Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

2 files changed, 2154 insertions, 0 deletions

diff --git a/vendor/crossbeam-deque/src/deque.rs b/vendor/crossbeam-deque/src/deque.rs
new file mode 100644
index 000000000..802a2fef5
--- /dev/null
+++ b/vendor/crossbeam-deque/src/deque.rs
@@ -0,0 +1,2044 @@
+use std::cell::{Cell, UnsafeCell};
+use std::cmp;
+use std::fmt;
+use std::iter::FromIterator;
+use std::marker::PhantomData;
+use std::mem::{self, MaybeUninit};
+use std::ptr;
+use std::sync::atomic::{self, AtomicIsize, AtomicPtr, AtomicUsize, Ordering};
+use std::sync::Arc;
+
+use crate::epoch::{self, Atomic, Owned};
+use crate::utils::{Backoff, CachePadded};
+
+// Minimum buffer capacity.
+const MIN_CAP: usize = 64;
+// Maximum number of tasks that can be stolen in `steal_batch()` and `steal_batch_and_pop()`.
+const MAX_BATCH: usize = 32;
+// If a buffer of at least this size is retired, thread-local garbage is flushed so that it gets
+// deallocated as soon as possible.
+const FLUSH_THRESHOLD_BYTES: usize = 1 << 10;
+
+/// A buffer that holds tasks in a worker queue.
+///
+/// This is just a pointer to the buffer and its length - dropping an instance of this struct will
+/// *not* deallocate the buffer.
+struct Buffer<T> {
+    /// Pointer to the allocated memory.
+    ptr: *mut T,
+
+    /// Capacity of the buffer. Always a power of two.
+    cap: usize,
+}
+
+unsafe impl<T> Send for Buffer<T> {}
+
+impl<T> Buffer<T> {
+    /// Allocates a new buffer with the specified capacity.
+    fn alloc(cap: usize) -> Buffer<T> {
+        debug_assert_eq!(cap, cap.next_power_of_two());
+
+        let mut v = Vec::with_capacity(cap);
+        let ptr = v.as_mut_ptr();
+        mem::forget(v);
+
+        Buffer { ptr, cap }
+    }
+
+    /// Deallocates the buffer.
+    unsafe fn dealloc(self) {
+        drop(Vec::from_raw_parts(self.ptr, 0, self.cap));
+    }
+
+    /// Returns a pointer to the task at the specified `index`.
+    unsafe fn at(&self, index: isize) -> *mut T {
+        // `self.cap` is always a power of two.
+        self.ptr.offset(index & (self.cap - 1) as isize)
+    }
+
+    /// Writes `task` into the specified `index`.
+    ///
+    /// This method might be concurrently called with another `read` at the same index, which is
+    /// technically speaking a data race and therefore UB. We should use an atomic store here, but
+    /// that would be more expensive and difficult to implement generically for all types `T`.
+    /// Hence, as a hack, we use a volatile write instead.
+    unsafe fn write(&self, index: isize, task: T) {
+        ptr::write_volatile(self.at(index), task)
+    }
+
+    /// Reads a task from the specified `index`.
+    ///
+    /// This method might be concurrently called with another `write` at the same index, which is
+    /// technically speaking a data race and therefore UB. We should use an atomic load here, but
+    /// that would be more expensive and difficult to implement generically for all types `T`.
+    /// Hence, as a hack, we use a volatile write instead.
+    unsafe fn read(&self, index: isize) -> T {
+        ptr::read_volatile(self.at(index))
+    }
+}
+
+impl<T> Clone for Buffer<T> {
+    fn clone(&self) -> Buffer<T> {
+        Buffer {
+            ptr: self.ptr,
+            cap: self.cap,
+        }
+    }
+}
+
+impl<T> Copy for Buffer<T> {}
+
+/// Internal queue data shared between the worker and stealers.
+///
+/// The implementation is based on the following work:
+///
+/// 1. [Chase and Lev. Dynamic circular work-stealing deque. SPAA 2005.][chase-lev]
+/// 2. [Le, Pop, Cohen, and Nardelli. Correct and efficient work-stealing for weak memory models.
+///    PPoPP 2013.][weak-mem]
+/// 3. [Norris and Demsky. CDSchecker: checking concurrent data structures written with C/C++
+///    atomics. OOPSLA 2013.][checker]
+///
+/// [chase-lev]: https://dl.acm.org/citation.cfm?id=1073974
+/// [weak-mem]: https://dl.acm.org/citation.cfm?id=2442524
+/// [checker]: https://dl.acm.org/citation.cfm?id=2509514
+struct Inner<T> {
+    /// The front index.
+    front: AtomicIsize,
+
+    /// The back index.
+    back: AtomicIsize,
+
+    /// The underlying buffer.
+    buffer: CachePadded<Atomic<Buffer<T>>>,
+}
+
+impl<T> Drop for Inner<T> {
+    fn drop(&mut self) {
+        // Load the back index, front index, and buffer.
+        let b = self.back.load(Ordering::Relaxed);
+        let f = self.front.load(Ordering::Relaxed);
+
+        unsafe {
+            let buffer = self.buffer.load(Ordering::Relaxed, epoch::unprotected());
+
+            // Go through the buffer from front to back and drop all tasks in the queue.
+            let mut i = f;
+            while i != b {
+                buffer.deref().at(i).drop_in_place();
+                i = i.wrapping_add(1);
+            }
+
+            // Free the memory allocated by the buffer.
+            buffer.into_owned().into_box().dealloc();
+        }
+    }
+}
+
+/// Worker queue flavor: FIFO or LIFO.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum Flavor {
+    /// The first-in first-out flavor.
+    Fifo,
+
+    /// The last-in first-out flavor.
+    Lifo,
+}
+
+/// A worker queue.
+///
+/// This is a FIFO or LIFO queue that is owned by a single thread, but other threads may steal
+/// tasks from it. Task schedulers typically create a single worker queue per thread.
+///
+/// # Examples
+///
+/// A FIFO worker:
+///
+/// ```
+/// use crossbeam_deque::{Steal, Worker};
+///
+/// let w = Worker::new_fifo();
+/// let s = w.stealer();
+///
+/// w.push(1);
+/// w.push(2);
+/// w.push(3);
+///
+/// assert_eq!(s.steal(), Steal::Success(1));
+/// assert_eq!(w.pop(), Some(2));
+/// assert_eq!(w.pop(), Some(3));
+/// ```
+///
+/// A LIFO worker:
+///
+/// ```
+/// use crossbeam_deque::{Steal, Worker};
+///
+/// let w = Worker::new_lifo();
+/// let s = w.stealer();
+///
+/// w.push(1);
+/// w.push(2);
+/// w.push(3);
+///
+/// assert_eq!(s.steal(), Steal::Success(1));
+/// assert_eq!(w.pop(), Some(3));
+/// assert_eq!(w.pop(), Some(2));
+/// ```
+pub struct Worker<T> {
+    /// A reference to the inner representation of the queue.
+    inner: Arc<CachePadded<Inner<T>>>,
+
+    /// A copy of `inner.buffer` for quick access.
+    buffer: Cell<Buffer<T>>,
+
+    /// The flavor of the queue.
+    flavor: Flavor,
+
+    /// Indicates that the worker cannot be shared among threads.
+    _marker: PhantomData<*mut ()>, // !Send + !Sync
+}
+
+unsafe impl<T: Send> Send for Worker<T> {}
+
+impl<T> Worker<T> {
+    /// Creates a FIFO worker queue.
+    ///
+    /// Tasks are pushed and popped from opposite ends.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::<i32>::new_fifo();
+    /// ```
+    pub fn new_fifo() -> Worker<T> {
+        let buffer = Buffer::alloc(MIN_CAP);
+
+        let inner = Arc::new(CachePadded::new(Inner {
+            front: AtomicIsize::new(0),
+            back: AtomicIsize::new(0),
+            buffer: CachePadded::new(Atomic::new(buffer)),
+        }));
+
+        Worker {
+            inner,
+            buffer: Cell::new(buffer),
+            flavor: Flavor::Fifo,
+            _marker: PhantomData,
+        }
+    }
+
+    /// Creates a LIFO worker queue.
