summaryrefslogtreecommitdiffstats
path: root/vendor/parking_lot/src/rwlock.rs
blob: 4167a858700ad8f6ecd0069dca7c9373d3bde657 (plain)
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
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

use crate::raw_rwlock::RawRwLock;
use lock_api;

/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// This lock uses a task-fair locking policy which avoids both reader and
/// writer starvation. This means that readers trying to acquire the lock will
/// block even if the lock is unlocked when there are writers waiting to acquire
/// the lock. Because of this, attempts to recursively acquire a read lock
/// within a single thread may result in a deadlock.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies `Send` to be shared across threads and `Sync` to
/// allow concurrent access through readers. The RAII guards returned from the
/// locking methods implement `Deref` (and `DerefMut` for the `write` methods)
/// to allow access to the contained of the lock.
///
/// # Fairness
///
/// A typical unfair lock can often end up in a situation where a single thread
/// quickly acquires and releases the same lock in succession, which can starve
/// other threads waiting to acquire the rwlock. While this improves throughput
/// because it doesn't force a context switch when a thread tries to re-acquire
/// a rwlock it has just released, this can starve other threads.
///
/// This rwlock uses [eventual fairness](https://trac.webkit.org/changeset/203350)
/// to ensure that the lock will be fair on average without sacrificing
/// throughput. This is done by forcing a fair unlock on average every 0.5ms,
/// which will force the lock to go to the next thread waiting for the rwlock.
///
/// Additionally, any critical section longer than 1ms will always use a fair
/// unlock, which has a negligible impact on throughput considering the length
/// of the critical section.
///
/// You can also force a fair unlock by calling `RwLockReadGuard::unlock_fair`
/// or `RwLockWriteGuard::unlock_fair` when unlocking a mutex instead of simply
/// dropping the guard.
///
/// # Differences from the standard library `RwLock`
///
/// - Supports atomically downgrading a write lock into a read lock.
/// - Task-fair locking policy instead of an unspecified platform default.
/// - No poisoning, the lock is released normally on panic.
/// - Only requires 1 word of space, whereas the standard library boxes the
///   `RwLock` due to platform limitations.
/// - Can be statically constructed.
/// - Does not require any drop glue when dropped.
/// - Inline fast path for the uncontended case.
/// - Efficient handling of micro-contention using adaptive spinning.
/// - Allows raw locking & unlocking without a guard.
/// - Supports eventual fairness so that the rwlock is fair on average.
/// - Optionally allows making the rwlock fair by calling
///   `RwLockReadGuard::unlock_fair` and `RwLockWriteGuard::unlock_fair`.
///
/// # Examples
///
/// ```
/// use parking_lot::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
///     let r1 = lock.read();
///     let r2 = lock.read();
///     assert_eq!(*r1, 5);
///     assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
///     let mut w = lock.write();
///     *w += 1;
///     assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
pub type RwLock<T> = lock_api::RwLock<RawRwLock, T>;

/// Creates a new instance of an `RwLock<T>` which is unlocked.
///
/// This allows creating a `RwLock<T>` in a constant context on stable Rust.
pub const fn const_rwlock<T>(val: T) -> RwLock<T> {
    RwLock::const_new(<RawRwLock as lock_api::RawRwLock>::INIT, val)
}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
pub type RwLockReadGuard<'a, T> = lock_api::RwLockReadGuard<'a, RawRwLock, T>;

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
pub type RwLockWriteGuard<'a, T> = lock_api::RwLockWriteGuard<'a, RawRwLock, T>;

/// An RAII read lock guard returned by `RwLockReadGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedRwLockReadGuard` and `RwLockReadGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedRwLockReadGuard<'a, T> = lock_api::MappedRwLockReadGuard<'a, RawRwLock, T>;

/// An RAII write lock guard returned by `RwLockWriteGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedRwLockWriteGuard` and `RwLockWriteGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedRwLockWriteGuard<'a, T> = lock_api::MappedRwLockWriteGuard<'a, RawRwLock, T>;

