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spsc_queue.rs
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spsc_queue.rs
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//! A single-producer single-consumer concurrent queue
//!
//! This module contains the implementation of an SPSC queue which can be used
//! concurrently between two threads. This data structure is safe to use and
//! enforces the semantics that there is one pusher and one popper.
// https://www.1024cores.net/home/lock-free-algorithms/queues/unbounded-spsc-queue
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
use core::cell::UnsafeCell;
use core::ptr;
use crate::boxed::Box;
use crate::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
use super::cache_aligned::CacheAligned;
// Node within the linked list queue of messages to send
struct Node<T> {
// FIXME: this could be an uninitialized T if we're careful enough, and
// that would reduce memory usage (and be a bit faster).
// is it worth it?
value: Option<T>, // nullable for re-use of nodes
cached: bool, // This node goes into the node cache
next: AtomicPtr<Node<T>>, // next node in the queue
}
/// The single-producer single-consumer queue. This structure is not cloneable,
/// but it can be safely shared in an Arc if it is guaranteed that there
/// is only one popper and one pusher touching the queue at any one point in
/// time.
pub struct Queue<T, ProducerAddition = (), ConsumerAddition = ()> {
// consumer fields
consumer: CacheAligned<Consumer<T, ConsumerAddition>>,
// producer fields
producer: CacheAligned<Producer<T, ProducerAddition>>,
}
struct Consumer<T, Addition> {
tail: UnsafeCell<*mut Node<T>>, // where to pop from
tail_prev: AtomicPtr<Node<T>>, // where to pop from
cache_bound: usize, // maximum cache size
cached_nodes: AtomicUsize, // number of nodes marked as cacheable
addition: Addition,
}
struct Producer<T, Addition> {
head: UnsafeCell<*mut Node<T>>, // where to push to
first: UnsafeCell<*mut Node<T>>, // where to get new nodes from
tail_copy: UnsafeCell<*mut Node<T>>, // between first/tail
addition: Addition,
}
unsafe impl<T: Send, P: Send + Sync, C: Send + Sync> Send for Queue<T, P, C> {}
unsafe impl<T: Send, P: Send + Sync, C: Send + Sync> Sync for Queue<T, P, C> {}
impl<T> Node<T> {
fn new() -> *mut Node<T> {
Box::into_raw(box Node {
value: None,
cached: false,
next: AtomicPtr::new(ptr::null_mut::<Node<T>>()),
})
}
}
impl<T, ProducerAddition, ConsumerAddition> Queue<T, ProducerAddition, ConsumerAddition> {
/// Creates a new queue. With given additional elements in the producer and
/// consumer portions of the queue.
///
/// Due to the performance implications of cache-contention,
/// we wish to keep fields used mainly by the producer on a separate cache
/// line than those used by the consumer.
/// Since cache lines are usually 64 bytes, it is unreasonably expensive to
/// allocate one for small fields, so we allow users to insert additional
/// fields into the cache lines already allocated by this for the producer
/// and consumer.
///
/// This is unsafe as the type system doesn't enforce a single
/// consumer-producer relationship. It also allows the consumer to `pop`
/// items while there is a `peek` active due to all methods having a
/// non-mutable receiver.
///
/// # Arguments
///
/// * `bound` - This queue implementation is implemented with a linked
/// list, and this means that a push is always a malloc. In
/// order to amortize this cost, an internal cache of nodes is
/// maintained to prevent a malloc from always being
/// necessary. This bound is the limit on the size of the
/// cache (if desired). If the value is 0, then the cache has
/// no bound. Otherwise, the cache will never grow larger than
/// `bound` (although the queue itself could be much larger.
pub unsafe fn with_additions(
bound: usize,
producer_addition: ProducerAddition,
consumer_addition: ConsumerAddition,
) -> Self {
let n1 = Node::new();
let n2 = Node::new();
(*n1).next.store(n2, Ordering::Relaxed);
Queue {
consumer: CacheAligned::new(Consumer {
tail: UnsafeCell::new(n2),
tail_prev: AtomicPtr::new(n1),
cache_bound: bound,
cached_nodes: AtomicUsize::new(0),
addition: consumer_addition,
}),
producer: CacheAligned::new(Producer {
head: UnsafeCell::new(n2),
first: UnsafeCell::new(n1),
tail_copy: UnsafeCell::new(n1),
addition: producer_addition,
}),
}
}
/// Pushes a new value onto this queue. Note that to use this function
/// safely, it must be externally guaranteed that there is only one pusher.
