                    while !scan.is_null() {
                        if (*scan).key.load(Ordering::Relaxed) == key {
                            result.have_more_threads = true;
                            break;
                        }
                        scan = (*scan).next_in_queue.get();
                    }
                }

                // Invoke the callback before waking up the thread
                result.unparked_threads = 1;
                result.be_fair = (*bucket.fair_timeout.get()).should_timeout();
                let token = callback(result);

                // Set the token for the target thread
                (*current).unpark_token.set(token);

                // This is a bit tricky: we first lock the ThreadParker to prevent
                // the thread from exiting and freeing its ThreadData if its wait
                // times out. Then we unlock the queue since we don't want to keep
                // the queue locked while we perform a system call. Finally we wake
                // up the parked thread.
                let handle = (*current).parker.unpark_lock();
                // SAFETY: We hold the lock here, as required
                bucket.mutex.unlock();
                handle.unpark();

                return result;
            } else {
                link = &(*current).next_in_queue;
                previous = current;
                current = link.get();
            }
        }

        // No threads with a matching key were found in the bucket
        callback(result);
        // SAFETY: We hold the lock here, as required
        bucket.mutex.unlock();
        result
    }

    /// Unparks all threads in the queue associated with the given key.
    ///
    /// The given `UnparkToken` is passed to all unparked threads.
    ///
    /// This function returns the number of threads that were unparked.
    ///
    /// # Safety
    ///
    /// You should only call this function with an address that you control, since
    /// you could otherwise interfere with the operation of other synchronization
    /// primitives.
    #[inline]
    pub unsafe fn unpark_all(key: usize, unpark_token: UnparkToken) -> usize {
        // Lock the bucket for the given key
        let bucket = lock_bucket(key);

        // Remove all threads with the given key in the bucket
        let mut link = &bucket.queue_head;
        let mut current = bucket.queue_head.get();
        let mut previous = ptr::null();
        let mut threads = SmallVec::<[_; 8]>::new();
        while !current.is_null() {
            if (*current).key.load(Ordering::Relaxed) == key {
                // Remove the thread from the queue
                let next = (*current).next_in_queue.get();
                link.set(next);
                if bucket.queue_tail.get() == current {
                    bucket.queue_tail.set(previous);
                }

                // Set the token for the target thread
                (*current).unpark_token.set(unpark_token);

                // Don't wake up threads while holding the queue lock. See comment
                // in unpark_one. For now just record which threads we need to wake
                // up.
                threads.push((*current).parker.unpark_lock());
                current = next;
            } else {
                link = &(*current).next_in_queue;
                previous = current;
                current = link.get();
            }
        }

        // Unlock the bucket
        // SAFETY: We hold the lock here, as required
        bucket.mutex.unlock();

        // Now that we are outside the lock, wake up all the threads that we removed
        // from the queue.
        let num_threads = threads.len();
        for handle in threads.into_iter() {
            handle.unpark();
        }

        num_threads
    }

    /// Removes all threads from the queue associated with `key_from`, optionally
    /// unparks the first one and requeues the rest onto the queue associated with
    /// `key_to`.
    ///
    /// The `validate` function is called while both queues are locked. Its return
    /// value will determine which operation is performed, or whether the operation
    /// should be aborted. See `RequeueOp` for details about the different possible
    /// return values.
    ///
    /// The `callback` function is also called while both queues are locked. It is
    /// passed the `RequeueOp` returned by `validate` and an `UnparkResult`
    /// indicating whether a thread was unparked and whether there are threads still
    /// parked in the new queue. This `UnparkResult` value is also returned by
    /// `unpark_requeue`.
    ///
    /// The `callback` function should return an `UnparkToken` value which will be
    /// passed to the thread that is unparked. If no thread is unparked then the
    /// returned value is ignored.
    ///
    /// # Safety
    ///
    /// You should only call this function with an address that you control, since
    /// you could otherwise interfere with the operation of other synchronization
    /// primitives.
    ///
    /// The `validate` and `callback` functions are called while the queue is locked
    /// and must not panic or call into any function in `parking_lot`.
    #[inline]
    pub unsafe fn unpark_requeue(
        key_from: usize,
        key_to: usize,
        validate: impl FnOnce() -> RequeueOp,
        callback: impl FnOnce(RequeueOp, UnparkResult) -> UnparkToken,
    ) -> UnparkResult {
        // Lock the two buckets for the given key
        let (bucket_from, bucket_to) = lock_bucket_pair(key_from, key_to);

