1use crate::loom::sync::atomic::AtomicUsize;
2
3use std::fmt;
4use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
5use std::usize;
6
7pub(super) struct State {
8 val: AtomicUsize,
9}
10
11/// Current state value.
12#[derive(Copy, Clone)]
13pub(super) struct Snapshot(usize);
14
15type UpdateResult = Result<Snapshot, Snapshot>;
16
17/// The task is currently being run.
18const RUNNING: usize = 0b0001;
19
20/// The task is complete.
21///
22/// Once this bit is set, it is never unset.
23const COMPLETE: usize = 0b0010;
24
25/// Extracts the task's lifecycle value from the state.
26const LIFECYCLE_MASK: usize = 0b11;
27
28/// Flag tracking if the task has been pushed into a run queue.
29const NOTIFIED: usize = 0b100;
30
31/// The join handle is still around.
32#[allow(clippy::unusual_byte_groupings)] // https://github.com/rust-lang/rust-clippy/issues/6556
33const JOIN_INTEREST: usize = 0b1_000;
34
35/// A join handle waker has been set.
36#[allow(clippy::unusual_byte_groupings)] // https://github.com/rust-lang/rust-clippy/issues/6556
37const JOIN_WAKER: usize = 0b10_000;
38
39/// The task has been forcibly cancelled.
40#[allow(clippy::unusual_byte_groupings)] // https://github.com/rust-lang/rust-clippy/issues/6556
41const CANCELLED: usize = 0b100_000;
42
43/// All bits.
44const STATE_MASK: usize = LIFECYCLE_MASK | NOTIFIED | JOIN_INTEREST | JOIN_WAKER | CANCELLED;
45
46/// Bits used by the ref count portion of the state.
47const REF_COUNT_MASK: usize = !STATE_MASK;
48
49/// Number of positions to shift the ref count.
50const REF_COUNT_SHIFT: usize = REF_COUNT_MASK.count_zeros() as usize;
51
52/// One ref count.
53const REF_ONE: usize = 1 << REF_COUNT_SHIFT;
54
55/// State a task is initialized with.
56///
57/// A task is initialized with three references:
58///
59/// * A reference that will be stored in an OwnedTasks or LocalOwnedTasks.
60/// * A reference that will be sent to the scheduler as an ordinary notification.
61/// * A reference for the JoinHandle.
62///
63/// As the task starts with a `JoinHandle`, `JOIN_INTEREST` is set.
64/// As the task starts with a `Notified`, `NOTIFIED` is set.
65const INITIAL_STATE: usize = (REF_ONE * 3) | JOIN_INTEREST | NOTIFIED;
66
67#[must_use]
68pub(super) enum TransitionToRunning {
69 Success,
70 Cancelled,
71 Failed,
72 Dealloc,
73}
74
75#[must_use]
76pub(super) enum TransitionToIdle {
77 Ok,
78 OkNotified,
79 OkDealloc,
80 Cancelled,
81}
82
83#[must_use]
84pub(super) enum TransitionToNotifiedByVal {
85 DoNothing,
86 Submit,
87 Dealloc,
88}
89
90#[must_use]
91pub(crate) enum TransitionToNotifiedByRef {
92 DoNothing,
93 Submit,
94}
95
96/// All transitions are performed via RMW operations. This establishes an
97/// unambiguous modification order.
98impl State {
99 /// Returns a task's initial state.
100 pub(super) fn new() -> State {
101 // The raw task returned by this method has a ref-count of three. See
102 // the comment on INITIAL_STATE for more.
103 State {
104 val: AtomicUsize::new(INITIAL_STATE),
105 }
106 }
107
108 /// Loads the current state, establishes `Acquire` ordering.
109 pub(super) fn load(&self) -> Snapshot {
110 Snapshot(self.val.load(Acquire))
111 }
112
113 /// Attempts to transition the lifecycle to `Running`. This sets the
114 /// notified bit to false so notifications during the poll can be detected.
