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 threads, 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.
38 // http://www.1024cores.net/home/lock-free-algorithms
39 // /queues/non-intrusive-mpsc-node-based-queue
41 pub use self::PopResult::*;
43 use alloc::boxed::Box;
45 use core::cell::UnsafeCell;
47 use sync::atomic::{AtomicPtr, Ordering};
49 /// A result of the `pop` function.
50 pub enum PopResult<T> {
51 /// Some data has been popped
53 /// The queue is empty
55 /// The queue is in an inconsistent state. Popping data should succeed, but
56 /// some pushers have yet to make enough progress in order allow a pop to
57 /// succeed. It is recommended that a pop() occur "in the near future" in
58 /// order to see if the sender has made progress or not
63 next: AtomicPtr<Node<T>>,
67 /// The multi-producer single-consumer structure. This is not cloneable, but it
68 /// may be safely shared so long as it is guaranteed that there is only one
69 /// popper at a time (many pushers are allowed).
71 head: AtomicPtr<Node<T>>,
72 tail: UnsafeCell<*mut Node<T>>,
75 unsafe impl<T: Send> Send for Queue<T> { }
76 unsafe impl<T: Send> Sync for Queue<T> { }
79 unsafe fn new(v: Option<T>) -> *mut Node<T> {
80 Box::into_raw(box Node {
81 next: AtomicPtr::new(ptr::null_mut()),
88 /// Creates a new queue that is safe to share among multiple producers and
90 pub fn new() -> Queue<T> {
91 let stub = unsafe { Node::new(None) };
93 head: AtomicPtr::new(stub),
94 tail: UnsafeCell::new(stub),
98 /// Pushes a new value onto this queue.
99 pub fn push(&self, t: T) {
101 let n = Node::new(Some(t));
102 let prev = self.head.swap(n, Ordering::AcqRel);
103 (*prev).next.store(n, Ordering::Release);
107 /// Pops some data from this queue.
109 /// Note that the current implementation means that this function cannot
110 /// return `Option<T>`. It is possible for this queue to be in an
111 /// inconsistent state where many pushes have succeeded and completely
112 /// finished, but pops cannot return `Some(t)`. This inconsistent state
113 /// happens when a pusher is pre-empted at an inopportune moment.
115 /// This inconsistent state means that this queue does indeed have data, but
116 /// it does not currently have access to it at this time.
117 pub fn pop(&self) -> PopResult<T> {
119 let tail = *self.tail.get();
120 let next = (*tail).next.load(Ordering::Acquire);
123 *self.tail.get() = next;
124 assert!((*tail).value.is_none());
125 assert!((*next).value.is_some());
126 let ret = (*next).value.take().unwrap();
127 let _: Box<Node<T>> = Box::from_raw(tail);
131 if self.head.load(Ordering::Acquire) == tail {Empty} else {Inconsistent}
136 impl<T> Drop for Queue<T> {
139 let mut cur = *self.tail.get();
140 while !cur.is_null() {
141 let next = (*cur).next.load(Ordering::Relaxed);
142 let _: Box<Node<T>> = Box::from_raw(cur);
153 use sync::mpsc::channel;
154 use super::{Queue, Data, Empty, Inconsistent};
160 let q: Queue<Box<_>> = Queue::new();
169 let q = Queue::new();
172 Inconsistent | Data(..) => panic!()
174 let (tx, rx) = channel();
177 for _ in 0..nthreads {
180 thread::spawn(move|| {
184 tx.send(()).unwrap();
189 while i < nthreads * nmsgs {
191 Empty | Inconsistent => {},
192 Data(_) => { i += 1 }
196 for _ in 0..nthreads {