/// RAII structure used to release the upgradable read access of a lock when
/// dropped.
pub type RwLockUpgradableReadGuard<'a, T> = lock_api::RwLockUpgradableReadGuard<'a, RawRwLock, T>;

#[cfg(test)]
mod tests {
    use crate::{RwLock, RwLockUpgradableReadGuard, RwLockWriteGuard};
    use rand::Rng;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use std::sync::mpsc::channel;
    use std::sync::Arc;
    use std::thread;
    use std::time::Duration;

    #[cfg(feature = "serde")]
    use bincode::{deserialize, serialize};

    #[derive(Eq, PartialEq, Debug)]
    struct NonCopy(i32);

    #[test]
    fn smoke() {
        let l = RwLock::new(());
        drop(l.read());
        drop(l.write());
        drop(l.upgradable_read());
        drop((l.read(), l.read()));
        drop((l.read(), l.upgradable_read()));
        drop(l.write());
    }

    #[test]
    fn frob() {
        const N: u32 = 10;
        const M: u32 = 1000;

        let r = Arc::new(RwLock::new(()));

        let (tx, rx) = channel::<()>();
        for _ in 0..N {
            let tx = tx.clone();
            let r = r.clone();
            thread::spawn(move || {
                let mut rng = rand::thread_rng();
                for _ in 0..M {
                    if rng.gen_bool(1.0 / N as f64) {
                        drop(r.write());
                    } else {
                        drop(r.read());
                    }
                }
                drop(tx);
            });
        }
        drop(tx);
        let _ = rx.recv();
    }

    #[test]
    fn test_rw_arc_no_poison_wr() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.write();
            panic!();
        })
        .join();
        let lock = arc.read();
        assert_eq!(*lock, 1);
    }

    #[test]
    fn test_rw_arc_no_poison_ww() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.write();
            panic!();
        })
        .join();
        let lock = arc.write();
        assert_eq!(*lock, 1);
    }

    #[test]
    fn test_rw_arc_no_poison_rr() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.read();
            panic!();
        })
        .join();
        let lock = arc.read();
        assert_eq!(*lock, 1);
    }

    #[test]
    fn test_rw_arc_no_poison_rw() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.read();
            panic!()
        })
        .join();
        let lock = arc.write();
        assert_eq!(*lock, 1);
    }

    #[test]
    fn test_ruw_arc() {
        let arc = Arc::new(RwLock::new(0));
        let arc2 = arc.clone();
        let (tx, rx) = channel();

        thread::spawn(move || {
            for _ in 0..10 {
                let mut lock = arc2.write();
                let tmp = *lock;
                *lock = -1;
                thread::yield_now();
                *lock = tmp + 1;
            }
            tx.send(()).unwrap();
        });

        let mut children = Vec::new();

        // Upgradable readers try to catch the writer in the act and also
        // try to touch the value
        for _ in 0..5 {
            let arc3 = arc.clone();
            children.push(thread::spawn(move || {
                let lock = arc3.upgradable_read();
                let tmp = *lock;
                assert!(tmp >= 0);
                thread::yield_now();
                let mut lock = RwLockUpgradableReadGuard::upgrade(lock);
                assert_eq!(tmp, *lock);
                *lock = -1;
                thread::yield_now();
                *lock = tmp + 1;
            }));
        }

        // Readers try to catch the writers in the act
        for _ in 0..5 {
            let arc4 = arc.clone();
            children.push(thread::spawn(move || {
                let lock = arc4.read();
                assert!(*lock >= 0);
            }));
        }

        // Wait for children to pass their asserts
        for r in children {
            assert!(r.join().is_ok());
        }

        // Wait for writer to finish
        rx.recv().unwrap();
        let lock = arc.read();
        assert_eq!(*lock, 15);
    }

    #[test]
    fn test_rw_arc() {
        let arc = Arc::new(RwLock::new(0));
        let arc2 = arc.clone();
        let (tx, rx) = channel();

        thread::spawn(move || {
            let mut lock = arc2.write();
            for _ in 0..10 {
                let tmp = *lock;
                *lock = -1;
                thread::yield_now();
                *lock = tmp + 1;
            }
            tx.send(()).unwrap();
        });

