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Diffstat (limited to 'vendor/crossbeam-deque/src/deque.rs')
| -rw-r--r-- | vendor/crossbeam-deque/src/deque.rs | 2195 |
1 files changed, 2195 insertions, 0 deletions
diff --git a/vendor/crossbeam-deque/src/deque.rs b/vendor/crossbeam-deque/src/deque.rs new file mode 100644 index 0000000..c37de2d --- /dev/null +++ b/vendor/crossbeam-deque/src/deque.rs @@ -0,0 +1,2195 @@ +use std::cell::{Cell, UnsafeCell}; +use std::cmp; +use std::fmt; +use std::iter::FromIterator; +use std::marker::PhantomData; +use std::mem::{self, ManuallyDrop, 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 = ManuallyDrop::new(Vec::with_capacity(cap)); + let ptr = v.as_mut_ptr(); + + 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. + // We do all the loads at `MaybeUninit` because we might realize, after loading, that we + // don't actually have the right to access this memory. + 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: MaybeUninit<T>) { + ptr::write_volatile(self.at(index).cast::<MaybeUninit<T>>(), 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 load instead. + unsafe fn read(&self, index: isize) -> MaybeUninit<T> { + ptr::read_volatile(self.at(index).cast::<MaybeUninit<T>>()) + } +} + +impl<T> Clone for Buffer<T> { + fn clone(&self) -> Buffer<T> { + *self + } +} + +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.get_mut(); + let f = *self.front.get_mut(); + + 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, MaybeUninit::new(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).assume_init(); + + // 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. Reset to `None`. + 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.map(|t| unsafe { t.assume_init() }) + } + } + } + } +} + +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. + return Steal::Retry; + } + + // Return the stolen task. + Steal::Success(unsafe { task.assume_init() }) + } + + /// 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<()> { + self.steal_batch_with_limit(dest, MAX_BATCH) + } + + /// Steals no more than `limit` 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 the given limit. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_deque::Worker; + /// + /// let w1 = Worker::new_fifo(); + /// w1.push(1); + /// w1.push(2); + /// w1.push(3); + /// w1.push(4); + /// w1.push(5); + /// w1.push(6); + /// + /// let s = w1.stealer(); + /// let w2 = Worker::new_fifo(); + /// + /// let _ = s.steal_batch_with_limit(&w2, 2); + /// assert_eq!(w2.pop(), Some(1)); + /// assert_eq!(w2.pop(), Some(2)); + /// assert_eq!(w2.pop(), None); + /// + /// w1.push(7); + /// w1.push(8); + /// // Setting a large limit does not guarantee that all elements will be popped. In this case, + /// // half of the elements are currently popped, but the number of popped elements is considered + /// // an implementation detail that may be changed in the future. + /// let _ = s.steal_batch_with_limit(&w2, std::usize::MAX); + /// assert_eq!(w2.len(), 3); + /// ``` + pub fn steal_batch_with_limit(&self, dest: &Worker<T>, limit: usize) -> Steal<()> { + assert!(limit > 0); + 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, limit); + 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. + 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> { + self.steal_batch_with_limit_and_pop(dest, MAX_BATCH) + } + + /// Steals no more than `limit` 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 the given limit. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_deque::{Steal, Worker}; + /// + /// let w1 = Worker::new_fifo(); + /// w1.push(1); + /// w1.push(2); + /// w1.push(3); + /// w1.push(4); + /// w1.push(5); + /// w1.push(6); + /// + /// let s = w1.stealer(); + /// let w2 = Worker::new_fifo(); + /// + /// assert_eq!(s.steal_batch_with_limit_and_pop(&w2, 2), Steal::Success(1)); + /// assert_eq!(w2.pop(), Some(2)); + /// assert_eq!(w2.pop(), None); + /// + /// w1.push(7); + /// w1.push(8); + /// // Setting a large limit does not guarantee that all elements will be popped. In this case, + /// // half of the elements are currently popped, but the number of popped elements is considered + /// // an implementation detail that may be changed in the future. + /// assert_eq!(s.steal_batch_with_limit_and_pop(&w2, std::usize::MAX), Steal::Success(3)); + /// assert_eq!(w2.pop(), Some(4)); + /// assert_eq!(w2.pop(), Some(5)); + /// assert_eq!