1#![forbid(unsafe_op_in_unsafe_fn)]
2use crate::pin::Pin;
3use crate::sync::atomic::Ordering::{Acquire, Release};
4use crate::sys::futex::{self, futex_wait, futex_wake};
5use crate::time::Duration;
6
7type Futex = futex::SmallFutex;
8type State = futex::SmallPrimitive;
9
10const PARKED: State = State::MAX;
11const EMPTY: State = 0;
12const NOTIFIED: State = 1;
13
14pub struct Parker {
15 state: Futex,
16}
17
18// Notes about memory ordering:
19//
20// Memory ordering is only relevant for the relative ordering of operations
21// between different variables. Even Ordering::Relaxed guarantees a
22// monotonic/consistent order when looking at just a single atomic variable.
23//
24// So, since this parker is just a single atomic variable, we only need to look
25// at the ordering guarantees we need to provide to the 'outside world'.
26//
27// The only memory ordering guarantee that parking and unparking provide, is
28// that things which happened before unpark() are visible on the thread
29// returning from park() afterwards. Otherwise, it was effectively unparked
30// before unpark() was called while still consuming the 'token'.
31//
32// In other words, unpark() needs to synchronize with the part of park() that
33// consumes the token and returns.
34//
35// This is done with a release-acquire synchronization, by using
36// Ordering::Release when writing NOTIFIED (the 'token') in unpark(), and using
37// Ordering::Acquire when checking for this state in park().
38impl Parker {
39 /// Constructs the futex parker. The UNIX parker implementation
40 /// requires this to happen in-place.
41 pub unsafe fn new_in_place(parker: *mut Parker) {
42 unsafe { parker.write(Self { state: Futex::new(EMPTY) }) };
43 }
44
45 // Assumes this is only called by the thread that owns the Parker,
46 // which means that `self.state != PARKED`.
47 pub unsafe fn park(self: Pin<&Self>) {
48 // Change NOTIFIED=>EMPTY or EMPTY=>PARKED, and directly return in the
49 // first case.
50 if self.state.fetch_sub(1, Acquire) == NOTIFIED {
51 return;
52 }
53 loop {
54 // Wait for something to happen, assuming it's still set to PARKED.
55 futex_wait(&self.state, PARKED, None);
56 // Change NOTIFIED=>EMPTY and return in that case.
57 if self.state.compare_exchange(NOTIFIED, EMPTY, Acquire, Acquire).is_ok() {
58 return;
59 } else {
60 // Spurious wake up. We loop to try again.
61 }
62 }
63 }
64
65 // Assumes this is only called by the thread that owns the Parker,
66 // which means that `self.state != PARKED`. This implementation doesn't
67 // require `Pin`, but other implementations do.
68 pub unsafe fn park_timeout(self: Pin<&Self>, timeout: Duration) {
69 // Change NOTIFIED=>EMPTY or EMPTY=>PARKED, and directly return in the
70 // first case.
71 if self.state.fetch_sub(1, Acquire) == NOTIFIED {
72 return;
73 }
74 // Wait for something to happen, assuming it's still set to PARKED.
75 futex_wait(&self.state, PARKED, Some(timeout));
76 // This is not just a store, because we need to establish a
77 // release-acquire ordering with unpark().
78 if self.state.swap(EMPTY, Acquire) == NOTIFIED {
79 // Woke up because of unpark().
80 } else {
81 // Timeout or spurious wake up.
82 // We return either way, because we can't easily tell if it was the
83 // timeout or not.
84 }
85 }
86
87 // This implementation doesn't require `Pin`, but other implementations do.
88 #[inline]
89 pub fn unpark(self: Pin<&Self>) {
90 // Change PARKED=>NOTIFIED, EMPTY=>NOTIFIED, or NOTIFIED=>NOTIFIED, and
91 // wake the thread in the first case.
92 //
93 // Note that even NOTIFIED=>NOTIFIED results in a write. This is on
94 // purpose, to make sure every unpark() has a release-acquire ordering
95 // with park().
96 if self.state.swap(NOTIFIED, Release) == PARKED {
97 futex_wake(&self.state);
98 }
99 }
100}
101

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