blob: f71b8ff877f6d13b4eddfbbded2d4337f79eda09 [file] [log] [blame]
use crate::task::AtomicWaker;
use std::cell::UnsafeCell;
use std::ptr;
use std::sync::Arc;
use std::sync::atomic::AtomicPtr;
use std::sync::atomic::Ordering::{Relaxed, Acquire, Release, AcqRel};
use super::abort::abort;
use super::task::Task;
pub(super) enum Dequeue<Fut> {
Data(*const Task<Fut>),
pub(super) struct ReadyToRunQueue<Fut> {
// The waker of the task using `FuturesUnordered`.
pub(super) waker: AtomicWaker,
// Head/tail of the readiness queue
pub(super) head: AtomicPtr<Task<Fut>>,
pub(super) tail: UnsafeCell<*const Task<Fut>>,
pub(super) stub: Arc<Task<Fut>>,
/// An MPSC queue into which the tasks containing the futures are inserted
/// whenever the future inside is scheduled for polling.
impl<Fut> ReadyToRunQueue<Fut> {
/// The enqueue function from the 1024cores intrusive MPSC queue algorithm.
pub(super) fn enqueue(&self, task: *const Task<Fut>) {
unsafe {
// This action does not require any coordination
(*task), Relaxed);
// Note that these atomic orderings come from 1024cores
let task = task as *mut _;
let prev = self.head.swap(task, AcqRel);
(*prev), Release);
/// The dequeue function from the 1024cores intrusive MPSC queue algorithm
/// Note that this is unsafe as it required mutual exclusion (only one
/// thread can call this) to be guaranteed elsewhere.
pub(super) unsafe fn dequeue(&self) -> Dequeue<Fut> {
let mut tail = *self.tail.get();
let mut next = (*tail).next_ready_to_run.load(Acquire);
if tail == self.stub() {
if next.is_null() {
return Dequeue::Empty;
*self.tail.get() = next;
tail = next;
next = (*next).next_ready_to_run.load(Acquire);
if !next.is_null() {
*self.tail.get() = next;
debug_assert!(tail != self.stub());
return Dequeue::Data(tail);
if self.head.load(Acquire) as *const _ != tail {
return Dequeue::Inconsistent;
next = (*tail).next_ready_to_run.load(Acquire);
if !next.is_null() {
*self.tail.get() = next;
return Dequeue::Data(tail);
pub(super) fn stub(&self) -> *const Task<Fut> {
impl<Fut> Drop for ReadyToRunQueue<Fut> {
fn drop(&mut self) {
// Once we're in the destructor for `Inner<Fut>` we need to clear out
// the ready to run queue of tasks if there's anything left in there.
// Note that each task has a strong reference count associated with it
// which is owned by the ready to run queue. All tasks should have had
// their futures dropped already by the `FuturesUnordered` destructor
// above, so we're just pulling out tasks and dropping their refcounts.
unsafe {
loop {
match self.dequeue() {
Dequeue::Empty => break,
Dequeue::Inconsistent => abort("inconsistent in drop"),
Dequeue::Data(ptr) => drop(Arc::from_raw(ptr)),