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
45 use alloc::owned::Box;
49 use atomics::{AtomicPtr, Release, Acquire, AcqRel, Relaxed};
51 /// A result of the `pop` function.
52 pub enum PopResult<T> {
53 /// Some data has been popped
55 /// The queue is empty
57 /// The queue is in an inconsistent state. Popping data should succeed, but
58 /// some pushers have yet to make enough progress in order allow a pop to
59 /// succeed. It is recommended that a pop() occur "in the near future" in
60 /// order to see if the sender has made progress or not
65 next: AtomicPtr<Node<T>>,
69 /// The multi-producer single-consumer structure. This is not cloneable, but it
70 /// may be safely shared so long as it is guaranteed that there is only one
71 /// popper at a time (many pushers are allowed).
73 head: AtomicPtr<Node<T>>,
74 tail: Unsafe<*mut Node<T>>,
78 unsafe fn new(v: Option<T>) -> *mut Node<T> {
79 mem::transmute(box Node {
80 next: AtomicPtr::new(0 as *mut Node<T>),
86 impl<T: Send> Queue<T> {
87 /// Creates a new queue that is safe to share among multiple producers and
89 pub fn new() -> Queue<T> {
90 let stub = unsafe { Node::new(None) };
92 head: AtomicPtr::new(stub),
93 tail: Unsafe::new(stub),
97 /// Pushes a new value onto this queue.
98 pub fn push(&self, t: T) {
100 let n = Node::new(Some(t));
101 let prev = self.head.swap(n, AcqRel);
102 (*prev).next.store(n, Release);
106 /// Pops some data from this queue.
108 /// Note that the current implementation means that this function cannot
109 /// return `Option<T>`. It is possible for this queue to be in an
110 /// inconsistent state where many pushes have succeeded and completely
111 /// finished, but pops cannot return `Some(t)`. This inconsistent state
112 /// happens when a pusher is pre-empted at an inopportune moment.
114 /// This inconsistent state means that this queue does indeed have data, but
115 /// it does not currently have access to it at this time.
116 pub fn pop(&self) -> PopResult<T> {
118 let tail = *self.tail.get();
119 let next = (*tail).next.load(Acquire);
122 *self.tail.get() = next;
123 assert!((*tail).value.is_none());
124 assert!((*next).value.is_some());
125 let ret = (*next).value.take_unwrap();
126 let _: Box<Node<T>> = mem::transmute(tail);
130 if self.head.load(Acquire) == tail {Empty} else {Inconsistent}
134 /// Attempts to pop data from this queue, but doesn't attempt too hard. This
135 /// will canonicalize inconsistent states to a `None` value.
136 pub fn casual_pop(&self) -> Option<T> {
139 Empty | Inconsistent => None,
145 impl<T: Send> Drop for Queue<T> {
148 let mut cur = *self.tail.get();
149 while !cur.is_null() {
150 let next = (*cur).next.load(Relaxed);
151 let _: Box<Node<T>> = mem::transmute(cur);
165 use super::{Queue, Data, Empty, Inconsistent};
169 let q = Queue::new();
178 let q = Queue::new();
181 Inconsistent | Data(..) => fail!()
183 let (tx, rx) = channel();
186 for _ in range(0, nthreads) {
189 native::task::spawn(proc() {
190 for i in range(0, nmsgs) {
198 while i < nthreads * nmsgs {
200 Empty | Inconsistent => {},
201 Data(_) => { i += 1 }
205 for _ in range(0, nthreads) {