+    ///
+    /// Tasks are pushed and popped from the same end.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::<i32>::new_lifo();
+    /// ```
+    pub fn new_lifo() -> Worker<T> {
+        let buffer = Buffer::alloc(MIN_CAP);
+
+        let inner = Arc::new(CachePadded::new(Inner {
+            front: AtomicIsize::new(0),
+            back: AtomicIsize::new(0),
+            buffer: CachePadded::new(Atomic::new(buffer)),
+        }));
+
+        Worker {
+            inner,
+            buffer: Cell::new(buffer),
+            flavor: Flavor::Lifo,
+            _marker: PhantomData,
+        }
+    }
+
+    /// Creates a stealer for this queue.
+    ///
+    /// The returned stealer can be shared among threads and cloned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::<i32>::new_lifo();
+    /// let s = w.stealer();
+    /// ```
+    pub fn stealer(&self) -> Stealer<T> {
+        Stealer {
+            inner: self.inner.clone(),
+            flavor: self.flavor,
+        }
+    }
+
+    /// Resizes the internal buffer to the new capacity of `new_cap`.
+    #[cold]
+    unsafe fn resize(&self, new_cap: usize) {
+        // Load the back index, front index, and buffer.
+        let b = self.inner.back.load(Ordering::Relaxed);
+        let f = self.inner.front.load(Ordering::Relaxed);
+        let buffer = self.buffer.get();
+
+        // Allocate a new buffer and copy data from the old buffer to the new one.
+        let new = Buffer::alloc(new_cap);
+        let mut i = f;
+        while i != b {
+            ptr::copy_nonoverlapping(buffer.at(i), new.at(i), 1);
+            i = i.wrapping_add(1);
+        }
+
+        let guard = &epoch::pin();
+
+        // Replace the old buffer with the new one.
+        self.buffer.replace(new);
+        let old =
+            self.inner
+                .buffer
+                .swap(Owned::new(new).into_shared(guard), Ordering::Release, guard);
+
+        // Destroy the old buffer later.
+        guard.defer_unchecked(move || old.into_owned().into_box().dealloc());
+
+        // If the buffer is very large, then flush the thread-local garbage in order to deallocate
+        // it as soon as possible.
+        if mem::size_of::<T>() * new_cap >= FLUSH_THRESHOLD_BYTES {
+            guard.flush();
+        }
+    }
+
+    /// Reserves enough capacity so that `reserve_cap` tasks can be pushed without growing the
+    /// buffer.
+    fn reserve(&self, reserve_cap: usize) {
+        if reserve_cap > 0 {
+            // Compute the current length.
+            let b = self.inner.back.load(Ordering::Relaxed);
+            let f = self.inner.front.load(Ordering::SeqCst);
+            let len = b.wrapping_sub(f) as usize;
+
+            // The current capacity.
+            let cap = self.buffer.get().cap;
+
+            // Is there enough capacity to push `reserve_cap` tasks?
+            if cap - len < reserve_cap {
+                // Keep doubling the capacity as much as is needed.
+                let mut new_cap = cap * 2;
+                while new_cap - len < reserve_cap {
+                    new_cap *= 2;
+                }
+
+                // Resize the buffer.
+                unsafe {
+                    self.resize(new_cap);
+                }
+            }
+        }
+    }
+
+    /// Returns `true` if the queue is empty.
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::new_lifo();
+    ///
+    /// assert!(w.is_empty());
+    /// w.push(1);
+    /// assert!(!w.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        let b = self.inner.back.load(Ordering::Relaxed);
+        let f = self.inner.front.load(Ordering::SeqCst);
+        b.wrapping_sub(f) <= 0
+    }
+
+    /// Returns the number of tasks in the deque.
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::new_lifo();
+    ///
+    /// assert_eq!(w.len(), 0);
+    /// w.push(1);
+    /// assert_eq!(w.len(), 1);
+    /// w.push(1);
+    /// assert_eq!(w.len(), 2);
+    /// ```
+    pub fn len(&self) -> usize {
+        let b = self.inner.back.load(Ordering::Relaxed);
+        let f = self.inner.front.load(Ordering::SeqCst);
+        b.wrapping_sub(f).max(0) as usize
+    }
+
+    /// Pushes a task into the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::new_lifo();
+    /// w.push(1);
+    /// w.push(2);
+    /// ```
+    pub fn push(&self, task: T) {
+        // Load the back index, front index, and buffer.
+        let b = self.inner.back.load(Ordering::Relaxed);
+        let f = self.inner.front.load(Ordering::Acquire);
+        let mut buffer = self.buffer.get();
+
+        // Calculate the length of the queue.
+        let len = b.wrapping_sub(f);
+
+        // Is the queue full?
+        if len >= buffer.cap as isize {
+            // Yes. Grow the underlying buffer.
+            unsafe {
+                self.resize(2 * buffer.cap);
+            }
+            buffer = self.buffer.get();
+        }
+
+        // Write `task` into the slot.
+        unsafe {
+            buffer.write(b, task);
+        }
+
+        atomic::fence(Ordering::Release);
+
+        // Increment the back index.
+        //
+        // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data
+        // races because it doesn't understand fences.
+        self.inner.back.store(b.wrapping_add(1), Ordering::Release);
+    }
+
+    /// Pops a task from the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::new_fifo();
+    /// w.push(1);
+    /// w.push(2);
+    ///
+    /// assert_eq!(w.pop(), Some(1));
+    /// assert_eq!(w.pop(), Some(2));
+    /// assert_eq!(w.pop(), None);
+    /// ```
+    pub fn pop(&self) -> Option<T> {
+        // Load the back and front index.
+        let b = self.inner.back.load(Ordering::Relaxed);
+        let f = self.inner.front.load(Ordering::Relaxed);
+
+        // Calculate the length of the queue.
+        let len = b.wrapping_sub(f);
+
+        // Is the queue empty?
+        if len <= 0 {
+            return None;
+        }
+
+        match self.flavor {
+            // Pop from the front of the queue.
+            Flavor::Fifo => {
+                // Try incrementing the front index to pop the task.
+                let f = self.inner.front.fetch_add(1, Ordering::SeqCst);
+                let new_f = f.wrapping_add(1);
+
+                if b.wrapping_sub(new_f) < 0 {
+                    self.inner.front.store(f, Ordering::Relaxed);
+                    return None;
+                }
+
+                unsafe {
+                    // Read the popped task.
+                    let buffer = self.buffer.get();
+                    let task = buffer.read(f);
+
+                    // Shrink the buffer if `len - 1` is less than one fourth of the capacity.
+                    if buffer.cap > MIN_CAP && len <= buffer.cap as isize / 4 {
+                        self.resize(buffer.cap / 2);
+                    }
+
+                    Some(task)
+                }
+            }
+
+            // Pop from the back of the queue.
+            Flavor::Lifo => {
+                // Decrement the back index.
+                let b = b.wrapping_sub(1);
+                self.inner.back.store(b, Ordering::Relaxed);
+
+                atomic::fence(Ordering::SeqCst);
+
+                // Load the front index.
+                let f = self.inner.front.load(Ordering::Relaxed);
+
+                // Compute the length after the back index was decremented.
+                let len = b.wrapping_sub(f);
+
+                if len < 0 {
+                    // The queue is empty. Restore the back index to the original task.
+                    self.inner.back.store(b.wrapping_add(1), Ordering::Relaxed);
+                    None
+                } else {
+                    // Read the task to be popped.
+                    let buffer = self.buffer.get();
+                    let mut task = unsafe { Some(buffer.read(b)) };
+
+                    // Are we popping the last task from the queue?
+                    if len == 0 {
+                        // Try incrementing the front index.
+                        if self
+                            .inner
+                            .front
+                            .compare_exchange(
+                                f,
+                                f.wrapping_add(1),
+                                Ordering::SeqCst,
+                                Ordering::Relaxed,
+                            )
+                            .is_err()
+                        {
+                            // Failed. We didn't pop anything.
+                            mem::forget(task.take());
+                        }
+
+                        // Restore the back index to the original task.
+                        self.inner.back.store(b.wrapping_add(1), Ordering::Relaxed);
+                    } else {
+                        // Shrink the buffer if `len` is less than one fourth of the capacity.