        // If the validation function fails, just return
        let mut result = UnparkResult::default();
        let op = validate();
        if op == RequeueOp::Abort {
            // SAFETY: Both buckets are locked, as required.
            unlock_bucket_pair(bucket_from, bucket_to);
            return result;
        }

        // Remove all threads with the given key in the source bucket
        let mut link = &bucket_from.queue_head;
        let mut current = bucket_from.queue_head.get();
        let mut previous = ptr::null();
        let mut requeue_threads: *const ThreadData = ptr::null();
        let mut requeue_threads_tail: *const ThreadData = ptr::null();
        let mut wakeup_thread = None;
        while !current.is_null() {
            if (*current).key.load(Ordering::Relaxed) == key_from {
                // Remove the thread from the queue
                let next = (*current).next_in_queue.get();
                link.set(next);
                if bucket_from.queue_tail.get() == current {
                    bucket_from.queue_tail.set(previous);
                }

                // Prepare the first thread for wakeup and requeue the rest.
                if (op == RequeueOp::UnparkOneRequeueRest || op == RequeueOp::UnparkOne)
                    && wakeup_thread.is_none()
                {
                    wakeup_thread = Some(current);
                    result.unparked_threads = 1;
                } else {
                    if !requeue_threads.is_null() {
                        (*requeue_threads_tail).next_in_queue.set(current);
                    } else {
                        requeue_threads = current;
                    }
                    requeue_threads_tail = current;
                    (*current).key.store(key_to, Ordering::Relaxed);
                    result.requeued_threads += 1;
                }
                if op == RequeueOp::UnparkOne || op == RequeueOp::RequeueOne {
                    // Scan the rest of the queue to see if there are any other
                    // entries with the given key.
                    let mut scan = next;
                    while !scan.is_null() {
                        if (*scan).key.load(Ordering::Relaxed) == key_from {
                            result.have_more_threads = true;
                            break;
                        }
                        scan = (*scan).next_in_queue.get();
                    }
                    break;
                }
                current = next;
            } else {
                link = &(*current).next_in_queue;
                previous = current;
                current = link.get();
            }
        }

        // Add the requeued threads to the destination bucket
        if !requeue_threads.is_null() {
            (*requeue_threads_tail).next_in_queue.set(ptr::null());
            if !bucket_to.queue_head.get().is_null() {
                (*bucket_to.queue_tail.get())
                    .next_in_queue
                    .set(requeue_threads);
            } else {
                bucket_to.queue_head.set(requeue_threads);
            }
            bucket_to.queue_tail.set(requeue_threads_tail);
        }

        // Invoke the callback before waking up the thread
        if result.unparked_threads != 0 {
            result.be_fair = (*bucket_from.fair_timeout.get()).should_timeout();
        }
        let token = callback(op, result);

        // See comment in unpark_one for why we mess with the locking
        if let Some(wakeup_thread) = wakeup_thread {
            (*wakeup_thread).unpark_token.set(token);
            let handle = (*wakeup_thread).parker.unpark_lock();
            // SAFETY: Both buckets are locked, as required.
            unlock_bucket_pair(bucket_from, bucket_to);
            handle.unpark();
        } else {
            // SAFETY: Both buckets are locked, as required.
            unlock_bucket_pair(bucket_from, bucket_to);
        }

