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,
16 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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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22 *
23 * The views and conclusions contained in the software and documentation are
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::cell::UnsafeCell;
47use std::ptr;
48use std::sync::atomic::{AtomicPtr, Ordering};
49use std::thread;
50
51/// A result of the `pop` function.
52pub(super) enum PopResult<T> {
53 /// Some data has been popped
54 Data(T),
55 /// The queue is empty
56 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
61 Inconsistent,
62}
63
64struct Node<T> {
65 next: AtomicPtr<Self>,
66 value: Option<T>,
67}
68
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).
72pub(super) struct Queue<T> {
73 head: AtomicPtr<Node<T>>,
74 tail: UnsafeCell<*mut Node<T>>,
75}
76
77unsafe impl<T: Send> Send for Queue<T> {}
78unsafe impl<T: Send> Sync for Queue<T> {}
79
80impl<T> Node<T> {
81 unsafe fn new(v: Option<T>) -> *mut Self {
82 Box::into_raw(Box::new(Self { next: AtomicPtr::new(ptr::null_mut()), value: v }))
83 }
84}
85
86impl<T> Queue<T> {
87 /// Creates a new queue that is safe to share among multiple producers and
88 /// one consumer.
89 pub(super) fn new() -> Self {
90 let stub = unsafe { Node::new(None) };
91 Self { head: AtomicPtr::new(stub), tail: UnsafeCell::new(stub) }
92 }
93
94 /// Pushes a new value onto this queue.
95 pub(super) fn push(&self, t: T) {
96 unsafe {
97 let n = Node::new(Some(t));
98 let prev = self.head.swap(n, Ordering::AcqRel);
99 (*prev).next.store(n, Ordering::Release);
100 }
101 }
102
103 /// Pops some data from this queue.
104 ///
105 /// Note that the current implementation means that this function cannot
106 /// return `Option<T>`. It is possible for this queue to be in an
107 /// inconsistent state where many pushes have succeeded and completely
108 /// finished, but pops cannot return `Some(t)`. This inconsistent state
109 /// happens when a pusher is preempted at an inopportune moment.
110 ///
111 /// This inconsistent state means that this queue does indeed have data, but
112 /// it does not currently have access to it at this time.
113 ///
114 /// This function is unsafe because only one thread can call it at a time.
115 pub(super) unsafe fn pop(&self) -> PopResult<T> {
116 let tail = *self.tail.get();
117 let next = (*tail).next.load(Ordering::Acquire);
118
119 if !next.is_null() {
120 *self.tail.get() = next;
121 assert!((*tail).value.is_none());
122 assert!((*next).value.is_some());
123 let ret = (*next).value.take().unwrap();
124 drop(Box::from_raw(tail));
125 return Data(ret);
126 }
127
128 if self.head.load(Ordering::Acquire) == tail {
129 Empty
130 } else {
131 Inconsistent
132 }
133 }
134
135 /// Pop an element similarly to `pop` function, but spin-wait on inconsistent
136 /// queue state instead of returning `Inconsistent`.
137 ///
138 /// This function is unsafe because only one thread can call it at a time.
139 pub(super) unsafe fn pop_spin(&self) -> Option<T> {
140 loop {
141 match self.pop() {
142 Empty => return None,
143 Data(t) => return Some(t),
144 // Inconsistent means that there will be a message to pop
145 // in a short time. This branch can only be reached if
146 // values are being produced from another thread, so there
147 // are a few ways that we can deal with this:
148 //
149 // 1) Spin
150 // 2) thread::yield_now()
151 // 3) task::current().unwrap() & return Pending
152 //
153 // For now, thread::yield_now() is used, but it would
154 // probably be better to spin a few times then yield.
155 Inconsistent => {
156 thread::yield_now();
157 }
158 }
159 }
160 }
161}
162
163impl<T> Drop for Queue<T> {
164 fn drop(&mut self) {
165 unsafe {
166 let mut cur = *self.tail.get();
167 while !cur.is_null() {
168 let next = (*cur).next.load(Ordering::Relaxed);
169 drop(Box::from_raw(cur));
170 cur = next;
171 }
172 }
173 }
174}
175