+                        if buffer.cap > MIN_CAP && len < buffer.cap as isize / 4 {
+                            unsafe {
+                                self.resize(buffer.cap / 2);
+                            }
+                        }
+                    }
+
+                    task
+                }
+            }
+        }
+    }
+}
+
+impl<T> fmt::Debug for Worker<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Worker { .. }")
+    }
+}
+
+/// A stealer handle of a worker queue.
+///
+/// Stealers can be shared among threads.
+///
+/// Task schedulers typically have a single worker queue per worker thread.
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_deque::{Steal, Worker};
+///
+/// let w = Worker::new_lifo();
+/// w.push(1);
+/// w.push(2);
+///
+/// let s = w.stealer();
+/// assert_eq!(s.steal(), Steal::Success(1));
+/// assert_eq!(s.steal(), Steal::Success(2));
+/// assert_eq!(s.steal(), Steal::Empty);
+/// ```
+pub struct Stealer<T> {
+    /// A reference to the inner representation of the queue.
+    inner: Arc<CachePadded<Inner<T>>>,
+
+    /// The flavor of the queue.
+    flavor: Flavor,
+}
+
+unsafe impl<T: Send> Send for Stealer<T> {}
+unsafe impl<T: Send> Sync for Stealer<T> {}
+
+impl<T> Stealer<T> {
+    /// Returns `true` if the queue is empty.
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::new_lifo();
+    /// let s = w.stealer();
+    ///
+    /// assert!(s.is_empty());
+    /// w.push(1);
+    /// assert!(!s.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        let f = self.inner.front.load(Ordering::Acquire);
+        atomic::fence(Ordering::SeqCst);
+        let b = self.inner.back.load(Ordering::Acquire);
+        b.wrapping_sub(f) <= 0
+    }
+
+    /// Returns the number of tasks in the deque.
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w = Worker::new_lifo();
+    /// let s = w.stealer();
+    ///
+    /// assert_eq!(s.len(), 0);
+    /// w.push(1);
+    /// assert_eq!(s.len(), 1);
+    /// w.push(2);
+    /// assert_eq!(s.len(), 2);
+    /// ```
+    pub fn len(&self) -> usize {
+        let f = self.inner.front.load(Ordering::Acquire);
+        atomic::fence(Ordering::SeqCst);
+        let b = self.inner.back.load(Ordering::Acquire);
+        b.wrapping_sub(f).max(0) as usize
+    }
+
+    /// Steals a task from the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::{Steal, Worker};
+    ///
+    /// let w = Worker::new_lifo();
+    /// w.push(1);
+    /// w.push(2);
+    ///
+    /// let s = w.stealer();
+    /// assert_eq!(s.steal(), Steal::Success(1));
+    /// assert_eq!(s.steal(), Steal::Success(2));
+    /// ```
+    pub fn steal(&self) -> Steal<T> {
+        // Load the front index.
+        let f = self.inner.front.load(Ordering::Acquire);
+
+        // A SeqCst fence is needed here.
+        //
+        // If the current thread is already pinned (reentrantly), we must manually issue the
+        // fence. Otherwise, the following pinning will issue the fence anyway, so we don't
+        // have to.
+        if epoch::is_pinned() {
+            atomic::fence(Ordering::SeqCst);
+        }
+
+        let guard = &epoch::pin();
+
+        // Load the back index.
+        let b = self.inner.back.load(Ordering::Acquire);
+
+        // Is the queue empty?
+        if b.wrapping_sub(f) <= 0 {
+            return Steal::Empty;
+        }
+
+        // Load the buffer and read the task at the front.
+        let buffer = self.inner.buffer.load(Ordering::Acquire, guard);
+        let task = unsafe { buffer.deref().read(f) };
+
+        // Try incrementing the front index to steal the task.
+        // If the buffer has been swapped or the increment fails, we retry.
+        if self.inner.buffer.load(Ordering::Acquire, guard) != buffer
+            || self
+                .inner
+                .front
+                .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed)
+                .is_err()
+        {
+            // We didn't steal this task, forget it.
+            mem::forget(task);
+            return Steal::Retry;
+        }
+
+        // Return the stolen task.
+        Steal::Success(task)
+    }
+
+    /// Steals a batch of tasks and pushes them into another worker.
+    ///
+    /// How many tasks exactly will be stolen is not specified. That said, this method will try to
+    /// steal around half of the tasks in the queue, but also not more than some constant limit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Worker;
+    ///
+    /// let w1 = Worker::new_fifo();
+    /// w1.push(1);
+    /// w1.push(2);
+    /// w1.push(3);
+    /// w1.push(4);
+    ///
+    /// let s = w1.stealer();
+    /// let w2 = Worker::new_fifo();
+    ///
+    /// let _ = s.steal_batch(&w2);
+    /// assert_eq!(w2.pop(), Some(1));
+    /// assert_eq!(w2.pop(), Some(2));
+    /// ```
+    pub fn steal_batch(&self, dest: &Worker<T>) -> Steal<()> {
+        if Arc::ptr_eq(&self.inner, &dest.inner) {
+            if dest.is_empty() {
+                return Steal::Empty;
+            } else {
+                return Steal::Success(());
+            }
+        }
+
+        // Load the front index.
+        let mut f = self.inner.front.load(Ordering::Acquire);
+
+        // A SeqCst fence is needed here.
+        //
+        // If the current thread is already pinned (reentrantly), we must manually issue the
+        // fence. Otherwise, the following pinning will issue the fence anyway, so we don't
+        // have to.
+        if epoch::is_pinned() {
+            atomic::fence(Ordering::SeqCst);
+        }
+
+        let guard = &epoch::pin();
+
+        // Load the back index.
+        let b = self.inner.back.load(Ordering::Acquire);
+
+        // Is the queue empty?
+        let len = b.wrapping_sub(f);
+        if len <= 0 {
+            return Steal::Empty;
+        }
+
+        // Reserve capacity for the stolen batch.
+        let batch_size = cmp::min((len as usize + 1) / 2, MAX_BATCH);
+        dest.reserve(batch_size);
+        let mut batch_size = batch_size as isize;
+
+        // Get the destination buffer and back index.
+        let dest_buffer = dest.buffer.get();
+        let mut dest_b = dest.inner.back.load(Ordering::Relaxed);
+
+        // Load the buffer.
+        let buffer = self.inner.buffer.load(Ordering::Acquire, guard);
+
+        match self.flavor {
+            // Steal a batch of tasks from the front at once.
+            Flavor::Fifo => {
+                // Copy the batch from the source to the destination buffer.
+                match dest.flavor {
+                    Flavor::Fifo => {
+                        for i in 0..batch_size {
+                            unsafe {
+                                let task = buffer.deref().read(f.wrapping_add(i));
+                                dest_buffer.write(dest_b.wrapping_add(i), task);
+                            }
+                        }
+                    }
+                    Flavor::Lifo => {
+                        for i in 0..batch_size {
+                            unsafe {
+                                let task = buffer.deref().read(f.wrapping_add(i));
+                                dest_buffer.write(dest_b.wrapping_add(batch_size - 1 - i), task);
+                            }
+                        }
+                    }
+                }
+
+                // Try incrementing the front index to steal the batch.
+                // If the buffer has been swapped or the increment fails, we retry.
+                if self.inner.buffer.load(Ordering::Acquire, guard) != buffer
+                    || self
+                        .inner
+                        .front
+                        .compare_exchange(
+                            f,
+                            f.wrapping_add(batch_size),
+                            Ordering::SeqCst,
+                            Ordering::Relaxed,
+                        )
+                        .is_err()
+                {
+                    return Steal::Retry;
+                }
+
+                dest_b = dest_b.wrapping_add(batch_size);
+            }
+
+            // Steal a batch of tasks from the front one by one.
+            Flavor::Lifo => {
+                // This loop may modify the batch_size, which triggers a clippy lint warning.
+                // Use a new variable to avoid the warning, and to make it clear we aren't
+                // modifying the loop exit condition during iteration.
+                let original_batch_size = batch_size;
+
+                for i in 0..original_batch_size {
+                    // If this is not the first steal, check whether the queue is empty.
+                    if i > 0 {
+                        // We've already got the current front index. Now execute the fence to
+                        // synchronize with other threads.
+                        atomic::fence(Ordering::SeqCst);
+
+                        // Load the back index.
+                        let b = self.inner.back.load(Ordering::Acquire);
+
+                        // Is the queue empty?