        result
    }

    /// Unparks a number of threads from the front of the queue associated with
    /// `key` depending on the results of a filter function which inspects the
    /// `ParkToken` associated with each thread.
    ///
    /// The `filter` function is called for each thread in the queue or until
    /// `FilterOp::Stop` is returned. This function is passed the `ParkToken`
    /// associated with a particular thread, which is unparked if `FilterOp::Unpark`
    /// is returned.
    ///
    /// The `callback` function is also called while both queues are locked. It is
    /// passed an `UnparkResult` indicating the number of threads that were unparked
    /// and whether there are still parked threads in the queue. This `UnparkResult`
    /// value is also returned by `unpark_filter`.
    ///
    /// The `callback` function should return an `UnparkToken` value which will be
    /// passed to all threads that are unparked. If no thread is unparked then the
    /// returned value is ignored.
    ///
    /// # Safety
    ///
    /// You should only call this function with an address that you control, since
    /// you could otherwise interfere with the operation of other synchronization
    /// primitives.
    ///
    /// The `filter` and `callback` functions are called while the queue is locked
    /// and must not panic or call into any function in `parking_lot`.
    #[inline]
    pub unsafe fn unpark_filter(
        key: usize,
        mut filter: impl FnMut(ParkToken) -> FilterOp,
        callback: impl FnOnce(UnparkResult) -> UnparkToken,
    ) -> UnparkResult {
        // Lock the bucket for the given key
        let bucket = lock_bucket(key);

        // Go through the queue looking for threads with a matching key
        let mut link = &bucket.queue_head;
        let mut current = bucket.queue_head.get();
        let mut previous = ptr::null();
        let mut threads = SmallVec::<[_; 8]>::new();
        let mut result = UnparkResult::default();
        while !current.is_null() {
            if (*current).key.load(Ordering::Relaxed) == key {
                // Call the filter function with the thread's ParkToken
                let next = (*current).next_in_queue.get();
                match filter((*current).park_token.get()) {
                    FilterOp::Unpark => {
                        // Remove the thread from the queue
                        link.set(next);
                        if bucket.queue_tail.get() == current {
                            bucket.queue_tail.set(previous);
                        }

                        // Add the thread to our list of threads to unpark
                        threads.push((current, None));

                        current = next;
                    }
                    FilterOp::Skip => {
                        result.have_more_threads = true;
                        link = &(*current).next_in_queue;
                        previous = current;
                        current = link.get();
                    }
                    FilterOp::Stop => {
                        result.have_more_threads = true;
                        break;
                    }
                }
            } else {
                link = &(*current).next_in_queue;
                previous = current;
                current = link.get();
            }
        }

        // Invoke the callback before waking up the threads
        result.unparked_threads = threads.len();
        if result.unparked_threads != 0 {
            result.be_fair = (*bucket.fair_timeout.get()).should_timeout();
        }
        let token = callback(result);

        // Pass the token to all threads that are going to be unparked and prepare
        // them for unparking.
        for t in threads.iter_mut() {
            (*t.0).unpark_token.set(token);
            t.1 = Some((*t.0).parker.unpark_lock());
        }

        // SAFETY: We hold the lock here, as required
        bucket.mutex.unlock();

        // Now that we are outside the lock, wake up all the threads that we removed
        // from the queue.
        for (_, handle) in threads.into_iter() {
            handle.unchecked_unwrap().unpark();
        }

        result
    }

    /// \[Experimental\] Deadlock detection
    ///
    /// Enabled via the `deadlock_detection` feature flag.
    pub mod deadlock {
        #[cfg(feature = "deadlock_detection")]
        use super::deadlock_impl;

        #[cfg(feature = "deadlock_detection")]
        pub(super) use super::deadlock_impl::DeadlockData;

        /// Acquire a resource identified by key in the deadlock detector
        /// Noop if deadlock_detection feature isn't enabled.
        ///
        /// # Safety
        ///
        /// Call after the resource is acquired
        #[inline]
        pub unsafe fn acquire_resource(_key: usize) {
            #[cfg(feature = "deadlock_detection")]
            deadlock_impl::acquire_resource(_key);
        }

