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/* 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.
//!
//! This module implements an intrusive MPSC queue. This queue is incredibly
//! unsafe (due to use of unsafe pointers for nodes), and hence is not public.
// http://www.1024cores.net/home/lock-free-algorithms
// /queues/intrusive-mpsc-node-based-queue
use std::cast;
use std::sync::atomics;
use std::ty::Unsafe;
// NB: all links are done as AtomicUint instead of AtomicPtr to allow for static
// initialization.
pub struct Node<T> {
next: atomics::AtomicUint,
data: T,
}
pub struct DummyNode {
next: atomics::AtomicUint,
}
pub struct Queue<T> {
head: atomics::AtomicUint,
tail: Unsafe<*mut Node<T>>,
stub: DummyNode,
}
impl<T: Send> Queue<T> {
pub fn new() -> Queue<T> {
Queue {
head: atomics::AtomicUint::new(0),
tail: Unsafe::new(0 as *mut Node<T>),
stub: DummyNode {
next: atomics::AtomicUint::new(0),
},
}
}
pub unsafe fn push(&self, node: *mut Node<T>) {
(*node).next.store(0, atomics::Release);
let prev = self.head.swap(node as uint, atomics::AcqRel);
// Note that this code is slightly modified to allow static
// initialization of these queues with rust's flavor of static
// initialization.
if prev == 0 {
self.stub.next.store(node as uint, atomics::Release);
} else {
let prev = prev as *mut Node<T>;
(*prev).next.store(node as uint, atomics::Release);
}
}
/// You'll note that the other MPSC queue in std::sync is non-intrusive and
/// returns a `PopResult` here to indicate when the queue is inconsistent.
/// An "inconsistent state" in the other queue means that a pusher has
/// pushed, but it hasn't finished linking the rest of the chain.
///
/// This queue also suffers from this problem, but I currently haven't been
/// able to detangle when this actually happens. This code is translated
/// verbatim from the website above, and is more complicated than the
/// non-intrusive version.
///
/// Right now consumers of this queue must be ready for this fact. Just
/// because `pop` returns `None` does not mean that there is not data
/// on the queue.
pub unsafe fn pop(&self) -> Option<*mut Node<T>> {
let tail = *self.tail.get();
let mut tail = if !tail.is_null() {tail} else {
cast::transmute(&self.stub)
};
let mut next = (*tail).next(atomics::Relaxed);
if tail as uint == &self.stub as *DummyNode as uint {
if next.is_null() {
return None;
}
*self.tail.get() = next;
tail = next;
next = (*next).next(atomics::Relaxed);
}
if !next.is_null() {
*self.tail.get() = next;
return Some(tail);
}
let head = self.head.load(atomics::Acquire) as *mut Node<T>;
if tail != head {
return None;
}
let stub = cast::transmute(&self.stub);
self.push(stub);
next = (*tail).next(atomics::Relaxed);
if !next.is_null() {
*self.tail.get() = next;
return Some(tail);
}
return None
}
}
impl<T: Send> Node<T> {
pub fn new(t: T) -> Node<T> {
Node {
data: t,
next: atomics::AtomicUint::new(0),
}
}
pub unsafe fn next(&self, ord: atomics::Ordering) -> *mut Node<T> {
cast::transmute::<uint, *mut Node<T>>(self.next.load(ord))
}
}
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