+                        if b.wrapping_sub(f) <= 0 {
+                            batch_size = i;
+                            break;
+                        }
+                    }
+
+                    // Read the task at the front.
+                    let task = unsafe { buffer.deref().read(f) };
+
+                    // Try incrementing the front index to steal the task.
+                    // If the buffer has been swapped or the increment fails, we retry.
+                    if self.inner.buffer.load(Ordering::Acquire, guard) != buffer
+                        || self
+                            .inner
+                            .front
+                            .compare_exchange(
+                                f,
+                                f.wrapping_add(1),
+                                Ordering::SeqCst,
+                                Ordering::Relaxed,
+                            )
+                            .is_err()
+                    {
+                        // We didn't steal this task, forget it and break from the loop.
+                        mem::forget(task);
+                        batch_size = i;
+                        break;
+                    }
+
+                    // Write the stolen task into the destination buffer.
+                    unsafe {
+                        dest_buffer.write(dest_b, task);
+                    }
+
+                    // Move the source front index and the destination back index one step forward.
+                    f = f.wrapping_add(1);
+                    dest_b = dest_b.wrapping_add(1);
+                }
+
+                // If we didn't steal anything, the operation needs to be retried.
+                if batch_size == 0 {
+                    return Steal::Retry;
+                }
+
+                // If stealing into a FIFO queue, stolen tasks need to be reversed.
+                if dest.flavor == Flavor::Fifo {
+                    for i in 0..batch_size / 2 {
+                        unsafe {
+                            let i1 = dest_b.wrapping_sub(batch_size - i);
+                            let i2 = dest_b.wrapping_sub(i + 1);
+                            let t1 = dest_buffer.read(i1);
+                            let t2 = dest_buffer.read(i2);
+                            dest_buffer.write(i1, t2);
+                            dest_buffer.write(i2, t1);
+                        }
+                    }
+                }
+            }
+        }
+
+        atomic::fence(Ordering::Release);
+
+        // Update the back index in the destination queue.
+        //
+        // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data
+        // races because it doesn't understand fences.
+        dest.inner.back.store(dest_b, Ordering::Release);
+
+        // Return with success.
+        Steal::Success(())
+    }
+
+    /// Steals a batch of tasks, pushes them into another worker, and pops a task from that worker.
+    ///
+    /// How many tasks exactly will be stolen is not specified. That said, this method will try to
+    /// steal around half of the tasks in the queue, but also not more than some constant limit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::{Steal, Worker};
+    ///
+    /// let w1 = Worker::new_fifo();
+    /// w1.push(1);
+    /// w1.push(2);
+    /// w1.push(3);
+    /// w1.push(4);
+    ///
+    /// let s = w1.stealer();
+    /// let w2 = Worker::new_fifo();
+    ///
+    /// assert_eq!(s.steal_batch_and_pop(&w2), Steal::Success(1));
+    /// assert_eq!(w2.pop(), Some(2));
+    /// ```
+    pub fn steal_batch_and_pop(&self, dest: &Worker<T>) -> Steal<T> {
+        if Arc::ptr_eq(&self.inner, &dest.inner) {
+            match dest.pop() {
+                None => return Steal::Empty,
+                Some(task) => return Steal::Success(task),
+            }
+        }
+
+        // Load the front index.
+        let mut f = self.inner.front.load(Ordering::Acquire);
+
+        // A SeqCst fence is needed here.
+        //
+        // If the current thread is already pinned (reentrantly), we must manually issue the
+        // fence. Otherwise, the following pinning will issue the fence anyway, so we don't
+        // have to.
+        if epoch::is_pinned() {
+            atomic::fence(Ordering::SeqCst);
+        }
+
+        let guard = &epoch::pin();
+
+        // Load the back index.
+        let b = self.inner.back.load(Ordering::Acquire);
+
+        // Is the queue empty?
+        let len = b.wrapping_sub(f);
+        if len <= 0 {
+            return Steal::Empty;
+        }
+
+        // Reserve capacity for the stolen batch.
+        let batch_size = cmp::min((len as usize - 1) / 2, MAX_BATCH - 1);
+        dest.reserve(batch_size);
+        let mut batch_size = batch_size as isize;
+
+        // Get the destination buffer and back index.
+        let dest_buffer = dest.buffer.get();
+        let mut dest_b = dest.inner.back.load(Ordering::Relaxed);
+
+        // Load the buffer
+        let buffer = self.inner.buffer.load(Ordering::Acquire, guard);
+
+        // Read the task at the front.
+        let mut task = unsafe { buffer.deref().read(f) };
+
+        match self.flavor {
+            // Steal a batch of tasks from the front at once.
+            Flavor::Fifo => {
+                // Copy the batch from the source to the destination buffer.
+                match dest.flavor {
+                    Flavor::Fifo => {
+                        for i in 0..batch_size {
+                            unsafe {
+                                let task = buffer.deref().read(f.wrapping_add(i + 1));
+                                dest_buffer.write(dest_b.wrapping_add(i), task);
+                            }
+                        }
+                    }
+                    Flavor::Lifo => {
+                        for i in 0..batch_size {
+                            unsafe {
+                                let task = buffer.deref().read(f.wrapping_add(i + 1));
+                                dest_buffer.write(dest_b.wrapping_add(batch_size - 1 - i), task);
+                            }
+                        }
+                    }
+                }
+
+                // Try incrementing the front index to steal the task.
+                // If the buffer has been swapped or the increment fails, we retry.
+                if self.inner.buffer.load(Ordering::Acquire, guard) != buffer
+                    || self
+                        .inner
+                        .front
+                        .compare_exchange(
+                            f,
+                            f.wrapping_add(batch_size + 1),
+                            Ordering::SeqCst,
+                            Ordering::Relaxed,
+                        )
+                        .is_err()
+                {
+                    // We didn't steal this task, forget it.
+                    mem::forget(task);
+                    return Steal::Retry;
+                }
+
+                dest_b = dest_b.wrapping_add(batch_size);
+            }
+
+            // Steal a batch of tasks from the front one by one.
+            Flavor::Lifo => {
+                // Try incrementing the front index to steal the task.
+                if self
+                    .inner
+                    .front
+                    .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed)
+                    .is_err()
+                {
+                    // We didn't steal this task, forget it.
+                    mem::forget(task);
+                    return Steal::Retry;
+                }
+
+                // Move the front index one step forward.
+                f = f.wrapping_add(1);
+
+                // Repeat the same procedure for the batch steals.
+                //
+                // This loop may modify the batch_size, which triggers a clippy lint warning.
+                // Use a new variable to avoid the warning, and to make it clear we aren't
+                // modifying the loop exit condition during iteration.
+                let original_batch_size = batch_size;
+                for i in 0..original_batch_size {
+                    // We've already got the current front index. Now execute the fence to
+                    // synchronize with other threads.
+                    atomic::fence(Ordering::SeqCst);
+
+                    // Load the back index.
+                    let b = self.inner.back.load(Ordering::Acquire);
+
+                    // Is the queue empty?
+                    if b.wrapping_sub(f) <= 0 {
+                        batch_size = i;
+                        break;
+                    }
+
+                    // Read the task at the front.
+                    let tmp = unsafe { buffer.deref().read(f) };
+
+                    // Try incrementing the front index to steal the task.
+                    // If the buffer has been swapped or the increment fails, we retry.
+                    if self.inner.buffer.load(Ordering::Acquire, guard) != buffer
+                        || self
+                            .inner
+                            .front
+                            .compare_exchange(
+                                f,
+                                f.wrapping_add(1),
+                                Ordering::SeqCst,
+                                Ordering::Relaxed,
+                            )
+                            .is_err()
+                    {
+                        // We didn't steal this task, forget it and break from the loop.
+                        mem::forget(tmp);
+                        batch_size = i;
+                        break;
+                    }
+
+                    // Write the previously stolen task into the destination buffer.
+                    unsafe {
+                        dest_buffer.write(dest_b, mem::replace(&mut task, tmp));
+                    }
+
+                    // Move the source front index and the destination back index one step forward.
+                    f = f.wrapping_add(1);
+                    dest_b = dest_b.wrapping_add(1);
+                }
+
+                // If stealing into a FIFO queue, stolen tasks need to be reversed.