        /// Release a resource identified by key in the deadlock detector.
        /// Noop if deadlock_detection feature isn't enabled.
        ///
        /// # Panics
        ///
        /// Panics if the resource was already released or wasn't acquired in this thread.
        ///
        /// # Safety
        ///
        /// Call before the resource is released
        #[inline]
        pub unsafe fn release_resource(_key: usize) {
            #[cfg(feature = "deadlock_detection")]
            deadlock_impl::release_resource(_key);
        }

        /// Returns all deadlocks detected *since* the last call.
        /// Each cycle consist of a vector of `DeadlockedThread`.
        #[cfg(feature = "deadlock_detection")]
        #[inline]
        pub fn check_deadlock() -> Vec<Vec<deadlock_impl::DeadlockedThread>> {
            deadlock_impl::check_deadlock()
        }

        #[inline]
        pub(super) unsafe fn on_unpark(_td: &super::ThreadData) {
            #[cfg(feature = "deadlock_detection")]
            deadlock_impl::on_unpark(_td);
        }
    }

    #[cfg(feature = "deadlock_detection")]
    mod deadlock_impl {
        use super::{get_hashtable, lock_bucket, with_thread_data, ThreadData, NUM_THREADS};
        use crate::thread_parker::{ThreadParkerT, UnparkHandleT};
        use crate::word_lock::WordLock;
        use backtrace::Backtrace;
        use petgraph;
        use petgraph::graphmap::DiGraphMap;
        use std::cell::{Cell, UnsafeCell};
        use std::collections::HashSet;
        use std::sync::atomic::Ordering;
        use std::sync::mpsc;
        use thread_id;

        /// Representation of a deadlocked thread
        pub struct DeadlockedThread {
            thread_id: usize,
            backtrace: Backtrace,
        }

        impl DeadlockedThread {
            /// The system thread id
            pub fn thread_id(&self) -> usize {
                self.thread_id
            }

            /// The thread backtrace
            pub fn backtrace(&self) -> &Backtrace {
                &self.backtrace
            }
        }

        pub struct DeadlockData {
            // Currently owned resources (keys)
            resources: UnsafeCell<Vec<usize>>,

            // Set when there's a pending callstack request
            deadlocked: Cell<bool>,

            // Sender used to report the backtrace
            backtrace_sender: UnsafeCell<Option<mpsc::Sender<DeadlockedThread>>>,

            // System thread id
            thread_id: usize,
        }

        impl DeadlockData {
            pub fn new() -> Self {
                DeadlockData {
                    resources: UnsafeCell::new(Vec::new()),
                    deadlocked: Cell::new(false),
                    backtrace_sender: UnsafeCell::new(None),
                    thread_id: thread_id::get(),
                }
            }
        }

        pub(super) unsafe fn on_unpark(td: &ThreadData) {
            if td.deadlock_data.deadlocked.get() {
                let sender = (*td.deadlock_data.backtrace_sender.get()).take().unwrap();
                sender
                    .send(DeadlockedThread {
                        thread_id: td.deadlock_data.thread_id,
                        backtrace: Backtrace::new(),
                    })
                    .unwrap();
                // make sure to close this sender
                drop(sender);

                // park until the end of the time
                td.parker.prepare_park();
                td.parker.park();
                unreachable!("unparked deadlocked thread!");
            }
        }

        pub unsafe fn acquire_resource(key: usize) {
            with_thread_data(|thread_data| {
                (*thread_data.deadlock_data.resources.get()).push(key);
            });
        }

        pub unsafe fn release_resource(key: usize) {
            with_thread_data(|thread_data| {
                let resources = &mut (*thread_data.deadlock_data.resources.get());

