1#![cfg_attr(not(feature = "full"), allow(dead_code))]
2
3use crate::loom::sync::atomic::AtomicUsize;
4use crate::loom::sync::{Arc, Condvar, Mutex};
5
6use std::sync::atomic::Ordering::SeqCst;
7use std::time::Duration;
8
9#[derive(Debug)]
10pub(crate) struct ParkThread {
11 inner: Arc<Inner>,
12}
13
14/// Unblocks a thread that was blocked by `ParkThread`.
15#[derive(Clone, Debug)]
16pub(crate) struct UnparkThread {
17 inner: Arc<Inner>,
18}
19
20#[derive(Debug)]
21struct Inner {
22 state: AtomicUsize,
23 mutex: Mutex<()>,
24 condvar: Condvar,
25}
26
27const EMPTY: usize = 0;
28const PARKED: usize = 1;
29const NOTIFIED: usize = 2;
30
31tokio_thread_local! {
32 static CURRENT_PARKER: ParkThread = ParkThread::new();
33}
34
35// Bit of a hack, but it is only for loom
36#[cfg(loom)]
37tokio_thread_local! {
38 static CURRENT_THREAD_PARK_COUNT: AtomicUsize = AtomicUsize::new(0);
39}
40
41// ==== impl ParkThread ====
42
43impl ParkThread {
44 pub(crate) fn new() -> Self {
45 Self {
46 inner: Arc::new(Inner {
47 state: AtomicUsize::new(EMPTY),
48 mutex: Mutex::new(()),
49 condvar: Condvar::new(),
50 }),
51 }
52 }
53
54 pub(crate) fn unpark(&self) -> UnparkThread {
55 let inner = self.inner.clone();
56 UnparkThread { inner }
57 }
58
59 pub(crate) fn park(&mut self) {
60 #[cfg(loom)]
61 CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));
62 self.inner.park();
63 }
64
65 pub(crate) fn park_timeout(&mut self, duration: Duration) {
66 #[cfg(loom)]
67 CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));
68
69 // Wasm doesn't have threads, so just sleep.
70 #[cfg(not(target_family = "wasm"))]
71 self.inner.park_timeout(duration);
72 #[cfg(target_family = "wasm")]
73 std::thread::sleep(duration);
74 }
75
76 pub(crate) fn shutdown(&mut self) {
77 self.inner.shutdown();
78 }
79}
80
81// ==== impl Inner ====
82
83impl Inner {
84 fn park(&self) {
85 // If we were previously notified then we consume this notification and
86 // return quickly.
87 if self
88 .state
89 .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
90 .is_ok()
91 {
92 return;
93 }
94
95 // Otherwise we need to coordinate going to sleep
96 let mut m = self.mutex.lock();
97
98 match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
99 Ok(_) => {}
100 Err(NOTIFIED) => {
101 // We must read here, even though we know it will be `NOTIFIED`.
102 // This is because `unpark` may have been called again since we read
103 // `NOTIFIED` in the `compare_exchange` above. We must perform an
104 // acquire operation that synchronizes with that `unpark` to observe
105 // any writes it made before the call to unpark. To do that we must
106 // read from the write it made to `state`.
107 let old = self.state.swap(EMPTY, SeqCst);
108 debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
109
110 return;
111 }
112 Err(actual) => panic!("inconsistent park state; actual = {}", actual),
113 }
114
115 loop {
116 m = self.condvar.wait(m).unwrap();
117
118 if self
119 .state
120 .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
121 .is_ok()
122 {
123 // got a notification
124 return;
125 }
126
127 // spurious wakeup, go back to sleep
128 }
129 }
130
131 /// Parks the current thread for at most `dur`.
132 fn park_timeout(&self, dur: Duration) {
133 // Like `park` above we have a fast path for an already-notified thread,
134 // and afterwards we start coordinating for a sleep. Return quickly.
135 if self
136 .state
137 .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
138 .is_ok()
139 {
140 return;
141 }
142
143 if dur == Duration::from_millis(0) {
144 return;
145 }
146
147 let m = self.mutex.lock();
148
149 match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
150 Ok(_) => {}
151 Err(NOTIFIED) => {
152 // We must read again here, see `park`.
153 let old = self.state.swap(EMPTY, SeqCst);
154 debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
155
156 return;
157 }
158 Err(actual) => panic!("inconsistent park_timeout state; actual = {}", actual),
159 }
160
161 // Wait with a timeout, and if we spuriously wake up or otherwise wake up
162 // from a notification, we just want to unconditionally set the state back to
163 // empty, either consuming a notification or un-flagging ourselves as
164 // parked.
165 let (_m, _result) = self.condvar.wait_timeout(m, dur).unwrap();
166
167 match self.state.swap(EMPTY, SeqCst) {
168 NOTIFIED => {} // got a notification, hurray!
169 PARKED => {} // no notification, alas
170 n => panic!("inconsistent park_timeout state: {}", n),
171 }
172 }
173
174 fn unpark(&self) {
175 // To ensure the unparked thread will observe any writes we made before
176 // this call, we must perform a release operation that `park` can
177 // synchronize with. To do that we must write `NOTIFIED` even if `state`
178 // is already `NOTIFIED`. That is why this must be a swap rather than a
179 // compare-and-swap that returns if it reads `NOTIFIED` on failure.