115 pub(super) fn transition_to_running(&self) -> TransitionToRunning {
116 self.fetch_update_action(|mut next| {
117 let action;
118 assert!(next.is_notified());
119
120 if !next.is_idle() {
121 // This happens if the task is either currently running or if it
122 // has already completed, e.g. if it was cancelled during
123 // shutdown. Consume the ref-count and return.
124 next.ref_dec();
125 if next.ref_count() == 0 {
126 action = TransitionToRunning::Dealloc;
127 } else {
128 action = TransitionToRunning::Failed;
129 }
130 } else {
131 // We are able to lock the RUNNING bit.
132 next.set_running();
133 next.unset_notified();
134
135 if next.is_cancelled() {
136 action = TransitionToRunning::Cancelled;
137 } else {
138 action = TransitionToRunning::Success;
139 }
140 }
141 (action, Some(next))
142 })
143 }
144
145 /// Transitions the task from `Running` -> `Idle`.
146 ///
147 /// Returns `true` if the transition to `Idle` is successful, `false` otherwise.
148 /// The transition to `Idle` fails if the task has been flagged to be
149 /// cancelled.
150 pub(super) fn transition_to_idle(&self) -> TransitionToIdle {
151 self.fetch_update_action(|curr| {
152 assert!(curr.is_running());
153
154 if curr.is_cancelled() {
155 return (TransitionToIdle::Cancelled, None);
156 }
157
158 let mut next = curr;
159 let action;
160 next.unset_running();
161
162 if !next.is_notified() {
163 // Polling the future consumes the ref-count of the Notified.
164 next.ref_dec();
165 if next.ref_count() == 0 {
166 action = TransitionToIdle::OkDealloc;
167 } else {
168 action = TransitionToIdle::Ok;
169 }
170 } else {
171 // The caller will schedule a new notification, so we create a
172 // new ref-count for the notification. Our own ref-count is kept
173 // for now, and the caller will drop it shortly.
174 next.ref_inc();
175 action = TransitionToIdle::OkNotified;
176 }
177
178 (action, Some(next))
179 })
180 }
181
182 /// Transitions the task from `Running` -> `Complete`.
183 pub(super) fn transition_to_complete(&self) -> Snapshot {
184 const DELTA: usize = RUNNING | COMPLETE;
185
186 let prev = Snapshot(self.val.fetch_xor(DELTA, AcqRel));
187 assert!(prev.is_running());
188 assert!(!prev.is_complete());
189
190 Snapshot(prev.0 ^ DELTA)
191 }
192
193 /// Transitions from `Complete` -> `Terminal`, decrementing the reference
194 /// count the specified number of times.
195 ///
196 /// Returns true if the task should be deallocated.
197 pub(super) fn transition_to_terminal(&self, count: usize) -> bool {
198 let prev = Snapshot(self.val.fetch_sub(count * REF_ONE, AcqRel));
199 assert!(
200 prev.ref_count() >= count,
201 "current: {}, sub: {}",
202 prev.ref_count(),
203 count
204 );
205 prev.ref_count() == count
206 }
207
208 /// Transitions the state to `NOTIFIED`.
209 ///
210 /// If no task needs to be submitted, a ref-count is consumed.
211 ///
212 /// If a task needs to be submitted, the ref-count is incremented for the
213 /// new Notified.
214 pub(super) fn transition_to_notified_by_val(&self) -> TransitionToNotifiedByVal {
215 self.fetch_update_action(|mut snapshot| {
216 let action;
217
218 if snapshot.is_running() {
219 // If the task is running, we mark it as notified, but we should
220 // not submit anything as the thread currently running the
221 // future is responsible for that.
222 snapshot.set_notified();
223 snapshot.ref_dec();
224
225 // The thread that set the running bit also holds a ref-count.
226 assert!(snapshot.ref_count() > 0);
227
228 action = TransitionToNotifiedByVal::DoNothing;
229 } else if snapshot.is_complete() || snapshot.is_notified() {
230 // We do not need to submit any notifications, but we have to
231 // decrement the ref-count.
