1use alloc::sync::{Arc, Weak};
2use core::cell::UnsafeCell;
3use core::sync::atomic::Ordering::{self, Relaxed, SeqCst};
4use core::sync::atomic::{AtomicBool, AtomicPtr};
5
6use super::abort::abort;
7use super::ReadyToRunQueue;
8use crate::task::{waker_ref, ArcWake, WakerRef};
9
10pub(super) struct Task<Fut> {
11 // The future
12 pub(super) future: UnsafeCell<Option<Fut>>,
13
14 // Next pointer for linked list tracking all active tasks (use
15 // `spin_next_all` to read when access is shared across threads)
16 pub(super) next_all: AtomicPtr<Task<Fut>>,
17
18 // Previous task in linked list tracking all active tasks
19 pub(super) prev_all: UnsafeCell<*const Task<Fut>>,
20
21 // Length of the linked list tracking all active tasks when this node was
22 // inserted (use `spin_next_all` to synchronize before reading when access
23 // is shared across threads)
24 pub(super) len_all: UnsafeCell<usize>,
25
26 // Next pointer in ready to run queue
27 pub(super) next_ready_to_run: AtomicPtr<Task<Fut>>,
28
29 // Queue that we'll be enqueued to when woken
30 pub(super) ready_to_run_queue: Weak<ReadyToRunQueue<Fut>>,
31
32 // Whether or not this task is currently in the ready to run queue
33 pub(super) queued: AtomicBool,
34
35 // Whether the future was awoken during polling
36 // It is possible for this flag to be set to true after the polling,
37 // but it will be ignored.
38 pub(super) woken: AtomicBool,
39}
40
41// `Task` can be sent across threads safely because it ensures that
42// the underlying `Fut` type isn't touched from any of its methods.
43//
44// The parent (`super`) module is trusted not to access `future`
45// across different threads.
46unsafe impl<Fut> Send for Task<Fut> {}
47unsafe impl<Fut> Sync for Task<Fut> {}
48
49impl<Fut> ArcWake for Task<Fut> {
50 fn wake_by_ref(arc_self: &Arc<Self>) {
51 let inner = match arc_self.ready_to_run_queue.upgrade() {
52 Some(inner) => inner,
53 None => return,
54 };
55
56 arc_self.woken.store(true, Relaxed);
57
58 // It's our job to enqueue this task it into the ready to run queue. To
59 // do this we set the `queued` flag, and if successful we then do the
60 // actual queueing operation, ensuring that we're only queued once.
61 //
62 // Once the task is inserted call `wake` to notify the parent task,
63 // as it'll want to come along and run our task later.
64 //
65 // Note that we don't change the reference count of the task here,
66 // we merely enqueue the raw pointer. The `FuturesUnordered`
67 // implementation guarantees that if we set the `queued` flag that
68 // there's a reference count held by the main `FuturesUnordered` queue
69 // still.
70 let prev = arc_self.queued.swap(true, SeqCst);
71 if !prev {
72 inner.enqueue(Arc::as_ptr(arc_self));
73 inner.waker.wake();
74 }
75 }
76}
77
78impl<Fut> Task<Fut> {
79 /// Returns a waker reference for this task without cloning the Arc.
80 pub(super) fn waker_ref(this: &Arc<Self>) -> WakerRef<'_> {
81 waker_ref(this)
82 }
83
84 /// Spins until `next_all` is no longer set to `pending_next_all`.
85 ///
86 /// The temporary `pending_next_all` value is typically overwritten fairly
87 /// quickly after a node is inserted into the list of all futures, so this
88 /// should rarely spin much.
89 ///
90 /// When it returns, the correct `next_all` value is returned.
91 ///
92 /// `Relaxed` or `Acquire` ordering can be used. `Acquire` ordering must be
93 /// used before `len_all` can be safely read.
94 #[inline]
95 pub(super) fn spin_next_all(
96 &self,
97 pending_next_all: *mut Self,
98 ordering: Ordering,
99 ) -> *const Self {
100 loop {
101 let next = self.next_all.load(ordering);
102 if next != pending_next_all {
103 return next;
104 }
105 }
106 }
107}
108
109impl<Fut> Drop for Task<Fut> {
110 fn drop(&mut self) {
111 // Since `Task<Fut>` is sent across all threads for any lifetime,
112 // regardless of `Fut`, we, to guarantee memory safety, can't actually
113 // touch `Fut` at any time except when we have a reference to the
114 // `FuturesUnordered` itself .
115 //
116 // Consequently it *should* be the case that we always drop futures from
117 // the `FuturesUnordered` instance. This is a bomb, just in case there's
118 // a bug in that logic.
119 unsafe {
120 if (*self.future.get()).is_some() {
121 abort("future still here when dropping");
122 }
123 }
124 }
125}
126