180 match self.state.swap(NOTIFIED, SeqCst) {
181 EMPTY => return, // no one was waiting
182 NOTIFIED => return, // already unparked
183 PARKED => {} // gotta go wake someone up
184 _ => panic!("inconsistent state in unpark"),
185 }
186
187 // There is a period between when the parked thread sets `state` to
188 // `PARKED` (or last checked `state` in the case of a spurious wake
189 // up) and when it actually waits on `cvar`. If we were to notify
190 // during this period it would be ignored and then when the parked
191 // thread went to sleep it would never wake up. Fortunately, it has
192 // `lock` locked at this stage so we can acquire `lock` to wait until
193 // it is ready to receive the notification.
194 //
195 // Releasing `lock` before the call to `notify_one` means that when the
196 // parked thread wakes it doesn't get woken only to have to wait for us
197 // to release `lock`.
198 drop(self.mutex.lock());
199
200 self.condvar.notify_one();
201 }
202
203 fn shutdown(&self) {
204 self.condvar.notify_all();
205 }
206}
207
208impl Default for ParkThread {
209 fn default() -> Self {
210 Self::new()
211 }
212}
213
214// ===== impl UnparkThread =====
215
216impl UnparkThread {
217 pub(crate) fn unpark(&self) {
218 self.inner.unpark();
219 }
220}
221
222use crate::loom::thread::AccessError;
223use std::future::Future;
224use std::marker::PhantomData;
225use std::rc::Rc;
226use std::task::{RawWaker, RawWakerVTable, Waker};
227
228/// Blocks the current thread using a condition variable.
229#[derive(Debug)]
230pub(crate) struct CachedParkThread {
231 _anchor: PhantomData<Rc<()>>,
232}
233
234impl CachedParkThread {
235 /// Creates a new `ParkThread` handle for the current thread.
236 ///
237 /// This type cannot be moved to other threads, so it should be created on
238 /// the thread that the caller intends to park.
239 pub(crate) fn new() -> CachedParkThread {
240 CachedParkThread {
241 _anchor: PhantomData,
242 }
243 }
244
245 pub(crate) fn waker(&self) -> Result<Waker, AccessError> {
246 self.unpark().map(UnparkThread::into_waker)
247 }
248
249 fn unpark(&self) -> Result<UnparkThread, AccessError> {
250 self.with_current(ParkThread::unpark)
251 }
252
253 pub(crate) fn park(&mut self) {
254 self.with_current(|park_thread| park_thread.inner.park())
255 .unwrap();
256 }
257
258 pub(crate) fn park_timeout(&mut self, duration: Duration) {
259 self.with_current(|park_thread| park_thread.inner.park_timeout(duration))
260 .unwrap();
261 }
262
263 /// Gets a reference to the `ParkThread` handle for this thread.
264 fn with_current<F, R>(&self, f: F) -> Result<R, AccessError>
265 where
266 F: FnOnce(&ParkThread) -> R,
267 {
268 CURRENT_PARKER.try_with(|inner| f(inner))
269 }
270
271 pub(crate) fn block_on<F: Future>(&mut self, f: F) -> Result<F::Output, AccessError> {
272 use std::task::Context;
273 use std::task::Poll::Ready;
274
275 // `get_unpark()` should not return a Result
276 let waker = self.waker()?;
277 let mut cx = Context::from_waker(&waker);
278
279 pin!(f);
280
281 loop {
282 if let Ready(v) = crate::runtime::coop::budget(|| f.as_mut().poll(&mut cx)) {
283 return Ok(v);
284 }
285
286 self.park();
287 }
288 }
289}
290
291impl UnparkThread {
292 pub(crate) fn into_waker(self) -> Waker {
293 unsafe {
294 let raw = unparker_to_raw_waker(self.inner);
295 Waker::from_raw(raw)
296 }
297 }
298}
299
300impl Inner {
301 #[allow(clippy::wrong_self_convention)]
302 fn into_raw(this: Arc<Inner>) -> *const () {
303 Arc::into_raw(this) as *const ()
304 }
305
306 unsafe fn from_raw(ptr: *const ()) -> Arc<Inner> {
307 Arc::from_raw(ptr as *const Inner)
308 }
309}
310
311unsafe fn unparker_to_raw_waker(unparker: Arc<Inner>) -> RawWaker {
312 RawWaker::new(
313 Inner::into_raw(unparker),
314 &RawWakerVTable::new(clone, wake, wake_by_ref, drop_waker),
315 )
316}
317
318unsafe fn clone(raw: *const ()) -> RawWaker {
319 Arc::increment_strong_count(raw as *const Inner);
320 unparker_to_raw_waker(Inner::from_raw(raw))
321}
322
323unsafe fn drop_waker(raw: *const ()) {
324 drop(Inner::from_raw(raw));
325}
326
327unsafe fn wake(raw: *const ()) {
328 let unparker = Inner::from_raw(raw);
329 unparker.unpark();
330}
331
332unsafe fn wake_by_ref(raw: *const ()) {
333 let raw = raw as *const Inner;
334 (*raw).unpark();
335}
336
337#[cfg(loom)]
338pub(crate) fn current_thread_park_count() -> usize {
339 CURRENT_THREAD_PARK_COUNT.with(|count| count.load(SeqCst))
340}
341