232 snapshot.ref_dec();
233
234 if snapshot.ref_count() == 0 {
235 action = TransitionToNotifiedByVal::Dealloc;
236 } else {
237 action = TransitionToNotifiedByVal::DoNothing;
238 }
239 } else {
240 // We create a new notified that we can submit. The caller
241 // retains ownership of the ref-count they passed in.
242 snapshot.set_notified();
243 snapshot.ref_inc();
244 action = TransitionToNotifiedByVal::Submit;
245 }
246
247 (action, Some(snapshot))
248 })
249 }
250
251 /// Transitions the state to `NOTIFIED`.
252 pub(super) fn transition_to_notified_by_ref(&self) -> TransitionToNotifiedByRef {
253 self.fetch_update_action(|mut snapshot| {
254 if snapshot.is_complete() || snapshot.is_notified() {
255 // There is nothing to do in this case.
256 (TransitionToNotifiedByRef::DoNothing, None)
257 } else if snapshot.is_running() {
258 // If the task is running, we mark it as notified, but we should
259 // not submit as the thread currently running the future is
260 // responsible for that.
261 snapshot.set_notified();
262 (TransitionToNotifiedByRef::DoNothing, Some(snapshot))
263 } else {
264 // The task is idle and not notified. We should submit a
265 // notification.
266 snapshot.set_notified();
267 snapshot.ref_inc();
268 (TransitionToNotifiedByRef::Submit, Some(snapshot))
269 }
270 })
271 }
272
273 /// Transitions the state to `NOTIFIED`, unconditionally increasing the ref count.
274 #[cfg(all(
275 tokio_unstable,
276 tokio_taskdump,
277 feature = "rt",
278 target_os = "linux",
279 any(target_arch = "aarch64", target_arch = "x86", target_arch = "x86_64")
280 ))]
281 pub(super) fn transition_to_notified_for_tracing(&self) {
282 self.fetch_update_action(|mut snapshot| {
283 snapshot.set_notified();
284 snapshot.ref_inc();
285 ((), Some(snapshot))
286 });
287 }
288
289 /// Sets the cancelled bit and transitions the state to `NOTIFIED` if idle.
290 ///
291 /// Returns `true` if the task needs to be submitted to the pool for
292 /// execution.
293 pub(super) fn transition_to_notified_and_cancel(&self) -> bool {
294 self.fetch_update_action(|mut snapshot| {
295 if snapshot.is_cancelled() || snapshot.is_complete() {
296 // Aborts to completed or cancelled tasks are no-ops.
297 (false, None)
298 } else if snapshot.is_running() {
299 // If the task is running, we mark it as cancelled. The thread
300 // running the task will notice the cancelled bit when it
301 // stops polling and it will kill the task.
302 //
303 // The set_notified() call is not strictly necessary but it will
304 // in some cases let a wake_by_ref call return without having
305 // to perform a compare_exchange.
306 snapshot.set_notified();
307 snapshot.set_cancelled();
308 (false, Some(snapshot))
309 } else {
310 // The task is idle. We set the cancelled and notified bits and
311 // submit a notification if the notified bit was not already
312 // set.
313 snapshot.set_cancelled();
314 if !snapshot.is_notified() {
315 snapshot.set_notified();
316 snapshot.ref_inc();
317 (true, Some(snapshot))
318 } else {
319 (false, Some(snapshot))
320 }
321 }
322 })
323 }
324
325 /// Sets the `CANCELLED` bit and attempts to transition to `Running`.
326 ///
327 /// Returns `true` if the transition to `Running` succeeded.
328 pub(super) fn transition_to_shutdown(&self) -> bool {
329 let mut prev = Snapshot(0);
330
331 let _ = self.fetch_update(|mut snapshot| {
332 prev = snapshot;
333
334 if snapshot.is_idle() {
335 snapshot.set_running();
336 }
337
338 // If the task was not idle, the thread currently running the task
339 // will notice the cancelled bit and cancel it once the poll
340 // completes.
341 snapshot.set_cancelled();
342 Some(snapshot)
343 });
344
345 prev.is_idle()
346 }
347
348 /// Optimistically tries to swap the state assuming the join handle is
349 /// __immediately__ dropped on spawn.