+                if dest.flavor == Flavor::Fifo {
+                    for i in 0..batch_size / 2 {
+                        unsafe {
+                            let i1 = dest_b.wrapping_sub(batch_size - i);
+                            let i2 = dest_b.wrapping_sub(i + 1);
+                            let t1 = dest_buffer.read(i1);
+                            let t2 = dest_buffer.read(i2);
+                            dest_buffer.write(i1, t2);
+                            dest_buffer.write(i2, t1);
+                        }
+                    }
+                }
+            }
+        }
+
+        atomic::fence(Ordering::Release);
+
+        // Update the back index in the destination queue.
+        //
+        // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data
+        // races because it doesn't understand fences.
+        dest.inner.back.store(dest_b, Ordering::Release);
+
+        // Return with success.
+        Steal::Success(task)
+    }
+}
+
+impl<T> Clone for Stealer<T> {
+    fn clone(&self) -> Stealer<T> {
+        Stealer {
+            inner: self.inner.clone(),
+            flavor: self.flavor,
+        }
+    }
+}
+
+impl<T> fmt::Debug for Stealer<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Stealer { .. }")
+    }
+}
+
+// Bits indicating the state of a slot:
+// * If a task has been written into the slot, `WRITE` is set.
+// * If a task has been read from the slot, `READ` is set.
+// * If the block is being destroyed, `DESTROY` is set.
+const WRITE: usize = 1;
+const READ: usize = 2;
+const DESTROY: usize = 4;
+
+// Each block covers one "lap" of indices.
+const LAP: usize = 64;
+// The maximum number of values a block can hold.
+const BLOCK_CAP: usize = LAP - 1;
+// How many lower bits are reserved for metadata.
+const SHIFT: usize = 1;
+// Indicates that the block is not the last one.
+const HAS_NEXT: usize = 1;
+
+/// A slot in a block.
+struct Slot<T> {
+    /// The task.
+    task: UnsafeCell<MaybeUninit<T>>,
+
+    /// The state of the slot.
+    state: AtomicUsize,
+}
+
+impl<T> Slot<T> {
+    /// Waits until a task is written into the slot.
+    fn wait_write(&self) {
+        let backoff = Backoff::new();
+        while self.state.load(Ordering::Acquire) & WRITE == 0 {
+            backoff.snooze();
+        }
+    }
+}
+
+/// A block in a linked list.
+///
+/// Each block in the list can hold up to `BLOCK_CAP` values.
+struct Block<T> {
+    /// The next block in the linked list.
+    next: AtomicPtr<Block<T>>,
+
+    /// Slots for values.
+    slots: [Slot<T>; BLOCK_CAP],
+}
+
+impl<T> Block<T> {
+    /// Creates an empty block that starts at `start_index`.
+    fn new() -> Block<T> {
+        // SAFETY: This is safe because:
+        //  [1] `Block::next` (AtomicPtr) may be safely zero initialized.
+        //  [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4].
+        //  [3] `Slot::task` (UnsafeCell) may be safely zero initialized because it
+        //       holds a MaybeUninit.
+        //  [4] `Slot::state` (AtomicUsize) may be safely zero initialized.
+        unsafe { MaybeUninit::zeroed().assume_init() }
+    }
+
+    /// Waits until the next pointer is set.
+    fn wait_next(&self) -> *mut Block<T> {
+        let backoff = Backoff::new();
+        loop {
+            let next = self.next.load(Ordering::Acquire);
+            if !next.is_null() {
+                return next;
+            }
+            backoff.snooze();
+        }
+    }
+
+    /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
+    unsafe fn destroy(this: *mut Block<T>, count: usize) {
+        // It is not necessary to set the `DESTROY` bit in the last slot because that slot has
+        // begun destruction of the block.
+        for i in (0..count).rev() {
+            let slot = (*this).slots.get_unchecked(i);
+
+            // Mark the `DESTROY` bit if a thread is still using the slot.
+            if slot.state.load(Ordering::Acquire) & READ == 0
+                && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
+            {
+                // If a thread is still using the slot, it will continue destruction of the block.
+                return;
+            }
+        }
+
+        // No thread is using the block, now it is safe to destroy it.
+        drop(Box::from_raw(this));
+    }
+}
+
+/// A position in a queue.
+struct Position<T> {
+    /// The index in the queue.
+    index: AtomicUsize,
+
+    /// The block in the linked list.
+    block: AtomicPtr<Block<T>>,
+}
+
+/// An injector queue.
+///
+/// This is a FIFO queue that can be shared among multiple threads. Task schedulers typically have
+/// a single injector queue, which is the entry point for new tasks.
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_deque::{Injector, Steal};
+///
+/// let q = Injector::new();
+/// q.push(1);
+/// q.push(2);
+///
+/// assert_eq!(q.steal(), Steal::Success(1));
+/// assert_eq!(q.steal(), Steal::Success(2));
+/// assert_eq!(q.steal(), Steal::Empty);
+/// ```
+pub struct Injector<T> {
+    /// The head of the queue.
+    head: CachePadded<Position<T>>,
+
+    /// The tail of the queue.
+    tail: CachePadded<Position<T>>,
+
+    /// Indicates that dropping a `Injector<T>` may drop values of type `T`.
+    _marker: PhantomData<T>,
+}
+
+unsafe impl<T: Send> Send for Injector<T> {}
+unsafe impl<T: Send> Sync for Injector<T> {}
+
+impl<T> Default for Injector<T> {
+    fn default() -> Self {
+        let block = Box::into_raw(Box::new(Block::<T>::new()));
+        Self {
+            head: CachePadded::new(Position {
+                block: AtomicPtr::new(block),
+                index: AtomicUsize::new(0),
+            }),
+            tail: CachePadded::new(Position {
+                block: AtomicPtr::new(block),
+                index: AtomicUsize::new(0),
+            }),
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<T> Injector<T> {
+    /// Creates a new injector queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Injector;
+    ///
+    /// let q = Injector::<i32>::new();
+    /// ```
+    pub fn new() -> Injector<T> {
+        Self::default()
+    }
+
+    /// Pushes a task into the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Injector;
+    ///
+    /// let w = Injector::new();
+    /// w.push(1);
+    /// w.push(2);
+    /// ```
+    pub fn push(&self, task: T) {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.index.load(Ordering::Acquire);
+        let mut block = self.tail.block.load(Ordering::Acquire);
+        let mut next_block = None;
+
+        loop {
+            // Calculate the offset of the index into the block.
+            let offset = (tail >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+                tail = self.tail.index.load(Ordering::Acquire);
+                block = self.tail.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // If we're going to have to install the next block, allocate it in advance in order to
+            // make the wait for other threads as short as possible.
+            if offset + 1 == BLOCK_CAP && next_block.is_none() {
+                next_block = Some(Box::new(Block::<T>::new()));
+            }
+
+            let new_tail = tail + (1 << SHIFT);
+
+            // Try advancing the tail forward.
+            match self.tail.index.compare_exchange_weak(
+                tail,
+                new_tail,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, install the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next_block = Box::into_raw(next_block.unwrap());
+                        let next_index = new_tail.wrapping_add(1 << SHIFT);
+
+                        self.tail.block.store(next_block, Ordering::Release);
+                        self.tail.index.store(next_index, Ordering::Release);
+                        (*block).next.store(next_block, Ordering::Release);
+                    }
+
+                    // Write the task into the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    slot.task.get().write(MaybeUninit::new(task));
+                    slot.state.fetch_or(WRITE, Ordering::Release);
+
+                    return;
+                },
+                Err(t) => {
+                    tail = t;
+                    block = self.tail.block.load(Ordering::Acquire);
+                    backoff.spin();
+                }
+            }
+        }
+    }
+
+    /// Steals a task from the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::{Injector, Steal};
+    ///
+    /// let q = Injector::new();
+    /// q.push(1);
+    /// q.push(2);
+    ///
+    /// assert_eq!(q.steal(), Steal::Success(1));
+    /// assert_eq!(q.steal(), Steal::Success(2));
+    /// assert_eq!(q.steal(), Steal::Empty);
+    /// ```
+    pub fn steal(&self) -> Steal<T> {
+        let mut head;
+        let mut block;
+        let mut offset;
+
+        let backoff = Backoff::new();
+        loop {
+            head = self.head.index.load(Ordering::Acquire);
+            block = self.head.block.load(Ordering::Acquire);
+
+            // Calculate the offset of the index into the block.