                // There is only one situation where we can fail to find the
                // resource: we are currently running TLS destructors and our
                // ThreadData has already been freed. There isn't much we can do
                // about it at this point, so just ignore it.
                if let Some(p) = resources.iter().rposition(|x| *x == key) {
                    resources.swap_remove(p);
                }
            });
        }

        pub fn check_deadlock() -> Vec<Vec<DeadlockedThread>> {
            unsafe {
                // fast pass
                if check_wait_graph_fast() {
                    // double check
                    check_wait_graph_slow()
                } else {
                    Vec::new()
                }
            }
        }

        // Simple algorithm that builds a wait graph f the threads and the resources,
        // then checks for the presence of cycles (deadlocks).
        // This variant isn't precise as it doesn't lock the entire table before checking
        unsafe fn check_wait_graph_fast() -> bool {
            let table = get_hashtable();
            let thread_count = NUM_THREADS.load(Ordering::Relaxed);
            let mut graph = DiGraphMap::<usize, ()>::with_capacity(thread_count * 2, thread_count * 2);

            for b in &(*table).entries[..] {
                b.mutex.lock();
                let mut current = b.queue_head.get();
                while !current.is_null() {
                    if !(*current).parked_with_timeout.get()
                        && !(*current).deadlock_data.deadlocked.get()
                    {
                        // .resources are waiting for their owner
                        for &resource in &(*(*current).deadlock_data.resources.get()) {
                            graph.add_edge(resource, current as usize, ());
                        }
                        // owner waits for resource .key
                        graph.add_edge(current as usize, (*current).key.load(Ordering::Relaxed), ());
                    }
                    current = (*current).next_in_queue.get();
                }
                // SAFETY: We hold the lock here, as required
                b.mutex.unlock();
            }

            petgraph::algo::is_cyclic_directed(&graph)
        }

        #[derive(Hash, PartialEq, Eq, PartialOrd, Ord, Copy, Clone)]
        enum WaitGraphNode {
            Thread(*const ThreadData),
            Resource(usize),
        }

        use self::WaitGraphNode::*;

        // Contrary to the _fast variant this locks the entries table before looking for cycles.
        // Returns all detected thread wait cycles.
        // Note that once a cycle is reported it's never reported again.
        unsafe fn check_wait_graph_slow() -> Vec<Vec<DeadlockedThread>> {
            static DEADLOCK_DETECTION_LOCK: WordLock = WordLock::new();
            DEADLOCK_DETECTION_LOCK.lock();

            let mut table = get_hashtable();
            loop {
                // Lock all buckets in the old table
                for b in &table.entries[..] {
                    b.mutex.lock();
                }

                // Now check if our table is still the latest one. Another thread could
                // have grown the hash table between us getting and locking the hash table.
                let new_table = get_hashtable();
                if new_table as *const _ == table as *const _ {
                    break;
                }

                // Unlock buckets and try again
                for b in &table.entries[..] {
                    // SAFETY: We hold the lock here, as required
                    b.mutex.unlock();
                }

                table = new_table;
            }

            let thread_count = NUM_THREADS.load(Ordering::Relaxed);
            let mut graph =
                DiGraphMap::<WaitGraphNode, ()>::with_capacity(thread_count * 2, thread_count * 2);

            for b in &table.entries[..] {
                let mut current = b.queue_head.get();
                while !current.is_null() {
                    if !(*current).parked_with_timeout.get()
                        && !(*current).deadlock_data.deadlocked.get()
                    {
                        // .resources are waiting for their owner
                        for &resource in &(*(*current).deadlock_data.resources.get()) {
                            graph.add_edge(Resource(resource), Thread(current), ());
                        }
                        // owner waits for resource .key
                        graph.add_edge(
                            Thread(current),
                            Resource((*current).key.load(Ordering::Relaxed)),
                            (),
                        );
                    }
                    current = (*current).next_in_queue.get();
                }
            }

            for b in &table.entries[..] {
                // SAFETY: We hold the lock here, as required
                b.mutex.unlock();
            }

