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:
4 *
5 * 1. Redistributions of source code must retain the above copyright notice,
6 * this list of conditions and the following disclaimer.
7 *
8 * 2. Redistributions in binary form must reproduce the above copyright
9 * notice, this list of conditions and the following disclaimer in the
10 * documentation and/or other materials provided with the distribution.
11 *
12 * THIS SOFTWARE IS PROVIDED BY DMITRY VYUKOV "AS IS" AND ANY EXPRESS OR IMPLIED
13 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
14 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
15 * SHALL DMITRY VYUKOV OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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17 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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20 * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
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24 * those of the authors and should not be interpreted as representing official
25 * policies, either expressed or implied, of Dmitry Vyukov.
26 */
27
28//! A mostly lock-free multi-producer, single consumer queue for sending
29//! messages between asynchronous tasks.
30//!
31//! The queue implementation is essentially the same one used for mpsc channels
32//! in the standard library.
33//!
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.
37
38// http://www.1024cores.net/home/lock-free-algorithms
39// /queues/non-intrusive-mpsc-node-based-queue
40
41// NOTE: this implementation is lifted from the standard library and only
42// slightly modified
43
44pub(super) use self::PopResult::*;
45
46use std::boxed::Box;
47use std::cell::UnsafeCell;
48use std::ptr;
49use std::sync::atomic::{AtomicPtr, Ordering};
50use std::thread;
51
52/// A result of the `pop` function.
53pub(super) enum PopResult<T> {
54 /// Some data has been popped
55 Data(T),
56 /// The queue is empty
57 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
62 Inconsistent,
63}
64
65struct Node<T> {
66 next: AtomicPtr<Self>,
67 value: Option<T>,
68}
69
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).
73pub(super) struct Queue<T> {
74 head: AtomicPtr<Node<T>>,
75 tail: UnsafeCell<*mut Node<T>>,
76}
77
78unsafe impl<T: Send> Send for Queue<T> {}
79unsafe impl<T: Send> Sync for Queue<T> {}
80
81impl<T> Node<T> {
82 unsafe fn new(v: Option<T>) -> *mut Self {
83 Box::into_raw(Box::new(Self { next: AtomicPtr::new(ptr::null_mut()), value: v }))
84 }
85}
86
87impl<T> Queue<T> {
88 /// Creates a new queue that is safe to share among multiple producers and
89 /// one consumer.
90 pub(super) fn new() -> Self {
91 let stub = unsafe { Node::new(None) };
92 Self { head: AtomicPtr::new(stub), tail: UnsafeCell::new(stub) }
93 }
94
95 /// Pushes a new value onto this queue.
96 pub(super) fn push(&self, t: T) {
97 unsafe {
98 let n = Node::new(Some(t));
99 let prev = self.head.swap(n, Ordering::AcqRel);
100 (*prev).next.store(n, Ordering::Release);
101 }
102 }
103
104 /// Pops some data from this queue.
105 ///
106 /// Note that the current implementation means that this function cannot
107 /// return `Option<T>`. It is possible for this queue to be in an
108 /// inconsistent state where many pushes have succeeded and completely
109 /// finished, but pops cannot return `Some(t)`. This inconsistent state
110 /// happens when a pusher is preempted at an inopportune moment.
111 ///
112 /// This inconsistent state means that this queue does indeed have data, but
113 /// it does not currently have access to it at this time.
114 ///
115 /// This function is unsafe because only one thread can call it at a time.
116 pub(super) unsafe fn pop(&self) -> PopResult<T> {
117 unsafe {
118 let tail = *self.tail.get();
119 let next = (*tail).next.load(Ordering::Acquire);
120
121 if !next.is_null() {
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 drop(Box::from_raw(tail));
127 return Data(ret);
128 }
129
130 if self.head.load(Ordering::Acquire) == tail {
131 Empty
132 } else {
133 Inconsistent
134 }
135 }
136 }
137
138 /// Pop an element similarly to `pop` function, but spin-wait on inconsistent
139 /// queue state instead of returning `Inconsistent`.
140 ///
141 /// This function is unsafe because only one thread can call it at a time.
142 pub(super) unsafe fn pop_spin(&self) -> Option<T> {
143 loop {
144 match unsafe { self.pop() } {
145 Empty => return None,
146 Data(t) => return Some(t),
147 // Inconsistent means that there will be a message to pop
148 // in a short time. This branch can only be reached if
149 // values are being produced from another thread, so there
150 // are a few ways that we can deal with this:
151 //
152 // 1) Spin
153 // 2) thread::yield_now()
154 // 3) task::current().unwrap() & return Pending
155 //
156 // For now, thread::yield_now() is used, but it would
157 // probably be better to spin a few times then yield.
158 Inconsistent => {
159 thread::yield_now();
160 }
161 }
162 }
163 }
164}
165
166impl<T> Drop for Queue<T> {
167 fn drop(&mut self) {
168 unsafe {
169 let mut cur: *mut Node = *self.tail.get();
170 while !cur.is_null() {
171 let next: *mut Node = (*cur).next.load(order:Ordering::Relaxed);
172 drop(Box::from_raw(cur));
173 cur = next;
174 }
175 }
176 }
177}
178