1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
use crate::loom::sync::atomic::AtomicUsize;

use std::fmt;
use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};

pub(super) struct State {
    val: AtomicUsize,
}

/// Current state value.
#[derive(Copy, Clone)]
pub(super) struct Snapshot(usize);

type UpdateResult = Result<Snapshot, Snapshot>;

/// The task is currently being run.
const RUNNING: usize = 0b0001;

/// The task is complete.
///
/// Once this bit is set, it is never unset.
const COMPLETE: usize = 0b0010;

/// Extracts the task's lifecycle value from the state.
const LIFECYCLE_MASK: usize = 0b11;

/// Flag tracking if the task has been pushed into a run queue.
const NOTIFIED: usize = 0b100;

/// The join handle is still around.
const JOIN_INTEREST: usize = 0b1_000;

/// A join handle waker has been set.
const JOIN_WAKER: usize = 0b10_000;

/// The task has been forcibly cancelled.
const CANCELLED: usize = 0b100_000;

/// All bits.
const STATE_MASK: usize = LIFECYCLE_MASK | NOTIFIED | JOIN_INTEREST | JOIN_WAKER | CANCELLED;

/// Bits used by the ref count portion of the state.
const REF_COUNT_MASK: usize = !STATE_MASK;

/// Number of positions to shift the ref count.
const REF_COUNT_SHIFT: usize = REF_COUNT_MASK.count_zeros() as usize;

/// One ref count.
const REF_ONE: usize = 1 << REF_COUNT_SHIFT;

/// State a task is initialized with.
///
/// A task is initialized with three references:
///
///  * A reference that will be stored in an `OwnedTasks` or `LocalOwnedTasks`.
///  * A reference that will be sent to the scheduler as an ordinary notification.
///  * A reference for the `JoinHandle`.
///
/// As the task starts with a `JoinHandle`, `JOIN_INTEREST` is set.
/// As the task starts with a `Notified`, `NOTIFIED` is set.
const INITIAL_STATE: usize = (REF_ONE * 3) | JOIN_INTEREST | NOTIFIED;

#[must_use]
pub(super) enum TransitionToRunning {
    Success,
    Cancelled,
    Failed,
    Dealloc,
}

#[must_use]
pub(super) enum TransitionToIdle {
    Ok,
    OkNotified,
    OkDealloc,
    Cancelled,
}

#[must_use]
pub(super) enum TransitionToNotifiedByVal {
    DoNothing,
    Submit,
    Dealloc,
}

#[must_use]
pub(crate) enum TransitionToNotifiedByRef {
    DoNothing,
    Submit,
}

/// All transitions are performed via RMW operations. This establishes an
/// unambiguous modification order.
impl State {
    /// Returns a task's initial state.
    pub(super) fn new() -> State {
        // The raw task returned by this method has a ref-count of three. See
        // the comment on INITIAL_STATE for more.
        State {
            val: AtomicUsize::new(INITIAL_STATE),
        }
    }

    /// Loads the current state, establishes `Acquire` ordering.
    pub(super) fn load(&self) -> Snapshot {
        Snapshot(self.val.load(Acquire))
    }

    /// Attempts to transition the lifecycle to `Running`. This sets the
    /// notified bit to false so notifications during the poll can be detected.
    pub(super) fn transition_to_running(&self) -> TransitionToRunning {
        self.fetch_update_action(|mut next| {
            let action;
            assert!(next.is_notified());

            if !next.is_idle() {
                // This happens if the task is either currently running or if it
                // has already completed, e.g. if it was cancelled during
                // shutdown. Consume the ref-count and return.
                next.ref_dec();
                if next.ref_count() == 0 {
                    action = TransitionToRunning::Dealloc;
                } else {
                    action = TransitionToRunning::Failed;
                }
            } else {
                // We are able to lock the RUNNING bit.
                next.set_running();
                next.unset_notified();

                if next.is_cancelled() {
                    action = TransitionToRunning::Cancelled;
                } else {
                    action = TransitionToRunning::Success;
                }
            }
            (action, Some(next))
        })
    }

    /// Transitions the task from `Running` -> `Idle`.
    ///
    /// The transition to `Idle` fails if the task has been flagged to be
    /// cancelled.
    pub(super) fn transition_to_idle(&self) -> TransitionToIdle {
        self.fetch_update_action(|curr| {
            assert!(curr.is_running());

            if curr.is_cancelled() {
                return (TransitionToIdle::Cancelled, None);
            }

            let mut next = curr;
            let action;
            next.unset_running();

            if !next.is_notified() {
                // Polling the future consumes the ref-count of the Notified.
                next.ref_dec();
                if next.ref_count() == 0 {
                    action = TransitionToIdle::OkDealloc;
                } else {
                    action = TransitionToIdle::Ok;
                }
            } else {
                // The caller will schedule a new notification, so we create a
                // new ref-count for the notification. Our own ref-count is kept
                // for now, and the caller will drop it shortly.
                next.ref_inc();
                action = TransitionToIdle::OkNotified;
            }

