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::*;
44 use core::prelude::v1::*;
46 use alloc::boxed::Box;
48 use core::cell::UnsafeCell;
50 use sync::atomic::{AtomicPtr, Ordering};
52 /// A result of the `pop` function.
53 pub enum PopResult<T> {
54 /// Some data has been popped
56 /// The queue is empty
58 /// The queue is in an inconsistent state. Popping data should succeed, but
59 /// some pushers have yet to make enough progress in order allow a pop to
60 /// succeed. It is recommended that a pop() occur "in the near future" in
61 /// order to see if the sender has made progress or not
66 next: AtomicPtr<Node<T>>,
70 /// The multi-producer single-consumer structure. This is not cloneable, but it
71 /// may be safely shared so long as it is guaranteed that there is only one
72 /// popper at a time (many pushers are allowed).
74 head: AtomicPtr<Node<T>>,
75 tail: UnsafeCell<*mut Node<T>>,
78 unsafe impl<T: Send> Send for Queue<T> { }
79 unsafe impl<T: Send> Sync for Queue<T> { }
82 unsafe fn new(v: Option<T>) -> *mut Node<T> {
83 Box::into_raw(box Node {
84 next: AtomicPtr::new(ptr::null_mut()),
91 /// Creates a new queue that is safe to share among multiple producers and
93 pub fn new() -> Queue<T> {
94 let stub = unsafe { Node::new(None) };
96 head: AtomicPtr::new(stub),
97 tail: UnsafeCell::new(stub),
101 /// Pushes a new value onto this queue.
102 pub fn push(&self, t: T) {
104 let n = Node::new(Some(t));
105 let prev = self.head.swap(n, Ordering::AcqRel);
106 (*prev).next.store(n, Ordering::Release);
110 /// Pops some data from this queue.
112 /// Note that the current implementation means that this function cannot
113 /// return `Option<T>`. It is possible for this queue to be in an
114 /// inconsistent state where many pushes have succeeded and completely
115 /// finished, but pops cannot return `Some(t)`. This inconsistent state
116 /// happens when a pusher is pre-empted at an inopportune moment.
118 /// This inconsistent state means that this queue does indeed have data, but
119 /// it does not currently have access to it at this time.
120 pub fn pop(&self) -> PopResult<T> {
122 let tail = *self.tail.get();
123 let next = (*tail).next.load(Ordering::Acquire);
126 *self.tail.get() = next;
127 assert!((*tail).value.is_none());
128 assert!((*next).value.is_some());
129 let ret = (*next).value.take().unwrap();
130 let _: Box<Node<T>> = Box::from_raw(tail);
134 if self.head.load(Ordering::Acquire) == tail {Empty} else {Inconsistent}
139 #[stable(feature = "rust1", since = "1.0.0")]
140 impl<T> Drop for Queue<T> {
143 let mut cur = *self.tail.get();
144 while !cur.is_null() {
145 let next = (*cur).next.load(Ordering::Relaxed);
146 let _: Box<Node<T>> = Box::from_raw(cur);
157 use sync::mpsc::channel;
158 use super::{Queue, Data, Empty, Inconsistent};
164 let q: Queue<Box<_>> = Queue::new();
173 let q = Queue::new();
176 Inconsistent | Data(..) => panic!()
178 let (tx, rx) = channel();
181 for _ in 0..nthreads {
184 thread::spawn(move|| {
188 tx.send(()).unwrap();
193 while i < nthreads * nmsgs {
195 Empty | Inconsistent => {},
196 Data(_) => { i += 1 }
200 for _ in 0..nthreads {