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//! The implementation is based on Dmitry Vyukov's bounded MPMC queue. //! //! Source: //! - http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue //! //! Copyright & License: //! - Copyright (c) 2010-2011 Dmitry Vyukov //! - Simplified BSD License and Apache License, Version 2.0 //! - http://www.1024cores.net/home/code-license use std::cell::UnsafeCell; use std::fmt; use std::marker::PhantomData; use std::mem; use std::ptr; use std::sync::atomic::{self, AtomicUsize, Ordering}; use crossbeam_utils::{Backoff, CachePadded}; use err::{PopError, PushError}; /// A slot in a queue. struct Slot<T> { /// The current stamp. /// /// If the stamp equals the tail, this node will be next written to. If it equals head + 1, /// this node will be next read from. stamp: AtomicUsize, /// The value in this slot. value: UnsafeCell<T>, } /// A bounded multi-producer multi-consumer queue. /// /// This queue allocates a fixed-capacity buffer on construction, which is used to store pushed /// elements. The queue cannot hold more elements than the buffer allows. Attempting to push an /// element into a full queue will fail. Having a buffer allocated upfront makes this queue a bit /// faster than [`SegQueue`]. /// /// [`SegQueue`]: struct.SegQueue.html /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PushError}; /// /// let q = ArrayQueue::new(2); /// /// assert_eq!(q.push('a'), Ok(())); /// assert_eq!(q.push('b'), Ok(())); /// assert_eq!(q.push('c'), Err(PushError('c'))); /// assert_eq!(q.pop(), Ok('a')); /// ``` pub struct ArrayQueue<T> { /// The head of the queue. /// /// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a /// single `usize`. The lower bits represent the index, while the upper bits represent the lap. /// /// Elements are popped from the head of the queue. head: CachePadded<AtomicUsize>, /// The tail of the queue. /// /// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a /// single `usize`. The lower bits represent the index, while the upper bits represent the lap. /// /// Elements are pushed into the tail of the queue. tail: CachePadded<AtomicUsize>, /// The buffer holding slots. buffer: *mut Slot<T>, /// The queue capacity. cap: usize, /// A stamp with the value of `{ lap: 1, index: 0 }`. one_lap: usize, /// Indicates that dropping an `ArrayQueue<T>` may drop elements of type `T`. _marker: PhantomData<T>, } unsafe impl<T: Send> Sync for ArrayQueue<T> {} unsafe impl<T: Send> Send for ArrayQueue<T> {} impl<T> ArrayQueue<T> { /// Creates a new bounded queue with the given capacity. /// /// # Panics /// /// Panics if the capacity is zero. /// /// # Examples /// /// ``` /// use crossbeam_queue::ArrayQueue; /// /// let q = ArrayQueue::<i32>::new(100); /// ``` pub fn new(cap: usize) -> ArrayQueue<T> { assert!(cap > 0, "capacity must be non-zero"); // Head is initialized to `{ lap: 0, index: 0 }`. // Tail is initialized to `{ lap: 0, index: 0 }`. let head = 0; let tail = 0; // Allocate a buffer of `cap` slots. let buffer = { let mut v = Vec::<Slot<T>>::with_capacity(cap); let ptr = v.as_mut_ptr(); mem::forget(v); ptr }; // Initialize stamps in the slots. for i in 0..cap { unsafe { // Set the stamp to `{ lap: 0, index: i }`. let slot = buffer.add(i); ptr::write(&mut (*slot).stamp, AtomicUsize::new(i)); } } // One lap is the smallest power of two greater than `cap`. let one_lap = (cap + 1).next_power_of_two(); ArrayQueue { buffer, cap, one_lap, head: CachePadded::new(AtomicUsize::new(head)), tail: CachePadded::new(AtomicUsize::new(tail)), _marker: PhantomData, } } /// Attempts to push an element into the queue. /// /// If the queue is full, the element is returned back as an error. /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PushError}; /// /// let q = ArrayQueue::new(1); /// /// assert_eq!(q.push(10), Ok(())); /// assert_eq!(q.push(20), Err(PushError(20))); /// ``` pub fn push(&self, value: T) -> Result<(), PushError<T>> { let backoff = Backoff::new(); let mut tail = self.tail.load(Ordering::Relaxed); loop { // Deconstruct the tail. let index = tail & (self.one_lap - 1); let lap = tail & !(self.one_lap - 1); // Inspect the corresponding slot. let slot = unsafe { &*self.buffer.add(index) }; let stamp = slot.stamp.load(Ordering::Acquire); // If the tail and the stamp match, we may attempt to push. if tail == stamp { let new_tail = if index + 1 < self.cap { // Same lap, incremented index. // Set to `{ lap: lap, index: index + 1 }`. tail + 1 } else { // One lap forward, index wraps around to zero. // Set to `{ lap: lap.wrapping_add(1), index: 0 }`. lap.wrapping_add(self.one_lap) }; // Try moving the tail. match self.tail.compare_exchange_weak( tail, new_tail, Ordering::SeqCst, Ordering::Relaxed, ) { Ok(_) => { // Write the value into the slot and update the stamp. unsafe { slot.value.get().write(value); } slot.stamp.store(tail + 1, Ordering::Release); return Ok(()); } Err(t) => { tail = t; backoff.spin(); } } } else if stamp.wrapping_add(self.one_lap) == tail + 1 { atomic::fence(Ordering::SeqCst); let head = self.head.load(Ordering::Relaxed); // If the head lags one lap behind the tail as well... if head.wrapping_add(self.one_lap) == tail { // ...then the queue is full. return Err(PushError(value)); } backoff.spin(); tail = self.tail.load(Ordering::Relaxed); } else { // Snooze because we need to wait for the stamp to get updated. backoff.snooze(); tail = self.tail.load(Ordering::Relaxed); } } } /// Attempts to pop an element from the queue. /// /// If the queue is empty, an error is returned. /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PopError}; /// /// let q = ArrayQueue::new(1); /// assert_eq!