            (action, Some(next))
        })
    }

    /// Transitions the task from `Running` -> `Complete`.
    pub(super) fn transition_to_complete(&self) -> Snapshot {
        const DELTA: usize = RUNNING | COMPLETE;

        let prev = Snapshot(self.val.fetch_xor(DELTA, AcqRel));
        assert!(prev.is_running());
        assert!(!prev.is_complete());

        Snapshot(prev.0 ^ DELTA)
    }

    /// Transitions from `Complete` -> `Terminal`, decrementing the reference
    /// count the specified number of times.
    ///
    /// Returns true if the task should be deallocated.
    pub(super) fn transition_to_terminal(&self, count: usize) -> bool {
        let prev = Snapshot(self.val.fetch_sub(count * REF_ONE, AcqRel));
        assert!(
            prev.ref_count() >= count,
            "current: {}, sub: {}",
            prev.ref_count(),
            count
        );
        prev.ref_count() == count
    }

    /// Transitions the state to `NOTIFIED`.
    ///
    /// If no task needs to be submitted, a ref-count is consumed.
    ///
    /// If a task needs to be submitted, the ref-count is incremented for the
    /// new Notified.
    pub(super) fn transition_to_notified_by_val(&self) -> TransitionToNotifiedByVal {
        self.fetch_update_action(|mut snapshot| {
            let action;

            if snapshot.is_running() {
                // If the task is running, we mark it as notified, but we should
                // not submit anything as the thread currently running the
                // future is responsible for that.
                snapshot.set_notified();
                snapshot.ref_dec();

                // The thread that set the running bit also holds a ref-count.
                assert!(snapshot.ref_count() > 0);

                action = TransitionToNotifiedByVal::DoNothing;
            } else if snapshot.is_complete() || snapshot.is_notified() {
                // We do not need to submit any notifications, but we have to
                // decrement the ref-count.
                snapshot.ref_dec();

                if snapshot.ref_count() == 0 {
                    action = TransitionToNotifiedByVal::Dealloc;
                } else {
                    action = TransitionToNotifiedByVal::DoNothing;
                }
            } else {
                // We create a new notified that we can submit. The caller
                // retains ownership of the ref-count they passed in.
                snapshot.set_notified();
                snapshot.ref_inc();
                action = TransitionToNotifiedByVal::Submit;
            }

            (action, Some(snapshot))
        })
    }

    /// Transitions the state to `NOTIFIED`.
    pub(super) fn transition_to_notified_by_ref(&self) -> TransitionToNotifiedByRef {
        self.fetch_update_action(|mut snapshot| {
            if snapshot.is_complete() || snapshot.is_notified() {
                // There is nothing to do in this case.
                (TransitionToNotifiedByRef::DoNothing, None)
            } else if snapshot.is_running() {
                // If the task is running, we mark it as notified, but we should
                // not submit as the thread currently running the future is
                // responsible for that.
                snapshot.set_notified();
                (TransitionToNotifiedByRef::DoNothing, Some(snapshot))
            } else {
                // The task is idle and not notified. We should submit a
                // notification.
                snapshot.set_notified();
                snapshot.ref_inc();
                (TransitionToNotifiedByRef::Submit, Some(snapshot))
            }
        })
    }

    /// Transitions the state to `NOTIFIED`, unconditionally increasing the ref
    /// count.
    ///
    /// Returns `true` if the notified bit was transitioned from `0` to `1`;
    /// otherwise `false.`
    #[cfg(all(
        tokio_unstable,
        tokio_taskdump,
        feature = "rt",
        target_os = "linux",
        any(target_arch = "aarch64", target_arch = "x86", target_arch = "x86_64")
    ))]
    pub(super) fn transition_to_notified_for_tracing(&self) -> bool {
        self.fetch_update_action(|mut snapshot| {
            if snapshot.is_notified() {
                (false, None)
            } else {
                snapshot.set_notified();
                snapshot.ref_inc();
                (true, Some(snapshot))
            }
        })
    }