pub fn push(&self, t: T) {
unsafe {
// Acquire a node (which either uses a cached one or allocates a new
// one), and then append this to the 'head' node.
let n = self.alloc();
assert!((*n).value.is_none());
(*n).value = Some(t);
(*n).next.store(ptr::null_mut(), Ordering::Relaxed);
(**self.producer.head.get()).next.store(n, Ordering::Release);
*(&self.producer.head).get() = n;
}
}
unsafe fn alloc(&self) -> *mut Node<T> {
// First try to see if we can consume the 'first' node for our uses.
if *self.producer.first.get() != *self.producer.tail_copy.get() {
let ret = *self.producer.first.get();
*self.producer.0.first.get() = (*ret).next.load(Ordering::Relaxed);
return ret;
}
// If the above fails, then update our copy of the tail and try
// again.
*self.producer.0.tail_copy.get() = self.consumer.tail_prev.load(Ordering::Acquire);
if *self.producer.first.get() != *self.producer.tail_copy.get() {
let ret = *self.producer.first.get();
*self.producer.0.first.get() = (*ret).next.load(Ordering::Relaxed);
return ret;
}
// If all of that fails, then we have to allocate a new node
// (there's nothing in the node cache).
Node::new()
}
/// Attempts to pop a value from this queue. Remember that to use this type
/// safely you must ensure that there is only one popper at a time.
pub fn pop(&self) -> Option<T> {
unsafe {
// The `tail` node is not actually a used node, but rather a
// sentinel from where we should start popping from. Hence, look at
// tail's next field and see if we can use it. If we do a pop, then
// the current tail node is a candidate for going into the cache.
let tail = *self.consumer.tail.get();
let next = (*tail).next.load(Ordering::Acquire);
if next.is_null() {
return None;
}
assert!((*next).value.is_some());
let ret = (*next).value.take();
*self.consumer.0.tail.get() = next;
if self.consumer.cache_bound == 0 {
self.consumer.tail_prev.store(tail, Ordering::Release);
} else {
let cached_nodes = self.consumer.cached_nodes.load(Ordering::Relaxed);
if cached_nodes < self.consumer.cache_bound && !(*tail).cached {
self.consumer.cached_nodes.store(cached_nodes, Ordering::Relaxed);
(*tail).cached = true;
}
if (*tail).cached {
self.consumer.tail_prev.store(tail, Ordering::Release);
} else {
(*self.consumer.tail_prev.load(Ordering::Relaxed))
.next
.store(next, Ordering::Relaxed);
// We have successfully erased all references to 'tail', so
// now we can safely drop it.
let _: Box<Node<T>> = Box::from_raw(tail);
}
}
ret
}
}
/// Attempts to peek at the head of the queue, returning `None` if the queue
/// has no data currently
///
/// # Warning
/// The reference returned is invalid if it is not used before the consumer
/// pops the value off the queue. If the producer then pushes another value
/// onto the queue, it will overwrite the value pointed to by the reference.
pub fn peek(&self) -> Option<&mut T> {
// This is essentially the same as above with all the popping bits
// stripped out.
unsafe {
let tail = *self.consumer.tail.get();
let next = (*tail).next.load(Ordering::Acquire);
if next.is_null() { None } else { (*next).value.as_mut() }
}
}
pub fn producer_addition(&self) -> &ProducerAddition {
&self.producer.addition
}
pub fn consumer_addition(&self) -> &ConsumerAddition {
&self.consumer.addition
}
}
impl<T, ProducerAddition, ConsumerAddition> Drop for Queue<T, ProducerAddition, ConsumerAddition> {
fn drop(&mut self) {
unsafe {
let mut cur = *self.producer.first.get();
while !cur.is_null() {
let next = (*cur).next.load(Ordering::Relaxed);
let _n: Box<Node<T>> = Box::from_raw(cur);
cur = next;
}
}
}
}