(q.push(10), Ok(())); /// /// assert_eq!(q.pop(), Ok(10)); /// assert_eq!(q.pop(), Err(PopError)); /// ``` pub fn pop(&self) -> Result<T, PopError> { let backoff = Backoff::new(); let mut head = self.head.load(Ordering::Relaxed); loop { // Deconstruct the head. let index = head & (self.one_lap - 1); let lap = head & !(self.one_lap - 1); // Inspect the corresponding slot. let slot = unsafe { &*self.buffer.add(index) }; let stamp = slot.stamp.load(Ordering::Acquire); // If the the stamp is ahead of the head by 1, we may attempt to pop. if head + 1 == stamp { let new = if index + 1 < self.cap { // Same lap, incremented index. // Set to `{ lap: lap, index: index + 1 }`. head + 1 } else { // One lap forward, index wraps around to zero. // Set to `{ lap: lap.wrapping_add(1), index: 0 }`. lap.wrapping_add(self.one_lap) }; // Try moving the head. match self.head.compare_exchange_weak( head, new, Ordering::SeqCst, Ordering::Relaxed, ) { Ok(_) => { // Read the value from the slot and update the stamp. let msg = unsafe { slot.value.get().read() }; slot.stamp .store(head.wrapping_add(self.one_lap), Ordering::Release); return Ok(msg); } Err(h) => { head = h; backoff.spin(); } } } else if stamp == head { atomic::fence(Ordering::SeqCst); let tail = self.tail.load(Ordering::Relaxed); // If the tail equals the head, that means the channel is empty. if tail == head { return Err(PopError); } backoff.spin(); head = self.head.load(Ordering::Relaxed); } else { // Snooze because we need to wait for the stamp to get updated. backoff.snooze(); head = self.head.load(Ordering::Relaxed); } } } /// Returns the capacity of the queue. /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PopError}; /// /// let q = ArrayQueue::<i32>::new(100); /// /// assert_eq!(q.capacity(), 100); /// ``` pub fn capacity(&self) -> usize { self.cap } /// Returns `true` if the queue is empty. /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PopError}; /// /// let q = ArrayQueue::new(100); /// /// assert!(q.is_empty()); /// q.push(1).unwrap(); /// assert!(!q.is_empty()); /// ``` pub fn is_empty(&self) -> bool { let head = self.head.load(Ordering::SeqCst); let tail = self.tail.load(Ordering::SeqCst); // Is the tail lagging one lap behind head? // Is the tail equal to the head? // // Note: If the head changes just before we load the tail, that means there was a moment // when the channel was not empty, so it is safe to just return `false`. tail == head } /// Returns `true` if the queue is full. /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PopError}; /// /// let q = ArrayQueue::new(1); /// /// assert!(!q.is_full()); /// q.push(1).unwrap(); /// assert!(q.is_full()); /// ``` pub fn is_full(&self) -> bool { let tail = self.tail.load(Ordering::SeqCst); let head = self.head.load(Ordering::SeqCst); // Is the head lagging one lap behind tail? // // Note: If the tail changes just before we load the head, that means there was a moment // when the queue was not full, so it is safe to just return `false`. head.wrapping_add(self.one_lap) == tail } /// Returns the number of elements in the queue. /// /// # Examples /// /// ``` /// use crossbeam_queue::{ArrayQueue, PopError}; /// /// let q = ArrayQueue::new(100); /// assert_eq!(q.len(), 0); /// /// q.push(10).unwrap(); /// assert_eq!(q.len(), 1); /// /// q.push(20).unwrap(); /// assert_eq!(q.len(), 2); /// ``` pub fn len(&self) -> usize { loop { // Load the tail, then load the head. let tail = self.tail.load(Ordering::SeqCst); let head = self.head.load(Ordering::SeqCst); // If the tail didn't change, we've got consistent values to work with. if self.tail.load(Ordering::SeqCst) == tail { let hix = head & (self.one_lap - 1); let tix = tail & (self.one_lap - 1); return if hix < tix { tix - hix } else if hix > tix { self.cap - hix + tix } else if tail == head { 0 } else { self.cap }; } } } } impl<T> Drop for ArrayQueue<T> { fn drop(&mut self) { // Get the index of the head. let hix = self.head.load(Ordering::Relaxed) & (self.one_lap - 1); // Loop over all slots that hold a message and drop them. for i in 0..self.len() { // Compute the index of the next slot holding a message. let index = if hix + i < self.cap { hix + i } else { hix + i - self.cap }; unsafe { self.buffer.add(index).drop_in_place(); } } // Finally, deallocate the buffer, but don't run any destructors. unsafe { Vec::from_raw_parts(self.buffer, 0, self.cap); } } } impl<T> fmt::Debug for ArrayQueue<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("ArrayQueue { .. }") } }