            // find cycles
            let cycles = graph_cycles(&graph);

            let mut results = Vec::with_capacity(cycles.len());

            for cycle in cycles {
                let (sender, receiver) = mpsc::channel();
                for td in cycle {
                    let bucket = lock_bucket((*td).key.load(Ordering::Relaxed));
                    (*td).deadlock_data.deadlocked.set(true);
                    *(*td).deadlock_data.backtrace_sender.get() = Some(sender.clone());
                    let handle = (*td).parker.unpark_lock();
                    // SAFETY: We hold the lock here, as required
                    bucket.mutex.unlock();
                    // unpark the deadlocked thread!
                    // on unpark it'll notice the deadlocked flag and report back
                    handle.unpark();
                }
                // make sure to drop our sender before collecting results
                drop(sender);
                results.push(receiver.iter().collect());
            }

            DEADLOCK_DETECTION_LOCK.unlock();

            results
        }

        // normalize a cycle to start with the "smallest" node
        fn normalize_cycle<T: Ord + Copy + Clone>(input: &[T]) -> Vec<T> {
            let min_pos = input
                .iter()
                .enumerate()
                .min_by_key(|&(_, &t)| t)
                .map(|(p, _)| p)
                .unwrap_or(0);
            input
                .iter()
                .cycle()
                .skip(min_pos)
                .take(input.len())
                .cloned()
                .collect()
        }

        // returns all thread cycles in the wait graph
        fn graph_cycles(g: &DiGraphMap<WaitGraphNode, ()>) -> Vec<Vec<*const ThreadData>> {
            use petgraph::visit::depth_first_search;
            use petgraph::visit::DfsEvent;
            use petgraph::visit::NodeIndexable;

            let mut cycles = HashSet::new();
            let mut path = Vec::with_capacity(g.node_bound());
            // start from threads to get the correct threads cycle
            let threads = g
                .nodes()
                .filter(|n| if let &Thread(_) = n { true } else { false });

            depth_first_search(g, threads, |e| match e {
                DfsEvent::Discover(Thread(n), _) => path.push(n),
                DfsEvent::Finish(Thread(_), _) => {
                    path.pop();
                }
                DfsEvent::BackEdge(_, Thread(n)) => {
                    let from = path.iter().rposition(|&i| i == n).unwrap();
                    cycles.insert(normalize_cycle(&path[from..]));
                }
                _ => (),
            });

            cycles.iter().cloned().collect()
        }
    }

    #[cfg(test)]
    mod tests {
        use super::{ThreadData, DEFAULT_PARK_TOKEN, DEFAULT_UNPARK_TOKEN};
        use std::{
            ptr,
            sync::{
                atomic::{AtomicIsize, AtomicPtr, AtomicUsize, Ordering},
                Arc,
            },
            thread,
            time::Duration,
        };

        /// Calls a closure for every `ThreadData` currently parked on a given key
        fn for_each(key: usize, mut f: impl FnMut(&ThreadData)) {
            let bucket = super::lock_bucket(key);

            let mut current: *const ThreadData = bucket.queue_head.get();
            while !current.is_null() {
                let current_ref = unsafe { &*current };
                if current_ref.key.load(Ordering::Relaxed) == key {
                    f(current_ref);
                }
                current = current_ref.next_in_queue.get();
            }

            // SAFETY: We hold the lock here, as required
            unsafe { bucket.mutex.unlock() };
        }

        macro_rules! test {
            ( $( $name:ident(
                repeats: $repeats:expr,
                latches: $latches:expr,
                delay: $delay:expr,
                threads: $threads:expr,
                single_unparks: $single_unparks:expr);
            )* ) => {
                $(#[test]
                fn $name() {
                    let delay = Duration::from_micros($delay);
                    for _ in 0..$repeats {
                        run_parking_test($latches, delay, $threads, $single_unparks);
                    }
                })*
            };
        }