    /// Sets the cancelled bit and transitions the state to `NOTIFIED` if idle.
    ///
    /// Returns `true` if the task needs to be submitted to the pool for
    /// execution.
    pub(super) fn transition_to_notified_and_cancel(&self) -> bool {
        self.fetch_update_action(|mut snapshot| {
            if snapshot.is_cancelled() || snapshot.is_complete() {
                // Aborts to completed or cancelled tasks are no-ops.
                (false, None)
            } else if snapshot.is_running() {
                // If the task is running, we mark it as cancelled. The thread
                // running the task will notice the cancelled bit when it
                // stops polling and it will kill the task.
                //
                // The set_notified() call is not strictly necessary but it will
                // in some cases let a wake_by_ref call return without having
                // to perform a compare_exchange.
                snapshot.set_notified();
                snapshot.set_cancelled();
                (false, Some(snapshot))
            } else {
                // The task is idle. We set the cancelled and notified bits and
                // submit a notification if the notified bit was not already
                // set.
                snapshot.set_cancelled();
                if !snapshot.is_notified() {
                    snapshot.set_notified();
                    snapshot.ref_inc();
                    (true, Some(snapshot))
                } else {
                    (false, Some(snapshot))
                }
            }
        })
    }

    /// Sets the `CANCELLED` bit and attempts to transition to `Running`.
    ///
    /// Returns `true` if the transition to `Running` succeeded.
    pub(super) fn transition_to_shutdown(&self) -> bool {
        let mut prev = Snapshot(0);

        let _ = self.fetch_update(|mut snapshot| {
            prev = snapshot;

            if snapshot.is_idle() {
                snapshot.set_running();
            }

            // If the task was not idle, the thread currently running the task
            // will notice the cancelled bit and cancel it once the poll
            // completes.
            snapshot.set_cancelled();
            Some(snapshot)
        });

        prev.is_idle()
    }

    /// Optimistically tries to swap the state assuming the join handle is
    /// __immediately__ dropped on spawn.
    pub(super) fn drop_join_handle_fast(&self) -> Result<(), ()> {
        use std::sync::atomic::Ordering::Relaxed;

        // Relaxed is acceptable as if this function is called and succeeds,
        // then nothing has been done w/ the join handle.
        //
        // The moment the join handle is used (polled), the `JOIN_WAKER` flag is
        // set, at which point the CAS will fail.
        //
        // Given this, there is no risk if this operation is reordered.
        self.val
            .compare_exchange_weak(
                INITIAL_STATE,
                (INITIAL_STATE - REF_ONE) & !JOIN_INTEREST,
                Release,
                Relaxed,
            )
            .map(|_| ())
            .map_err(|_| ())
    }

    /// Tries to unset the `JOIN_INTEREST` flag.
    ///
    /// Returns `Ok` if the operation happens before the task transitions to a
    /// completed state, `Err` otherwise.
    pub(super) fn unset_join_interested(&self) -> UpdateResult {
        self.fetch_update(|curr| {
            assert!(curr.is_join_interested());

            if curr.is_complete() {
                return None;
            }

            let mut next = curr;
            next.unset_join_interested();

            Some(next)
        })
    }

    /// Sets the `JOIN_WAKER` bit.
    ///
    /// Returns `Ok` if the bit is set, `Err` otherwise. This operation fails if
    /// the task has completed.
    pub(super) fn set_join_waker(&self) -> UpdateResult {
        self.fetch_update(|curr| {
            assert!(curr.is_join_interested());
            assert!(!curr.is_join_waker_set());

            if curr.is_complete() {
                return None;
            }

            let mut next = curr;
            next.set_join_waker();

            Some(next)
        })
    }

    /// Unsets the `JOIN_WAKER` bit.
    ///
    /// Returns `Ok` has been unset, `Err` otherwise. This operation fails if
    /// the task has completed.
    pub(super) fn unset_waker(&self) -> UpdateResult {
        self.fetch_update(|curr| {
            assert!(curr.is_join_interested());
            assert!(curr.is_join_waker_set());

            if curr.is_complete() {
                return None;
            }

            let mut next = curr;
            next.unset_join_waker();

            Some(next)
        })
    }

    pub(super) fn ref_inc(&self) {
        use std::process;
        use std::sync::atomic::Ordering::Relaxed;

        // Using a relaxed ordering is alright here, as knowledge of the
        // original reference prevents other threads from erroneously deleting
        // the object.
        //
        // As explained in the [Boost documentation][1], Increasing the
        // reference counter can always be done with memory_order_relaxed: New
        // references to an object can only be formed from an existing
        // reference, and passing an existing reference from one thread to
        // another must already provide any required synchronization.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        let prev = self.val.fetch_add(REF_ONE, Relaxed);

