1 /* Copyright (c) 2010-2011 Dmitry Vyukov. All rights reserved.
2 * Redistribution and use in source and binary forms, with or without
3 * modification, are permitted provided that the following conditions are met:
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28 //! A mostly lock-free multi-producer, single consumer queue.
30 //! This module contains an implementation of a concurrent MPSC queue. This
31 //! queue can be used to share data between tasks, and is also used as the
32 //! building block of channels in rust.
34 //! Note that the current implementation of this queue has a caveat of the `pop`
35 //! method, and see the method for more information about it. Due to this
36 //! caveat, this queue may not be appropriate for all use-cases.
40 // http://www.1024cores.net/home/lock-free-algorithms
41 // /queues/non-intrusive-mpsc-node-based-queue
43 pub use self::PopResult::*;
47 use alloc::boxed::Box;
49 use core::cell::UnsafeCell;
51 use sync::atomic::{AtomicPtr, Ordering};
53 /// A result of the `pop` function.
54 pub enum PopResult<T> {
55 /// Some data has been popped
57 /// The queue is empty
59 /// The queue is in an inconsistent state. Popping data should succeed, but
60 /// some pushers have yet to make enough progress in order allow a pop to
61 /// succeed. It is recommended that a pop() occur "in the near future" in
62 /// order to see if the sender has made progress or not
67 next: AtomicPtr<Node<T>>,
71 /// The multi-producer single-consumer structure. This is not cloneable, but it
72 /// may be safely shared so long as it is guaranteed that there is only one
73 /// popper at a time (many pushers are allowed).
75 head: AtomicPtr<Node<T>>,
76 tail: UnsafeCell<*mut Node<T>>,
79 unsafe impl<T:Send> Send for Queue<T> { }
80 unsafe impl<T:Send> Sync for Queue<T> { }
83 unsafe fn new(v: Option<T>) -> *mut Node<T> {
84 mem::transmute(box Node {
85 next: AtomicPtr::new(0 as *mut Node<T>),
91 impl<T: Send> Queue<T> {
92 /// Creates a new queue that is safe to share among multiple producers and
94 pub fn new() -> Queue<T> {
95 let stub = unsafe { Node::new(None) };
97 head: AtomicPtr::new(stub),
98 tail: UnsafeCell::new(stub),
102 /// Pushes a new value onto this queue.
103 pub fn push(&self, t: T) {
105 let n = Node::new(Some(t));
106 let prev = self.head.swap(n, Ordering::AcqRel);
107 (*prev).next.store(n, Ordering::Release);
111 /// Pops some data from this queue.
113 /// Note that the current implementation means that this function cannot
114 /// return `Option<T>`. It is possible for this queue to be in an
115 /// inconsistent state where many pushes have succeeded and completely
116 /// finished, but pops cannot return `Some(t)`. This inconsistent state
117 /// happens when a pusher is pre-empted at an inopportune moment.
119 /// This inconsistent state means that this queue does indeed have data, but
120 /// it does not currently have access to it at this time.
121 pub fn pop(&self) -> PopResult<T> {
123 let tail = *self.tail.get();
124 let next = (*tail).next.load(Ordering::Acquire);
127 *self.tail.get() = next;
128 assert!((*tail).value.is_none());
129 assert!((*next).value.is_some());
130 let ret = (*next).value.take().unwrap();
131 let _: Box<Node<T>> = mem::transmute(tail);
135 if self.head.load(Ordering::Acquire) == tail {Empty} else {Inconsistent}
141 impl<T: Send> Drop for Queue<T> {
144 let mut cur = *self.tail.get();
145 while !cur.is_null() {
146 let next = (*cur).next.load(Ordering::Relaxed);
147 let _: Box<Node<T>> = mem::transmute(cur);
158 use sync::mpsc::channel;
159 use super::{Queue, Data, Empty, Inconsistent};
165 let q = Queue::new();
174 let q = Queue::new();
177 Inconsistent | Data(..) => panic!()
179 let (tx, rx) = channel();
182 for _ in range(0, nthreads) {
185 Thread::spawn(move|| {
186 for i in range(0, nmsgs) {
189 tx.send(()).unwrap();
194 while i < nthreads * nmsgs {
196 Empty | Inconsistent => {},
197 Data(_) => { i += 1 }
201 for _ in range(0, nthreads) {