        // Readers try to catch the writer in the act
        let mut children = Vec::new();
        for _ in 0..5 {
            let arc3 = arc.clone();
            children.push(thread::spawn(move || {
                let lock = arc3.read();
                assert!(*lock >= 0);
            }));
        }

        // Wait for children to pass their asserts
        for r in children {
            assert!(r.join().is_ok());
        }

        // Wait for writer to finish
        rx.recv().unwrap();
        let lock = arc.read();
        assert_eq!(*lock, 10);
    }

    #[test]
    fn test_rw_arc_access_in_unwind() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _ = thread::spawn(move || {
            struct Unwinder {
                i: Arc<RwLock<isize>>,
            }
            impl Drop for Unwinder {
                fn drop(&mut self) {
                    let mut lock = self.i.write();
                    *lock += 1;
                }
            }
            let _u = Unwinder { i: arc2 };
            panic!();
        })
        .join();
        let lock = arc.read();
        assert_eq!(*lock, 2);
    }

    #[test]
    fn test_rwlock_unsized() {
        let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
        {
            let b = &mut *rw.write();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*rw.read(), comp);
    }

    #[test]
    fn test_rwlock_try_read() {
        let lock = RwLock::new(0isize);
        {
            let read_guard = lock.read();

            let read_result = lock.try_read();
            assert!(
                read_result.is_some(),
                "try_read should succeed while read_guard is in scope"
            );

            drop(read_guard);
        }
        {
            let upgrade_guard = lock.upgradable_read();

            let read_result = lock.try_read();
            assert!(
                read_result.is_some(),
                "try_read should succeed while upgrade_guard is in scope"
            );

            drop(upgrade_guard);
        }
        {
            let write_guard = lock.write();

            let read_result = lock.try_read();
            assert!(
                read_result.is_none(),
                "try_read should fail while write_guard is in scope"
            );

            drop(write_guard);
        }
    }

    #[test]
    fn test_rwlock_try_write() {
        let lock = RwLock::new(0isize);
        {
            let read_guard = lock.read();

            let write_result = lock.try_write();
            assert!(
                write_result.is_none(),
                "try_write should fail while read_guard is in scope"
            );
            assert!(lock.is_locked());
            assert!(!lock.is_locked_exclusive());

            drop(read_guard);
        }
        {
            let upgrade_guard = lock.upgradable_read();

            let write_result = lock.try_write();
            assert!(
                write_result.is_none(),
                "try_write should fail while upgrade_guard is in scope"
            );
            assert!(lock.is_locked());
            assert!(!lock.is_locked_exclusive());

            drop(upgrade_guard);
        }
        {
            let write_guard = lock.write();

            let write_result = lock.try_write();
            assert!(
                write_result.is_none(),
                "try_write should fail while write_guard is in scope"
            );
            assert!(lock.is_locked());
            assert!(lock.is_locked_exclusive());

            drop(write_guard);
        }
    }

    #[test]
    fn test_rwlock_try_upgrade() {
        let lock = RwLock::new(0isize);
        {
            let read_guard = lock.read();

            let upgrade_result = lock.try_upgradable_read();
            assert!(
                upgrade_result.is_some(),
                "try_upgradable_read should succeed while read_guard is in scope"
            );

            drop(read_guard);
        }
        {
            let upgrade_guard = lock.upgradable_read();

            let upgrade_result = lock.try_upgradable_read();
            assert!(
                upgrade_result.is_none(),
                "try_upgradable_read should fail while upgrade_guard is in scope"
            );

            drop(upgrade_guard);
        }
        {
            let write_guard = lock.write();

            let upgrade_result = lock.try_upgradable_read();
            assert!(
                upgrade_result.is_none(),
                "try_upgradable should fail while write_guard is in scope"
            );

            drop(write_guard);
        }
    }

    #[test]
    fn test_into_inner() {
        let m = RwLock::new(NonCopy(10));
        assert_eq!(m.into_inner(), NonCopy(10));
    }