(w2.pop(), None); + /// ``` + pub fn steal_batch_with_limit_and_pop(&self, dest: &Worker<T>, limit: usize) -> Steal<T> { + assert!(limit > 0); + 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, limit - 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. + 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. + 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. + 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(unsafe { task.assume_init() }) + } +} + +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> { + const UNINIT: Self = Self { + task: UnsafeCell::new(MaybeUninit::uninit()), + state: AtomicUsize::new(0), + }; + + /// 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> { + Self { + next: AtomicPtr::new(ptr::null_mut()), + slots: [Slot::UNINIT; BLOCK_CAP], + } + } + + /// 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<()> { + self.steal_batch_with_limit(dest, MAX_BATCH) + } + + /// Steals no more than 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); + /// q.push(5); + /// q.push(6); + /// + /// let w = Worker::new_fifo(); + /// let _ = q.steal_batch_with_limit(&w, 2); + /// assert_eq!(w.pop(), Some(1)); + /// assert_eq!(w.pop(), Some(2)); + /// assert_eq!(w.pop(), None); + /// + /// q.push(7); + /// q.push(8); + /// // Setting a large limit does not guarantee that all elements will be popped. In this case, + /// // half of the elements are currently popped, but the number of popped elements is considered + /// // an implementation detail that may be changed in the future. + /// let _ = q.steal_batch_with_limit(&w, std::usize::MAX); + /// assert_eq!(w.len(), 3); + /// ``` + pub fn steal_batch_with_limit(&self, dest: &Worker<T>, limit: usize) -> Steal<()> { + assert!(limit > 0); + 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(limit); + } else { + let len = (tail - head) >> SHIFT; + // Steal half of the available tasks. + advance = ((len + 1) / 2).min(limit); + } + } else { + // We can steal all tasks till the end of the block. + advance = (BLOCK_CAP - offset).min(limit); + } + + 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(); + + // 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(); + + // 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> { + // TODO: we use `MAX_BATCH + 1` as the hard limit for Injecter as the performance is slightly + // better, but we may change it in the future to be compatible with the same method in Stealer. + self.steal_batch_with_limit_and_pop(dest, MAX_BATCH + 1) + } + + /// Steals no more than `limit` 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 the given limit. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_deque::{Injector, Steal, Worker}; + /// + /// let q = Injector::new(); + /// q.push(1); + /// q.push(2); + /// q.push(3); + /// q.push(4); + /// q.push(5); + /// q.push(6); + /// + /// let w = Worker::new_fifo(); + /// assert_eq!(q.steal_batch_with_limit_and_pop(&w, 2), Steal::Success(1)); + /// assert_eq!(w.pop(), Some(2)); + /// assert_eq!(w.pop(), None); + /// + /// q.push(7); + /// // Setting a large limit does not guarantee that all elements will be popped. In this case, + /// // half of the elements are currently popped, but the number of popped elements is considered + /// // an implementation detail that may be changed in the future. + /// assert_eq!(q.steal_batch_with_limit_and_pop(&w, std::usize::MAX), Steal::Success(3)); + /// assert_eq!(w.pop(), Some(4)); + /// assert_eq!(w.pop(), Some(5)); + /// assert_eq!(w.pop(), None); + /// ``` + pub fn steal_batch_with_limit_and_pop(&self, dest: &Worker<T>, limit: usize) -> Steal<T> { + assert!(limit > 0); + 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(limit); + } else { + let len = (tail - head) >> SHIFT; + // Steal half of the available tasks. + advance = ((len + 1) / 2).min(limit); + } + } else { + // We can steal all tasks till the end of the block. + advance = (BLOCK_CAP - offset).min(limit); + } + + 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(); + + 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(); + + // 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(); + + // 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.assume_init()) + } + } + + /// 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.get_mut(); + let mut tail = *self.tail.index.get_mut(); + let mut block = *self.head.block.get_mut(); + + // 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.get_mut(); + 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 + } + } +} |