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 18 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF 19 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE 20 * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF 21 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 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 44 pub(super) use self::PopResult::*; 45 46 use std::cell::UnsafeCell; 47 use std::ptr; 48 use std::sync::atomic::{AtomicPtr, Ordering}; 49 use std::thread; 50 51 /// A result of the `pop` function. 52 pub(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 64 struct 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). 72 pub(super) struct Queue<T> { 73 head: AtomicPtr<Node<T>>, 74 tail: UnsafeCell<*mut Node<T>>, 75 } 76 77 unsafe impl<T: Send> Send for Queue<T> {} 78 unsafe impl<T: Send> Sync for Queue<T> {} 79 80 impl<T> Node<T> { new(v: Option<T>) -> *mut Self81 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 86 impl<T> Queue<T> { 87 /// Creates a new queue that is safe to share among multiple producers and 88 /// one consumer. new() -> Self89 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. push(&self, t: T)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. pop(&self) -> PopResult<T>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. pop_spin(&self) -> Option<T>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 } 162 163 impl<T> Drop for Queue<T> { drop(&mut self)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