1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
|
/* Copyright (c) 2010-2011 Dmitry Vyukov. All rights reserved.
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY DMITRY VYUKOV "AS IS" AND ANY EXPRESS OR IMPLIED
* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
* SHALL DMITRY VYUKOV OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
* OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* The views and conclusions contained in the software and documentation are
* those of the authors and should not be interpreted as representing official
* policies, either expressed or implied, of Dmitry Vyukov.
*/
//! A mostly lock-free multi-producer, single consumer queue for sending
//! messages between asynchronous tasks.
//!
//! The queue implementation is essentially the same one used for mpsc channels
//! in the standard library.
//!
//! Note that the current implementation of this queue has a caveat of the `pop`
//! method, and see the method for more information about it. Due to this
//! caveat, this queue may not be appropriate for all use-cases.
// http://www.1024cores.net/home/lock-free-algorithms
// /queues/non-intrusive-mpsc-node-based-queue
// NOTE: this implementation is lifted from the standard library and only
// slightly modified
pub(super) use self::PopResult::*;
use std::cell::UnsafeCell;
use std::ptr;
use std::sync::atomic::{AtomicPtr, Ordering};
use std::thread;
/// A result of the `pop` function.
pub(super) enum PopResult<T> {
/// Some data has been popped
Data(T),
/// The queue is empty
Empty,
/// The queue is in an inconsistent state. Popping data should succeed, but
/// some pushers have yet to make enough progress in order allow a pop to
/// succeed. It is recommended that a pop() occur "in the near future" in
/// order to see if the sender has made progress or not
Inconsistent,
}
struct Node<T> {
next: AtomicPtr<Self>,
value: Option<T>,
}
/// The multi-producer single-consumer structure. This is not cloneable, but it
/// may be safely shared so long as it is guaranteed that there is only one
/// popper at a time (many pushers are allowed).
pub(super) struct Queue<T> {
head: AtomicPtr<Node<T>>,
tail: UnsafeCell<*mut Node<T>>,
}
unsafe impl<T: Send> Send for Queue<T> {}
unsafe impl<T: Send> Sync for Queue<T> {}
impl<T> Node<T> {
unsafe fn new(v: Option<T>) -> *mut Self {
Box::into_raw(Box::new(Self { next: AtomicPtr::new(ptr::null_mut()), value: v }))
}
}
impl<T> Queue<T> {
/// Creates a new queue that is safe to share among multiple producers and
/// one consumer.
pub(super) fn new() -> Self {
let stub = unsafe { Node::new(None) };
Self { head: AtomicPtr::new(stub), tail: UnsafeCell::new(stub) }
}
/// Pushes a new value onto this queue.
pub(super) fn push(&self, t: T) {
unsafe {
let n = Node::new(Some(t));
let prev = self.head.swap(n, Ordering::AcqRel);
(*prev).next.store(n, Ordering::Release);
}
}
/// Pops some data from this queue.
///
/// Note that the current implementation means that this function cannot
/// return `Option<T>`. It is possible for this queue to be in an
/// inconsistent state where many pushes have succeeded and completely
/// finished, but pops cannot return `Some(t)`. This inconsistent state
/// happens when a pusher is preempted at an inopportune moment.
///
/// This inconsistent state means that this queue does indeed have data, but
/// it does not currently have access to it at this time.
///
/// This function is unsafe because only one thread can call it at a time.
pub(super) unsafe fn pop(&self) -> PopResult<T> {
let tail = *self.tail.get();
let next = (*tail).next.load(Ordering::Acquire);
if !next.is_null() {
*self.tail.get() = next;
assert!((*tail).value.is_none());
assert!((*next).value.is_some());
let ret = (*next).value.take().unwrap();
drop(Box::from_raw(tail));
return Data(ret);
}
if self.head.load(Ordering::Acquire) == tail {
Empty
} else {
Inconsistent
}
}
/// Pop an element similarly to `pop` function, but spin-wait on inconsistent
/// queue state instead of returning `Inconsistent`.
///
/// This function is unsafe because only one thread can call it at a time.
pub(super) unsafe fn pop_spin(&self) -> Option<T> {
loop {
match self.pop() {
Empty => return None,
Data(t) => return Some(t),
// Inconsistent means that there will be a message to pop
// in a short time. This branch can only be reached if
// values are being produced from another thread, so there
// are a few ways that we can deal with this:
//
// 1) Spin
// 2) thread::yield_now()
// 3) task::current().unwrap() & return Pending
//
// For now, thread::yield_now() is used, but it would
// probably be better to spin a few times then yield.
Inconsistent => {
thread::yield_now();
}
}
}
}
}
impl<T> Drop for Queue<T> {
fn drop(&mut self) {
unsafe {
let mut cur = *self.tail.get();
while !cur.is_null() {
let next = (*cur).next.load(Ordering::Relaxed);
drop(Box::from_raw(cur));
cur = next;
}
}
}
}
|