350 pub(super) fn drop_join_handle_fast(&self) -> Result<(), ()> {
351 use std::sync::atomic::Ordering::Relaxed;
352
353 // Relaxed is acceptable as if this function is called and succeeds,
354 // then nothing has been done w/ the join handle.
355 //
356 // The moment the join handle is used (polled), the `JOIN_WAKER` flag is
357 // set, at which point the CAS will fail.
358 //
359 // Given this, there is no risk if this operation is reordered.
360 self.val
361 .compare_exchange_weak(
362 INITIAL_STATE,
363 (INITIAL_STATE - REF_ONE) & !JOIN_INTEREST,
364 Release,
365 Relaxed,
366 )
367 .map(|_| ())
368 .map_err(|_| ())
369 }
370
371 /// Tries to unset the JOIN_INTEREST flag.
372 ///
373 /// Returns `Ok` if the operation happens before the task transitions to a
374 /// completed state, `Err` otherwise.
375 pub(super) fn unset_join_interested(&self) -> UpdateResult {
376 self.fetch_update(|curr| {
377 assert!(curr.is_join_interested());
378
379 if curr.is_complete() {
380 return None;
381 }
382
383 let mut next = curr;
384 next.unset_join_interested();
385
386 Some(next)
387 })
388 }
389
390 /// Sets the `JOIN_WAKER` bit.
391 ///
392 /// Returns `Ok` if the bit is set, `Err` otherwise. This operation fails if
393 /// the task has completed.
394 pub(super) fn set_join_waker(&self) -> UpdateResult {
395 self.fetch_update(|curr| {
396 assert!(curr.is_join_interested());
397 assert!(!curr.is_join_waker_set());
398
399 if curr.is_complete() {
400 return None;
401 }
402
403 let mut next = curr;
404 next.set_join_waker();
405
406 Some(next)
407 })
408 }
409
410 /// Unsets the `JOIN_WAKER` bit.
411 ///
412 /// Returns `Ok` has been unset, `Err` otherwise. This operation fails if
413 /// the task has completed.
414 pub(super) fn unset_waker(&self) -> UpdateResult {
415 self.fetch_update(|curr| {
416 assert!(curr.is_join_interested());
417 assert!(curr.is_join_waker_set());
418
419 if curr.is_complete() {
420 return None;
421 }
422
423 let mut next = curr;
424 next.unset_join_waker();
425
426 Some(next)
427 })
428 }
429
430 pub(super) fn ref_inc(&self) {
431 use std::process;
432 use std::sync::atomic::Ordering::Relaxed;
433
434 // Using a relaxed ordering is alright here, as knowledge of the
435 // original reference prevents other threads from erroneously deleting
436 // the object.
437 //
438 // As explained in the [Boost documentation][1], Increasing the
439 // reference counter can always be done with memory_order_relaxed: New
440 // references to an object can only be formed from an existing
441 // reference, and passing an existing reference from one thread to
442 // another must already provide any required synchronization.
443 //
444 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
445 let prev = self.val.fetch_add(REF_ONE, Relaxed);
446
447 // If the reference count overflowed, abort.
448 if prev > isize::MAX as usize {
449 process::abort();
450 }
451 }
452
453 /// Returns `true` if the task should be released.
454 pub(super) fn ref_dec(&self) -> bool {
455 let prev = Snapshot(self.val.fetch_sub(REF_ONE, AcqRel));
456 assert!(prev.ref_count() >= 1);
457 prev.ref_count() == 1
458 }
459
460 /// Returns `true` if the task should be released.