+            offset = (head >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+            } else {
+                break;
+            }
+        }
+
+        let mut new_head = head + (1 << SHIFT);
+
+        if new_head & HAS_NEXT == 0 {
+            atomic::fence(Ordering::SeqCst);
+            let tail = self.tail.index.load(Ordering::Relaxed);
+
+            // If the tail equals the head, that means the queue is empty.
+            if head >> SHIFT == tail >> SHIFT {
+                return Steal::Empty;
+            }
+
+            // If head and tail are not in the same block, set `HAS_NEXT` in head.
+            if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
+                new_head |= HAS_NEXT;
+            }
+        }
+
+        // Try moving the head index forward.
+        if self
+            .head
+            .index
+            .compare_exchange_weak(head, new_head, Ordering::SeqCst, Ordering::Acquire)
+            .is_err()
+        {
+            return Steal::Retry;
+        }
+
+        unsafe {
+            // If we've reached the end of the block, move to the next one.
+            if offset + 1 == BLOCK_CAP {
+                let next = (*block).wait_next();
+                let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT);
+                if !(*next).next.load(Ordering::Relaxed).is_null() {
+                    next_index |= HAS_NEXT;
+                }
+
+                self.head.block.store(next, Ordering::Release);
+                self.head.index.store(next_index, Ordering::Release);
+            }
+
+            // Read the task.
+            let slot = (*block).slots.get_unchecked(offset);
+            slot.wait_write();
+            let task = slot.task.get().read().assume_init();
+
+            // Destroy the block if we've reached the end, or if another thread wanted to destroy
+            // but couldn't because we were busy reading from the slot.
+            if (offset + 1 == BLOCK_CAP)
+                || (slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0)
+            {
+                Block::destroy(block, offset);
+            }
+
+            Steal::Success(task)
+        }
+    }
+
+    /// Steals a batch of tasks and pushes them into a worker.
+    ///
+    /// How many tasks exactly will be stolen is not specified. That said, this method will try to
+    /// steal around half of the tasks in the queue, but also not more than some constant limit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::{Injector, Worker};
+    ///
+    /// let q = Injector::new();
+    /// q.push(1);
+    /// q.push(2);
+    /// q.push(3);
+    /// q.push(4);
+    ///
+    /// let w = Worker::new_fifo();
+    /// let _ = q.steal_batch(&w);
+    /// assert_eq!(w.pop(), Some(1));
+    /// assert_eq!(w.pop(), Some(2));
+    /// ```
+    pub fn steal_batch(&self, dest: &Worker<T>) -> Steal<()> {
+        let mut head;
+        let mut block;
+        let mut offset;
+
+        let backoff = Backoff::new();
+        loop {
+            head = self.head.index.load(Ordering::Acquire);
+            block = self.head.block.load(Ordering::Acquire);
+
+            // Calculate the offset of the index into the block.
+            offset = (head >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+            } else {
+                break;
+            }
+        }
+
+        let mut new_head = head;
+        let advance;
+
+        if new_head & HAS_NEXT == 0 {
+            atomic::fence(Ordering::SeqCst);
+            let tail = self.tail.index.load(Ordering::Relaxed);
+
+            // If the tail equals the head, that means the queue is empty.
+            if head >> SHIFT == tail >> SHIFT {
+                return Steal::Empty;
+            }
+
+            // If head and tail are not in the same block, set `HAS_NEXT` in head. Also, calculate
+            // the right batch size to steal.
+            if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
+                new_head |= HAS_NEXT;
+                // We can steal all tasks till the end of the block.
+                advance = (BLOCK_CAP - offset).min(MAX_BATCH);
+            } else {
+                let len = (tail - head) >> SHIFT;
+                // Steal half of the available tasks.
+                advance = ((len + 1) / 2).min(MAX_BATCH);
+            }
+        } else {
+            // We can steal all tasks till the end of the block.
+            advance = (BLOCK_CAP - offset).min(MAX_BATCH);
+        }
+
+        new_head += advance << SHIFT;
+        let new_offset = offset + advance;
+
+        // Try moving the head index forward.
+        if self
+            .head
+            .index
+            .compare_exchange_weak(head, new_head, Ordering::SeqCst, Ordering::Acquire)
+            .is_err()
+        {
+            return Steal::Retry;
+        }
+
+        // Reserve capacity for the stolen batch.
+        let batch_size = new_offset - offset;
+        dest.reserve(batch_size);
+
+        // Get the destination buffer and back index.
+        let dest_buffer = dest.buffer.get();
+        let dest_b = dest.inner.back.load(Ordering::Relaxed);
+
+        unsafe {
+            // If we've reached the end of the block, move to the next one.
+            if new_offset == BLOCK_CAP {
+                let next = (*block).wait_next();
+                let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT);
+                if !(*next).next.load(Ordering::Relaxed).is_null() {
+                    next_index |= HAS_NEXT;
+                }
+
+                self.head.block.store(next, Ordering::Release);
+                self.head.index.store(next_index, Ordering::Release);
+            }
+
+            // Copy values from the injector into the destination queue.
+            match dest.flavor {
+                Flavor::Fifo => {
+                    for i in 0..batch_size {
+                        // Read the task.
+                        let slot = (*block).slots.get_unchecked(offset + i);
+                        slot.wait_write();
+                        let task = slot.task.get().read().assume_init();
+
+                        // Write it into the destination queue.
+                        dest_buffer.write(dest_b.wrapping_add(i as isize), task);
+                    }
+                }
+
+                Flavor::Lifo => {
+                    for i in 0..batch_size {
+                        // Read the task.
+                        let slot = (*block).slots.get_unchecked(offset + i);
+                        slot.wait_write();
+                        let task = slot.task.get().read().assume_init();
+
+                        // Write it into the destination queue.
+                        dest_buffer.write(dest_b.wrapping_add((batch_size - 1 - i) as isize), task);
+                    }
+                }
+            }
+
+            atomic::fence(Ordering::Release);
+
+            // Update the back index in the destination queue.
+            //
+            // This ordering could be `Relaxed`, but then thread sanitizer would falsely report
+            // data races because it doesn't understand fences.
+            dest.inner
+                .back
+                .store(dest_b.wrapping_add(batch_size as isize), Ordering::Release);
+
+            // Destroy the block if we've reached the end, or if another thread wanted to destroy
+            // but couldn't because we were busy reading from the slot.
+            if new_offset == BLOCK_CAP {
+                Block::destroy(block, offset);
+            } else {
+                for i in offset..new_offset {
+                    let slot = (*block).slots.get_unchecked(i);
+
+                    if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
+                        Block::destroy(block, offset);
+                        break;
+                    }
+                }
+            }
+
+            Steal::Success(())
+        }
+    }
+
+    /// Steals a batch of tasks, pushes them into a worker, and pops a task from that worker.
+    ///
+    /// How many tasks exactly will be stolen is not specified. That said, this method will try to
+    /// steal around half of the tasks in the queue, but also not more than some constant limit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::{Injector, Steal, Worker};
+    ///
+    /// let q = Injector::new();
+    /// q.push(1);
+    /// q.push(2);
+    /// q.push(3);
+    /// q.push(4);
+    ///
+    /// let w = Worker::new_fifo();
+    /// assert_eq!(q.steal_batch_and_pop(&w), Steal::Success(1));
+    /// assert_eq!(w.pop(), Some(2));
+    /// ```
+    pub fn steal_batch_and_pop(&self, dest: &Worker<T>) -> Steal<T> {
+        let mut head;
+        let mut block;
+        let mut offset;
+
+        let backoff = Backoff::new();
+        loop {
+            head = self.head.index.load(Ordering::Acquire);
+            block = self.head.block.load(Ordering::Acquire);
+
+            // Calculate the offset of the index into the block.
+            offset = (head >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+            } else {
+                break;
+            }
+        }
+
+        let mut new_head = head;
+        let advance;
+
+        if new_head & HAS_NEXT == 0 {
+            atomic::fence(Ordering::SeqCst);
+            let tail = self.tail.index.load(Ordering::Relaxed);
+
+            // If the tail equals the head, that means the queue is empty.
+            if head >> SHIFT == tail >> SHIFT {
+                return Steal::Empty;
+            }
+
+            // If head and tail are not in the same block, set `HAS_NEXT` in head.