        // If the reference count overflowed, abort.
        if prev > isize::MAX as usize {
            process::abort();
        }
    }

    /// Returns `true` if the task should be released.
    pub(super) fn ref_dec(&self) -> bool {
        let prev = Snapshot(self.val.fetch_sub(REF_ONE, AcqRel));
        assert!(prev.ref_count() >= 1);
        prev.ref_count() == 1
    }

    /// Returns `true` if the task should be released.
    pub(super) fn ref_dec_twice(&self) -> bool {
        let prev = Snapshot(self.val.fetch_sub(2 * REF_ONE, AcqRel));
        assert!(prev.ref_count() >= 2);
        prev.ref_count() == 2
    }

    fn fetch_update_action<F, T>(&self, mut f: F) -> T
    where
        F: FnMut(Snapshot) -> (T, Option<Snapshot>),
    {
        let mut curr = self.load();

        loop {
            let (output, next) = f(curr);
            let next = match next {
                Some(next) => next,
                None => return output,
            };

            let res = self.val.compare_exchange(curr.0, next.0, AcqRel, Acquire);

            match res {
                Ok(_) => return output,
                Err(actual) => curr = Snapshot(actual),
            }
        }
    }

    fn fetch_update<F>(&self, mut f: F) -> Result<Snapshot, Snapshot>
    where
        F: FnMut(Snapshot) -> Option<Snapshot>,
    {
        let mut curr = self.load();

        loop {
            let next = match f(curr) {
                Some(next) => next,
                None => return Err(curr),
            };

            let res = self.val.compare_exchange(curr.0, next.0, AcqRel, Acquire);

            match res {
                Ok(_) => return Ok(next),
                Err(actual) => curr = Snapshot(actual),
            }
        }
    }
}

// ===== impl Snapshot =====

impl Snapshot {
    /// Returns `true` if the task is in an idle state.
    pub(super) fn is_idle(self) -> bool {
        self.0 & (RUNNING | COMPLETE) == 0
    }

    /// Returns `true` if the task has been flagged as notified.
    pub(super) fn is_notified(self) -> bool {
        self.0 & NOTIFIED == NOTIFIED
    }

    fn unset_notified(&mut self) {
        self.0 &= !NOTIFIED;
    }

    fn set_notified(&mut self) {
        self.0 |= NOTIFIED;
    }

    pub(super) fn is_running(self) -> bool {
        self.0 & RUNNING == RUNNING
    }

    fn set_running(&mut self) {
        self.0 |= RUNNING;
    }

    fn unset_running(&mut self) {
        self.0 &= !RUNNING;
    }

    pub(super) fn is_cancelled(self) -> bool {
        self.0 & CANCELLED == CANCELLED
    }

    fn set_cancelled(&mut self) {
        self.0 |= CANCELLED;
    }

    /// Returns `true` if the task's future has completed execution.
    pub(super) fn is_complete(self) -> bool {
        self.0 & COMPLETE == COMPLETE
    }

    pub(super) fn is_join_interested(self) -> bool {
        self.0 & JOIN_INTEREST == JOIN_INTEREST
    }

    fn unset_join_interested(&mut self) {
        self.0 &= !JOIN_INTEREST;
    }

    pub(super) fn is_join_waker_set(self) -> bool {
        self.0 & JOIN_WAKER == JOIN_WAKER
    }

    fn set_join_waker(&mut self) {
        self.0 |= JOIN_WAKER;
    }

    fn unset_join_waker(&mut self) {
        self.0 &= !JOIN_WAKER;
    }

    pub(super) fn ref_count(self) -> usize {
        (self.0 & REF_COUNT_MASK) >> REF_COUNT_SHIFT
    }

    fn ref_inc(&mut self) {
        assert!(self.0 <= isize::MAX as usize);
        self.0 += REF_ONE;
    }

    pub(super) fn ref_dec(&mut self) {
        assert!(self.ref_count() > 0);
        self.0 -= REF_ONE;
    }
}

impl fmt::Debug for State {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        let snapshot = self.load();
        snapshot.fmt(fmt)
    }
}

impl fmt::Debug for Snapshot {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Snapshot")
            .field("is_running", &self.is_running())
            .field("is_complete", &self.is_complete())
            .field("is_notified", &self.is_notified())
            .field("is_cancelled", &self.is_cancelled())
            .field("is_join_interested", &self.is_join_interested())
            .field("is_join_waker_set", &self.is_join_waker_set())
            .field("ref_count", &self.ref_count())
            .finish()
    }
}