        test! {
            unpark_all_one_fast(
                repeats: 10000, latches: 1, delay: 0, threads: 1, single_unparks: 0
            );
            unpark_all_hundred_fast(
                repeats: 100, latches: 1, delay: 0, threads: 100, single_unparks: 0
            );
            unpark_one_one_fast(
                repeats: 1000, latches: 1, delay: 0, threads: 1, single_unparks: 1
            );
            unpark_one_hundred_fast(
                repeats: 20, latches: 1, delay: 0, threads: 100, single_unparks: 100
            );
            unpark_one_fifty_then_fifty_all_fast(
                repeats: 50, latches: 1, delay: 0, threads: 100, single_unparks: 50
            );
            unpark_all_one(
                repeats: 100, latches: 1, delay: 10000, threads: 1, single_unparks: 0
            );
            unpark_all_hundred(
                repeats: 100, latches: 1, delay: 10000, threads: 100, single_unparks: 0
            );
            unpark_one_one(
                repeats: 10, latches: 1, delay: 10000, threads: 1, single_unparks: 1
            );
            unpark_one_fifty(
                repeats: 1, latches: 1, delay: 10000, threads: 50, single_unparks: 50
            );
            unpark_one_fifty_then_fifty_all(
                repeats: 2, latches: 1, delay: 10000, threads: 100, single_unparks: 50
            );
            hundred_unpark_all_one_fast(
                repeats: 100, latches: 100, delay: 0, threads: 1, single_unparks: 0
            );
            hundred_unpark_all_one(
                repeats: 1, latches: 100, delay: 10000, threads: 1, single_unparks: 0
            );
        }

        fn run_parking_test(
            num_latches: usize,
            delay: Duration,
            num_threads: usize,
            num_single_unparks: usize,
        ) {
            let mut tests = Vec::with_capacity(num_latches);

            for _ in 0..num_latches {
                let test = Arc::new(SingleLatchTest::new(num_threads));
                let mut threads = Vec::with_capacity(num_threads);
                for _ in 0..num_threads {
                    let test = test.clone();
                    threads.push(thread::spawn(move || test.run()));
                }
                tests.push((test, threads));
            }

            for unpark_index in 0..num_single_unparks {
                thread::sleep(delay);
                for (test, _) in &tests {
                    test.unpark_one(unpark_index);
                }
            }

            for (test, threads) in tests {
                test.finish(num_single_unparks);
                for thread in threads {
                    thread.join().expect("Test thread panic");
                }
            }
        }

        struct SingleLatchTest {
            semaphore: AtomicIsize,
            num_awake: AtomicUsize,
            /// Holds the pointer to the last *unprocessed* woken up thread.
            last_awoken: AtomicPtr<ThreadData>,
            /// Total number of threads participating in this test.
            num_threads: usize,
        }

        impl SingleLatchTest {
            pub fn new(num_threads: usize) -> Self {
                Self {
                    // This implements a fair (FIFO) semaphore, and it starts out unavailable.
                    semaphore: AtomicIsize::new(0),
                    num_awake: AtomicUsize::new(0),
                    last_awoken: AtomicPtr::new(ptr::null_mut()),
                    num_threads,
                }
            }

            pub fn run(&self) {
                // Get one slot from the semaphore
                self.down();

                // Report back to the test verification code that this thread woke up
                let this_thread_ptr = super::with_thread_data(|t| t as *const _ as *mut _);
                self.last_awoken.store(this_thread_ptr, Ordering::SeqCst);
                self.num_awake.fetch_add(1, Ordering::SeqCst);
            }

            pub fn unpark_one(&self, single_unpark_index: usize) {
                // last_awoken should be null at all times except between self.up() and at the bottom
                // of this method where it's reset to null again
                assert!(self.last_awoken.load(Ordering::SeqCst).is_null());

                let mut queue: Vec<*mut ThreadData> = Vec::with_capacity(self.num_threads);
                for_each(self.semaphore_addr(), |thread_data| {
                    queue.push(thread_data as *const _ as *mut _);
                });
                assert!(queue.len() <= self.num_threads - single_unpark_index);

                let num_awake_before_up = self.num_awake.load(Ordering::SeqCst);

                self.up();