+            if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
+                new_head |= HAS_NEXT;
+                // We can steal all tasks till the end of the block.
+                advance = (BLOCK_CAP - offset).min(MAX_BATCH + 1);
+            } else {
+                let len = (tail - head) >> SHIFT;
+                // Steal half of the available tasks.
+                advance = ((len + 1) / 2).min(MAX_BATCH + 1);
+            }
+        } else {
+            // We can steal all tasks till the end of the block.
+            advance = (BLOCK_CAP - offset).min(MAX_BATCH + 1);
+        }
+
+        new_head += advance << SHIFT;
+        let new_offset = offset + advance;
+
+        // Try moving the head index forward.
+        if self
+            .head
+            .index
+            .compare_exchange_weak(head, new_head, Ordering::SeqCst, Ordering::Acquire)
+            .is_err()
+        {
+            return Steal::Retry;
+        }
+
+        // Reserve capacity for the stolen batch.
+        let batch_size = new_offset - offset - 1;
+        dest.reserve(batch_size);
+
+        // Get the destination buffer and back index.
+        let dest_buffer = dest.buffer.get();
+        let dest_b = dest.inner.back.load(Ordering::Relaxed);
+
+        unsafe {
+            // If we've reached the end of the block, move to the next one.
+            if new_offset == BLOCK_CAP {
+                let next = (*block).wait_next();
+                let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT);
+                if !(*next).next.load(Ordering::Relaxed).is_null() {
+                    next_index |= HAS_NEXT;
+                }
+
+                self.head.block.store(next, Ordering::Release);
+                self.head.index.store(next_index, Ordering::Release);
+            }
+
+            // Read the task.
+            let slot = (*block).slots.get_unchecked(offset);
+            slot.wait_write();
+            let task = slot.task.get().read().assume_init();
+
+            match dest.flavor {
+                Flavor::Fifo => {
+                    // Copy values from the injector into the destination queue.
+                    for i in 0..batch_size {
+                        // Read the task.
+                        let slot = (*block).slots.get_unchecked(offset + i + 1);
+                        slot.wait_write();
+                        let task = slot.task.get().read().assume_init();
+
+                        // Write it into the destination queue.
+                        dest_buffer.write(dest_b.wrapping_add(i as isize), task);
+                    }
+                }
+
+                Flavor::Lifo => {
+                    // Copy values from the injector into the destination queue.
+                    for i in 0..batch_size {
+                        // Read the task.
+                        let slot = (*block).slots.get_unchecked(offset + i + 1);
+                        slot.wait_write();
+                        let task = slot.task.get().read().assume_init();
+
+                        // Write it into the destination queue.
+                        dest_buffer.write(dest_b.wrapping_add((batch_size - 1 - i) as isize), task);
+                    }
+                }
+            }
+
+            atomic::fence(Ordering::Release);
+
+            // Update the back index in the destination queue.
+            //
+            // This ordering could be `Relaxed`, but then thread sanitizer would falsely report
+            // data races because it doesn't understand fences.
+            dest.inner
+                .back
+                .store(dest_b.wrapping_add(batch_size as isize), Ordering::Release);
+
+            // Destroy the block if we've reached the end, or if another thread wanted to destroy
+            // but couldn't because we were busy reading from the slot.
+            if new_offset == BLOCK_CAP {
+                Block::destroy(block, offset);
+            } else {
+                for i in offset..new_offset {
+                    let slot = (*block).slots.get_unchecked(i);
+
+                    if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
+                        Block::destroy(block, offset);
+                        break;
+                    }
+                }
+            }
+
+            Steal::Success(task)
+        }
+    }
+
+    /// Returns `true` if the queue is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Injector;
+    ///
+    /// let q = Injector::new();
+    ///
+    /// assert!(q.is_empty());
+    /// q.push(1);
+    /// assert!(!q.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        let head = self.head.index.load(Ordering::SeqCst);
+        let tail = self.tail.index.load(Ordering::SeqCst);
+        head >> SHIFT == tail >> SHIFT
+    }
+
+    /// Returns the number of tasks in the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Injector;
+    ///
+    /// let q = Injector::new();
+    ///
+    /// assert_eq!(q.len(), 0);
+    /// q.push(1);
+    /// assert_eq!(q.len(), 1);
+    /// q.push(1);
+    /// assert_eq!(q.len(), 2);
+    /// ```
+    pub fn len(&self) -> usize {
+        loop {
+            // Load the tail index, then load the head index.
+            let mut tail = self.tail.index.load(Ordering::SeqCst);
+            let mut head = self.head.index.load(Ordering::SeqCst);
+
+            // If the tail index didn't change, we've got consistent indices to work with.
+            if self.tail.index.load(Ordering::SeqCst) == tail {
+                // Erase the lower bits.
+                tail &= !((1 << SHIFT) - 1);
+                head &= !((1 << SHIFT) - 1);
+
+                // Fix up indices if they fall onto block ends.
+                if (tail >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    tail = tail.wrapping_add(1 << SHIFT);
+                }
+                if (head >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    head = head.wrapping_add(1 << SHIFT);
+                }
+
+                // Rotate indices so that head falls into the first block.
+                let lap = (head >> SHIFT) / LAP;
+                tail = tail.wrapping_sub((lap * LAP) << SHIFT);
+                head = head.wrapping_sub((lap * LAP) << SHIFT);
+
+                // Remove the lower bits.
+                tail >>= SHIFT;
+                head >>= SHIFT;
+
+                // Return the difference minus the number of blocks between tail and head.
+                return tail - head - tail / LAP;
+            }
+        }
+    }
+}
+
+impl<T> Drop for Injector<T> {
+    fn drop(&mut self) {
+        let mut head = self.head.index.load(Ordering::Relaxed);
+        let mut tail = self.tail.index.load(Ordering::Relaxed);
+        let mut block = self.head.block.load(Ordering::Relaxed);
+
+        // Erase the lower bits.
+        head &= !((1 << SHIFT) - 1);
+        tail &= !((1 << SHIFT) - 1);
+
+        unsafe {
+            // Drop all values between `head` and `tail` and deallocate the heap-allocated blocks.
+            while head != tail {
+                let offset = (head >> SHIFT) % LAP;
+
+                if offset < BLOCK_CAP {
+                    // Drop the task in the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    let p = &mut *slot.task.get();
+                    p.as_mut_ptr().drop_in_place();
+                } else {
+                    // Deallocate the block and move to the next one.
+                    let next = (*block).next.load(Ordering::Relaxed);
+                    drop(Box::from_raw(block));
+                    block = next;
+                }
+
+                head = head.wrapping_add(1 << SHIFT);
+            }
+
+            // Deallocate the last remaining block.
+            drop(Box::from_raw(block));
+        }
+    }
+}
+
+impl<T> fmt::Debug for Injector<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Worker { .. }")
+    }
+}
+
+/// Possible outcomes of a steal operation.
+///
+/// # Examples
+///
+/// There are lots of ways to chain results of steal operations together:
+///
+/// ```
+/// use crossbeam_deque::Steal::{self, Empty, Retry, Success};
+///
+/// let collect = |v: Vec<Steal<i32>>| v.into_iter().collect::<Steal<i32>>();
+///
+/// assert_eq!(collect(vec![Empty, Empty, Empty]), Empty);
+/// assert_eq!(collect(vec![Empty, Retry, Empty]), Retry);
+/// assert_eq!(collect(vec![Retry, Success(1), Empty]), Success(1));
+///
+/// assert_eq!(collect(vec![Empty, Empty]).or_else(|| Retry), Retry);
+/// assert_eq!(collect(vec![Retry, Empty]).or_else(|| Success(1)), Success(1));
+/// ```
+#[must_use]
+#[derive(PartialEq, Eq, Copy, Clone)]
+pub enum Steal<T> {
+    /// The queue was empty at the time of stealing.
+    Empty,
+
+    /// At least one task was successfully stolen.
+    Success(T),
+
+    /// The steal operation needs to be retried.