461 pub(super) fn ref_dec_twice(&self) -> bool {
462 let prev = Snapshot(self.val.fetch_sub(2 * REF_ONE, AcqRel));
463 assert!(prev.ref_count() >= 2);
464 prev.ref_count() == 2
465 }
466
467 fn fetch_update_action<F, T>(&self, mut f: F) -> T
468 where
469 F: FnMut(Snapshot) -> (T, Option<Snapshot>),
470 {
471 let mut curr = self.load();
472
473 loop {
474 let (output, next) = f(curr);
475 let next = match next {
476 Some(next) => next,
477 None => return output,
478 };
479
480 let res = self.val.compare_exchange(curr.0, next.0, AcqRel, Acquire);
481
482 match res {
483 Ok(_) => return output,
484 Err(actual) => curr = Snapshot(actual),
485 }
486 }
487 }
488
489 fn fetch_update<F>(&self, mut f: F) -> Result<Snapshot, Snapshot>
490 where
491 F: FnMut(Snapshot) -> Option<Snapshot>,
492 {
493 let mut curr = self.load();
494
495 loop {
496 let next = match f(curr) {
497 Some(next) => next,
498 None => return Err(curr),
499 };
500
501 let res = self.val.compare_exchange(curr.0, next.0, AcqRel, Acquire);
502
503 match res {
504 Ok(_) => return Ok(next),
505 Err(actual) => curr = Snapshot(actual),
506 }
507 }
508 }
509}
510
511// ===== impl Snapshot =====
512
513impl Snapshot {
514 /// Returns `true` if the task is in an idle state.
515 pub(super) fn is_idle(self) -> bool {
516 self.0 & (RUNNING | COMPLETE) == 0
517 }
518
519 /// Returns `true` if the task has been flagged as notified.
520 pub(super) fn is_notified(self) -> bool {
521 self.0 & NOTIFIED == NOTIFIED
522 }
523
524 fn unset_notified(&mut self) {
525 self.0 &= !NOTIFIED
526 }
527
528 fn set_notified(&mut self) {
529 self.0 |= NOTIFIED
530 }
531
532 pub(super) fn is_running(self) -> bool {
533 self.0 & RUNNING == RUNNING
534 }
535
536 fn set_running(&mut self) {
537 self.0 |= RUNNING;
538 }
539
540 fn unset_running(&mut self) {
541 self.0 &= !RUNNING;
542 }
543
544 pub(super) fn is_cancelled(self) -> bool {
545 self.0 & CANCELLED == CANCELLED
546 }
547
548 fn set_cancelled(&mut self) {
549 self.0 |= CANCELLED;
550 }
551
552 /// Returns `true` if the task's future has completed execution.
553 pub(super) fn is_complete(self) -> bool {
554 self.0 & COMPLETE == COMPLETE
555 }
556
557 pub(super) fn is_join_interested(self) -> bool {
558 self.0 & JOIN_INTEREST == JOIN_INTEREST
559 }
560
561 fn unset_join_interested(&mut self) {
562 self.0 &= !JOIN_INTEREST
563 }
564
565 pub(super) fn is_join_waker_set(self) -> bool {
566 self.0 & JOIN_WAKER == JOIN_WAKER
567 }
568
569 fn set_join_waker(&mut self) {
570 self.0 |= JOIN_WAKER;
571 }
572
573 fn unset_join_waker(&mut self) {
574 self.0 &= !JOIN_WAKER
575 }
576
577 pub(super) fn ref_count(self) -> usize {
578 (self.0 & REF_COUNT_MASK) >> REF_COUNT_SHIFT
579 }
580
581 fn ref_inc(&mut self) {
582 assert!(self.0 <= isize::MAX as usize);
583 self.0 += REF_ONE;
584 }
585
586 pub(super) fn ref_dec(&mut self) {
587 assert!(self.ref_count() > 0);
588 self.0 -= REF_ONE
589 }
590}
591
592impl fmt::Debug for State {
593 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
594 let snapshot: Snapshot = self.load();
595 snapshot.fmt(fmt)
596 }
597}
598
599impl fmt::Debug for Snapshot {
600 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
601 fmt&mut DebugStruct<'_, '_>.debug_struct("Snapshot")
602 .field("is_running", &self.is_running())
603 .field("is_complete", &self.is_complete())
604 .field("is_notified", &self.is_notified())
605 .field("is_cancelled", &self.is_cancelled())
606 .field("is_join_interested", &self.is_join_interested())
607 .field("is_join_waker_set", &self.is_join_waker_set())
608 .field(name:"ref_count", &self.ref_count())
609 .finish()
610 }
611}
612