                // Wait for a parked thread to wake up and update num_awake + last_awoken.
                while self.num_awake.load(Ordering::SeqCst) != num_awake_before_up + 1 {
                    thread::yield_now();
                }

                // At this point the other thread should have set last_awoken inside the run() method
                let last_awoken = self.last_awoken.load(Ordering::SeqCst);
                assert!(!last_awoken.is_null());
                if !queue.is_empty() && queue[0] != last_awoken {
                    panic!(
                        "Woke up wrong thread:\n\tqueue: {:?}\n\tlast awoken: {:?}",
                        queue, last_awoken
                    );
                }
                self.last_awoken.store(ptr::null_mut(), Ordering::SeqCst);
            }

            pub fn finish(&self, num_single_unparks: usize) {
                // The amount of threads not unparked via unpark_one
                let mut num_threads_left = self.num_threads.checked_sub(num_single_unparks).unwrap();

                // Wake remaining threads up with unpark_all. Has to be in a loop, because there might
                // still be threads that has not yet parked.
                while num_threads_left > 0 {
                    let mut num_waiting_on_address = 0;
                    for_each(self.semaphore_addr(), |_thread_data| {
                        num_waiting_on_address += 1;
                    });
                    assert!(num_waiting_on_address <= num_threads_left);

                    let num_awake_before_unpark = self.num_awake.load(Ordering::SeqCst);

                    let num_unparked =
                        unsafe { super::unpark_all(self.semaphore_addr(), DEFAULT_UNPARK_TOKEN) };
                    assert!(num_unparked >= num_waiting_on_address);
                    assert!(num_unparked <= num_threads_left);

                    // Wait for all unparked threads to wake up and update num_awake + last_awoken.
                    while self.num_awake.load(Ordering::SeqCst)
                        != num_awake_before_unpark + num_unparked
                    {
                        thread::yield_now()
                    }

                    num_threads_left = num_threads_left.checked_sub(num_unparked).unwrap();
                }
                // By now, all threads should have been woken up
                assert_eq!(self.num_awake.load(Ordering::SeqCst), self.num_threads);

                // Make sure no thread is parked on our semaphore address
                let mut num_waiting_on_address = 0;
                for_each(self.semaphore_addr(), |_thread_data| {
                    num_waiting_on_address += 1;
                });
                assert_eq!(num_waiting_on_address, 0);
            }

            pub fn down(&self) {
                let old_semaphore_value = self.semaphore.fetch_sub(1, Ordering::SeqCst);

                if old_semaphore_value > 0 {
                    // We acquired the semaphore. Done.
                    return;
                }

                // We need to wait.
                let validate = || true;
                let before_sleep = || {};
                let timed_out = |_, _| {};
                unsafe {
                    super::park(
                        self.semaphore_addr(),
                        validate,
                        before_sleep,
                        timed_out,
                        DEFAULT_PARK_TOKEN,
                        None,
                    );
                }
            }

            pub fn up(&self) {
                let old_semaphore_value = self.semaphore.fetch_add(1, Ordering::SeqCst);

                // Check if anyone was waiting on the semaphore. If they were, then pass ownership to them.
                if old_semaphore_value < 0 {
                    // We need to continue until we have actually unparked someone. It might be that
                    // the thread we want to pass ownership to has decremented the semaphore counter,
                    // but not yet parked.
                    loop {
                        match unsafe {
                            super::unpark_one(self.semaphore_addr(), |_| DEFAULT_UNPARK_TOKEN)
                                .unparked_threads
                        } {
                            1 => break,
                            0 => (),
                            i => panic!("Should not wake up {} threads", i),
                        }
                    }
                }
            }

            fn semaphore_addr(&self) -> usize {
                &self.semaphore as *const _ as usize
            }
        }
    }