+    Retry,
+}
+
+impl<T> Steal<T> {
+    /// Returns `true` if the queue was empty at the time of stealing.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Steal::{Empty, Retry, Success};
+    ///
+    /// assert!(!Success(7).is_empty());
+    /// assert!(!Retry::<i32>.is_empty());
+    ///
+    /// assert!(Empty::<i32>.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        match self {
+            Steal::Empty => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if at least one task was stolen.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Steal::{Empty, Retry, Success};
+    ///
+    /// assert!(!Empty::<i32>.is_success());
+    /// assert!(!Retry::<i32>.is_success());
+    ///
+    /// assert!(Success(7).is_success());
+    /// ```
+    pub fn is_success(&self) -> bool {
+        match self {
+            Steal::Success(_) => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the steal operation needs to be retried.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Steal::{Empty, Retry, Success};
+    ///
+    /// assert!(!Empty::<i32>.is_retry());
+    /// assert!(!Success(7).is_retry());
+    ///
+    /// assert!(Retry::<i32>.is_retry());
+    /// ```
+    pub fn is_retry(&self) -> bool {
+        match self {
+            Steal::Retry => true,
+            _ => false,
+        }
+    }
+
+    /// Returns the result of the operation, if successful.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Steal::{Empty, Retry, Success};
+    ///
+    /// assert_eq!(Empty::<i32>.success(), None);
+    /// assert_eq!(Retry::<i32>.success(), None);
+    ///
+    /// assert_eq!(Success(7).success(), Some(7));
+    /// ```
+    pub fn success(self) -> Option<T> {
+        match self {
+            Steal::Success(res) => Some(res),
+            _ => None,
+        }
+    }
+
+    /// If no task was stolen, attempts another steal operation.
+    ///
+    /// Returns this steal result if it is `Success`. Otherwise, closure `f` is invoked and then:
+    ///
+    /// * If the second steal resulted in `Success`, it is returned.
+    /// * If both steals were unsuccessful but any resulted in `Retry`, then `Retry` is returned.
+    /// * If both resulted in `None`, then `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_deque::Steal::{Empty, Retry, Success};
+    ///
+    /// assert_eq!(Success(1).or_else(|| Success(2)), Success(1));
+    /// assert_eq!(Retry.or_else(|| Success(2)), Success(2));
+    ///
+    /// assert_eq!(Retry.or_else(|| Empty), Retry::<i32>);
+    /// assert_eq!(Empty.or_else(|| Retry), Retry::<i32>);
+    ///
+    /// assert_eq!(Empty.or_else(|| Empty), Empty::<i32>);
+    /// ```
+    pub fn or_else<F>(self, f: F) -> Steal<T>
+    where
+        F: FnOnce() -> Steal<T>,
+    {
+        match self {
+            Steal::Empty => f(),
+            Steal::Success(_) => self,
+            Steal::Retry => {
+                if let Steal::Success(res) = f() {
+                    Steal::Success(res)
+                } else {
+                    Steal::Retry
+                }
+            }
+        }
+    }
+}
+
+impl<T> fmt::Debug for Steal<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Steal::Empty => f.pad("Empty"),
+            Steal::Success(_) => f.pad("Success(..)"),
+            Steal::Retry => f.pad("Retry"),
+        }
+    }
+}
+
+impl<T> FromIterator<Steal<T>> for Steal<T> {
+    /// Consumes items until a `Success` is found and returns it.
+    ///
+    /// If no `Success` was found, but there was at least one `Retry`, then returns `Retry`.
+    /// Otherwise, `Empty` is returned.
+    fn from_iter<I>(iter: I) -> Steal<T>
+    where
+        I: IntoIterator<Item = Steal<T>>,
+    {
+        let mut retry = false;
+        for s in iter {
+            match &s {
+                Steal::Empty => {}
+                Steal::Success(_) => return s,
+                Steal::Retry => retry = true,
+            }
+        }
+
+        if retry {
+            Steal::Retry
+        } else {
+            Steal::Empty
+        }
+    }
+}
diff --git a/vendor/crossbeam-deque/src/lib.rs b/vendor/crossbeam-deque/src/lib.rs
new file mode 100644
index 000000000..16bc728b3
--- /dev/null
+++ b/vendor/crossbeam-deque/src/lib.rs
@@ -0,0 +1,110 @@
+//! Concurrent work-stealing deques.
+//!
+//! These data structures are most commonly used in work-stealing schedulers. The typical setup
+//! involves a number of threads, each having its own FIFO or LIFO queue (*worker*). There is also
+//! one global FIFO queue (*injector*) and a list of references to *worker* queues that are able to
+//! steal tasks (*stealers*).
+//!
+//! We spawn a new task onto the scheduler by pushing it into the *injector* queue. Each worker
+//! thread waits in a loop until it finds the next task to run and then runs it. To find a task, it
+//! first looks into its local *worker* queue, and then into the *injector* and *stealers*.
+//!
+//! # Queues
+//!
+//! [`Injector`] is a FIFO queue, where tasks are pushed and stolen from opposite ends. It is
+//! shared among threads and is usually the entry point for new tasks.
+//!
+//! [`Worker`] has two constructors:
+//!
+//! * [`new_fifo()`] - Creates a FIFO queue, in which tasks are pushed and popped from opposite
+//!   ends.
+//! * [`new_lifo()`] - Creates a LIFO queue, in which tasks are pushed and popped from the same
+//!   end.
+//!
+//! Each [`Worker`] is owned by a single thread and supports only push and pop operations.
+//!
+//! Method [`stealer()`] creates a [`Stealer`] that may be shared among threads and can only steal
+//! tasks from its [`Worker`]. Tasks are stolen from the end opposite to where they get pushed.
+//!
+//! # Stealing
+//!
+//! Steal operations come in three flavors:
+//!
+//! 1. [`steal()`] - Steals one task.
+//! 2. [`steal_batch()`] - Steals a batch of tasks and moves them into another worker.
+//! 3. [`steal_batch_and_pop()`] - Steals a batch of tasks, moves them into another queue, and pops
+//!    one task from that worker.
+//!
+//! In contrast to push and pop operations, stealing can spuriously fail with [`Steal::Retry`], in
+//! which case the steal operation needs to be retried.
+//!
+//! # Examples
+//!
+//! Suppose a thread in a work-stealing scheduler is idle and looking for the next task to run. To
+//! find an available task, it might do the following:
+//!
+//! 1. Try popping one task from the local worker queue.
+//! 2. Try stealing a batch of tasks from the global injector queue.
+//! 3. Try stealing one task from another thread using the stealer list.
+//!
+//! An implementation of this work-stealing strategy:
+//!
+//! ```
+//! use crossbeam_deque::{Injector, Stealer, Worker};
+//! use std::iter;
+//!
+//! fn find_task<T>(
+//!     local: &Worker<T>,
+//!     global: &Injector<T>,
+//!     stealers: &[Stealer<T>],
+//! ) -> Option<T> {
+//!     // Pop a task from the local queue, if not empty.
+//!     local.pop().or_else(|| {
+//!         // Otherwise, we need to look for a task elsewhere.
+//!         iter::repeat_with(|| {
+//!             // Try stealing a batch of tasks from the global queue.
+//!             global.steal_batch_and_pop(local)
+//!                 // Or try stealing a task from one of the other threads.
+//!                 .or_else(|| stealers.iter().map(|s| s.steal()).collect())
+//!         })
+//!         // Loop while no task was stolen and any steal operation needs to be retried.
+//!         .find(|s| !s.is_retry())
+//!         // Extract the stolen task, if there is one.
+//!         .and_then(|s| s.success())
+//!     })
+//! }
+//! ```
+//!
+//! [`new_fifo()`]: Worker::new_fifo
+//! [`new_lifo()`]: Worker::new_lifo
+//! [`stealer()`]: Worker::stealer
+//! [`steal()`]: Stealer::steal
+//! [`steal_batch()`]: Stealer::steal_batch
+//! [`steal_batch_and_pop()`]: Stealer::steal_batch_and_pop
+
+#![doc(test(
+    no_crate_inject,
+    attr(
+        deny(warnings, rust_2018_idioms),
+        allow(dead_code, unused_assignments, unused_variables)
+    )
+))]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use cfg_if::cfg_if;
+
+cfg_if! {
+    if #[cfg(feature = "std")] {
+        use crossbeam_epoch as epoch;
+        use crossbeam_utils as utils;
+
+        mod deque;
+        pub use crate::deque::{Injector, Steal, Stealer, Worker};
+    }
+}