    #[test]
    fn test_into_inner_drop() {
        struct Foo(Arc<AtomicUsize>);
        impl Drop for Foo {
            fn drop(&mut self) {
                self.0.fetch_add(1, Ordering::SeqCst);
            }
        }
        let num_drops = Arc::new(AtomicUsize::new(0));
        let m = RwLock::new(Foo(num_drops.clone()));
        assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        {
            let _inner = m.into_inner();
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        }
        assert_eq!(num_drops.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn test_get_mut() {
        let mut m = RwLock::new(NonCopy(10));
        *m.get_mut() = NonCopy(20);
        assert_eq!(m.into_inner(), NonCopy(20));
    }

    #[test]
    fn test_rwlockguard_sync() {
        fn sync<T: Sync>(_: T) {}

        let rwlock = RwLock::new(());
        sync(rwlock.read());
        sync(rwlock.write());
    }

    #[test]
    fn test_rwlock_downgrade() {
        let x = Arc::new(RwLock::new(0));
        let mut handles = Vec::new();
        for _ in 0..8 {
            let x = x.clone();
            handles.push(thread::spawn(move || {
                for _ in 0..100 {
                    let mut writer = x.write();
                    *writer += 1;
                    let cur_val = *writer;
                    let reader = RwLockWriteGuard::downgrade(writer);
                    assert_eq!(cur_val, *reader);
                }
            }));
        }
        for handle in handles {
            handle.join().unwrap()
        }
        assert_eq!(*x.read(), 800);
    }

    #[test]
    fn test_rwlock_recursive() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let lock1 = arc.read();
        let t = thread::spawn(move || {
            let _lock = arc2.write();
        });

        if cfg!(not(all(target_env = "sgx", target_vendor = "fortanix"))) {
            thread::sleep(Duration::from_millis(100));
        } else {
            // FIXME: https://github.com/fortanix/rust-sgx/issues/31
            for _ in 0..100 {
                thread::yield_now();
            }
        }

        // A normal read would block here since there is a pending writer
        let lock2 = arc.read_recursive();

        // Unblock the thread and join it.
        drop(lock1);
        drop(lock2);
        t.join().unwrap();
    }

    #[test]
    fn test_rwlock_debug() {
        let x = RwLock::new(vec![0u8, 10]);

        assert_eq!(format!("{:?}", x), "RwLock { data: [0, 10] }");
        let _lock = x.write();
        assert_eq!(format!("{:?}", x), "RwLock { data: <locked> }");
    }

    #[test]
    fn test_clone() {
        let rwlock = RwLock::new(Arc::new(1));
        let a = rwlock.read_recursive();
        let b = a.clone();
        assert_eq!(Arc::strong_count(&b), 2);
    }

    #[cfg(feature = "serde")]
    #[test]
    fn test_serde() {
        let contents: Vec<u8> = vec![0, 1, 2];
        let mutex = RwLock::new(contents.clone());

        let serialized = serialize(&mutex).unwrap();
        let deserialized: RwLock<Vec<u8>> = deserialize(&serialized).unwrap();

        assert_eq!(*(mutex.read()), *(deserialized.read()));
        assert_eq!(contents, *(deserialized.read()));
    }

    #[test]
    fn test_issue_203() {
        struct Bar(RwLock<()>);

        impl Drop for Bar {
            fn drop(&mut self) {
                let _n = self.0.write();
            }
        }

        thread_local! {
            static B: Bar = Bar(RwLock::new(()));
        }

        thread::spawn(|| {
            B.with(|_| ());

            let a = RwLock::new(());
            let _a = a.read();
        })
        .join()
        .unwrap();
    }

    #[test]
    fn test_rw_write_is_locked() {
        let lock = RwLock::new(0isize);
        {
            let _read_guard = lock.read();

            assert!(lock.is_locked());
            assert!(!lock.is_locked_exclusive());
        }

        {
            let _write_guard = lock.write();

            assert!(lock.is_locked());
            assert!(lock.is_locked_exclusive());
        }
    }
}