futures_util/stream/futures_unordered/
ready_to_run_queue.rs

1use crate::task::AtomicWaker;
2use alloc::sync::Arc;
3use core::cell::UnsafeCell;
4use core::ptr;
5use core::sync::atomic::AtomicPtr;
6use core::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release};
7
8use super::abort::abort;
9use super::task::Task;
10
11pub(super) enum Dequeue<Fut> {
12    Data(*const Task<Fut>),
13    Empty,
14    Inconsistent,
15}
16
17pub(super) struct ReadyToRunQueue<Fut> {
18    // The waker of the task using `FuturesUnordered`.
19    pub(super) waker: AtomicWaker,
20
21    // Head/tail of the readiness queue
22    pub(super) head: AtomicPtr<Task<Fut>>,
23    pub(super) tail: UnsafeCell<*const Task<Fut>>,
24    pub(super) stub: Arc<Task<Fut>>,
25}
26
27/// An MPSC queue into which the tasks containing the futures are inserted
28/// whenever the future inside is scheduled for polling.
29impl<Fut> ReadyToRunQueue<Fut> {
30    // FIXME: this takes raw pointer without safety conditions.
31
32    /// The enqueue function from the 1024cores intrusive MPSC queue algorithm.
33    pub(super) fn enqueue(&self, task: *const Task<Fut>) {
34        unsafe {
35            debug_assert!((*task).queued.load(Relaxed));
36
37            // This action does not require any coordination
38            (*task).next_ready_to_run.store(ptr::null_mut(), Relaxed);
39
40            // Note that these atomic orderings come from 1024cores
41            let task = task as *mut _;
42            let prev = self.head.swap(task, AcqRel);
43            (*prev).next_ready_to_run.store(task, Release);
44        }
45    }
46
47    /// The dequeue function from the 1024cores intrusive MPSC queue algorithm
48    ///
49    /// Note that this is unsafe as it required mutual exclusion (only one
50    /// thread can call this) to be guaranteed elsewhere.
51    pub(super) unsafe fn dequeue(&self) -> Dequeue<Fut> {
52        unsafe {
53            let mut tail = *self.tail.get();
54            let mut next = (*tail).next_ready_to_run.load(Acquire);
55
56            if tail == self.stub() {
57                if next.is_null() {
58                    return Dequeue::Empty;
59                }
60
61                *self.tail.get() = next;
62                tail = next;
63                next = (*next).next_ready_to_run.load(Acquire);
64            }
65
66            if !next.is_null() {
67                *self.tail.get() = next;
68                debug_assert!(tail != self.stub());
69                return Dequeue::Data(tail);
70            }
71
72            if self.head.load(Acquire) as *const _ != tail {
73                return Dequeue::Inconsistent;
74            }
75
76            self.enqueue(self.stub());
77
78            next = (*tail).next_ready_to_run.load(Acquire);
79
80            if !next.is_null() {
81                *self.tail.get() = next;
82                return Dequeue::Data(tail);
83            }
84
85            Dequeue::Inconsistent
86        }
87    }
88
89    pub(super) fn stub(&self) -> *const Task<Fut> {
90        Arc::as_ptr(&self.stub)
91    }
92}
93
94impl<Fut> Drop for ReadyToRunQueue<Fut> {
95    fn drop(&mut self) {
96        // Once we're in the destructor for `Inner<Fut>` we need to clear out
97        // the ready to run queue of tasks if there's anything left in there.
98        //
99        // Note that each task has a strong reference count associated with it
100        // which is owned by the ready to run queue. All tasks should have had
101        // their futures dropped already by the `FuturesUnordered` destructor
102        // above, so we're just pulling out tasks and dropping their refcounts.
103        unsafe {
104            loop {
105                match self.dequeue() {
106                    Dequeue::Empty => break,
107                    Dequeue::Inconsistent => abort("inconsistent in drop"),
108                    Dequeue::Data(ptr) => drop(Arc::from_raw(ptr)),
109                }
110            }
111        }
112    }
113}
futures_util/stream/futures_unordered/ready_to_run_queue.rs

futures_util/stream/futures_unordered/
ready_to_run_queue.rs
