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
use core::fmt;
use core::mem;

use scopeguard::defer;

use crate::atomic::Shared;
use crate::collector::Collector;
use crate::deferred::Deferred;
use crate::internal::Local;

/// A guard that keeps the current thread pinned.
///
/// # Pinning
///
/// The current thread is pinned by calling [`pin`], which returns a new guard:
///
/// ```
/// use crossbeam_epoch as epoch;
///
/// // It is often convenient to prefix a call to `pin` with a `&` in order to create a reference.
/// // This is not really necessary, but makes passing references to the guard a bit easier.
/// let guard = &epoch::pin();
/// ```
///
/// When a guard gets dropped, the current thread is automatically unpinned.
///
/// # Pointers on the stack
///
/// Having a guard allows us to create pointers on the stack to heap-allocated objects.
/// For example:
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// // Create a heap-allocated number.
/// let a = Atomic::new(777);
///
/// // Pin the current thread.
/// let guard = &epoch::pin();
///
/// // Load the heap-allocated object and create pointer `p` on the stack.
/// let p = a.load(SeqCst, guard);
///
/// // Dereference the pointer and print the value:
/// if let Some(num) = unsafe { p.as_ref() } {
///     println!("The number is {}.", num);
/// }
/// ```
///
/// # Multiple guards
///
/// Pinning is reentrant and it is perfectly legal to create multiple guards. In that case, the
/// thread will actually be pinned only when the first guard is created and unpinned when the last
/// one is dropped:
///
/// ```
/// use crossbeam_epoch as epoch;
///
/// let guard1 = epoch::pin();
/// let guard2 = epoch::pin();
/// assert!(epoch::is_pinned());
/// drop(guard1);
/// assert!(epoch::is_pinned());
/// drop(guard2);
/// assert!(!epoch::is_pinned());
/// ```
///
/// [`pin`]: super::pin
pub struct Guard {
    pub(crate) local: *const Local,
}

impl Guard {
    /// Stores a function so that it can be executed at some point after all currently pinned
    /// threads get unpinned.
    ///
    /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache
    /// becomes full, some functions are moved into the global cache. At the same time, some
    /// functions from both local and global caches may get executed in order to incrementally
    /// clean up the caches as they fill up.
    ///
    /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it
    /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might
    /// never run, but the epoch-based garbage collection will make an effort to execute it
    /// reasonably soon.
    ///
    /// If this method is called from an [`unprotected`] guard, the function will simply be
    /// executed immediately.
    pub fn defer<F, R>(&self, f: F)
    where
        F: FnOnce() -> R,
        F: Send + 'static,
    {
        unsafe {
            self.defer_unchecked(f);
        }
    }

    /// Stores a function so that it can be executed at some point after all currently pinned
    /// threads get unpinned.
    ///
    /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache
    /// becomes full, some functions are moved into the global cache. At the same time, some
    /// functions from both local and global caches may get executed in order to incrementally
    /// clean up the caches as they fill up.
    ///
    /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it
    /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might
    /// never run, but the epoch-based garbage collection will make an effort to execute it
    /// reasonably soon.
    ///
    /// If this method is called from an [`unprotected`] guard, the function will simply be
    /// executed immediately.
    ///
    /// # Safety
    ///
    /// The given function must not hold reference onto the stack. It is highly recommended that
    /// the passed function is **always** marked with `move` in order to prevent accidental
    /// borrows.
    ///
    /// ```
    /// use crossbeam_epoch as epoch;
    ///
    /// let guard = &epoch::pin();
    /// let message = "Hello!";
    /// unsafe {
    ///     // ALWAYS use `move` when sending a closure into `defer_unchecked`.
    ///     guard.defer_unchecked(move || {
    ///         println!("{}", message);
    ///     });
    /// }
    /// ```
    ///
    /// Apart from that, keep in mind that another thread may execute `f`, so anything accessed by
    /// the closure must be `Send`.
    ///
    /// We intentionally didn't require `F: Send`, because Rust's type systems usually cannot prove
    /// `F: Send` for typical use cases. For example, consider the following code snippet, which
    /// exemplifies the typical use case of deferring the deallocation of a shared reference:
    ///
    /// ```ignore
    /// let shared = Owned::new(7i32).into_shared(guard);
    /// guard.defer_unchecked(move || shared.into_owned()); // `Shared` is not `Send`!
    /// ```
    ///
    /// While `Shared` is not `Send`, it's safe for another thread to call the deferred function,
    /// because it's called only after the grace period and `shared` is no longer shared with other
    /// threads. But we don't expect type systems to prove this.
    ///
    /// # Examples
    ///
    /// When a heap-allocated object in a data structure becomes unreachable, it has to be
    /// deallocated. However, the current thread and other threads may be still holding references
    /// on the stack to that same object. Therefore it cannot be deallocated before those references
    /// get dropped. This method can defer deallocation until all those threads get unpinned and
    /// consequently drop all their references on the stack.
    ///
    /// ```
    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
    /// use std::sync::atomic::Ordering::SeqCst;
    ///
    /// let a = Atomic::new("foo");
    ///
    /// // Now suppose that `a` is shared among multiple threads and concurrently
    /// // accessed and modified...
    ///
    /// // Pin the current thread.
    /// let guard = &epoch::pin();
    ///
    /// // Steal the object currently stored in `a` and swap it with another one.
    /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard);
    ///
    /// if !p.is_null() {
    ///     // The object `p` is pointing to is now unreachable.
    ///     // Defer its deallocation until all currently pinned threads get unpinned.
    ///     unsafe {
    ///         // ALWAYS use `move` when sending a closure into `defer_unchecked`.
    ///         guard.defer_unchecked(move || {
    ///             println!("{} is now being deallocated.", p.deref());
    ///             // Now we have unique access to the object pointed to by `p` and can turn it
    ///             // into an `Owned`. Dropping the `Owned` will deallocate the object.
    ///             drop(p.into_owned());
    ///         });
    ///     }
    /// }
    /// ```
    pub unsafe fn defer_unchecked<F, R>(&self, f: F)
    where
        F: FnOnce() -> R,
    {
        if let Some(local) = self.local.as_ref() {
            local.defer(Deferred::new(move || drop(f())), self);
        } else {
            drop(f());
        }
    }

    /// Stores a destructor for an object so that it can be deallocated and dropped at some point
    /// after all currently pinned threads get unpinned.
    ///
    /// This method first stores the destructor into the thread-local (or handle-local) cache. If
    /// this cache becomes full, some destructors are moved into the global cache. At the same
    /// time, some destructors from both local and global caches may get executed in order to
    /// incrementally clean up the caches as they fill up.
    ///
    /// There is no guarantee when exactly the destructor will be executed. The only guarantee is
    /// that it won't be executed until all currently pinned threads get unpinned. In theory, the
    /// destructor might never run, but the epoch-based garbage collection will make an effort to
    /// execute it reasonably soon.
    ///
    /// If this method is called from an [`unprotected`] guard, the destructor will simply be
    /// executed immediately.
    ///
    /// # Safety
    ///
    /// The object must not be reachable by other threads anymore, otherwise it might be still in
    /// use when the destructor runs.
    ///
    /// Apart from that, keep in mind that another thread may execute the destructor, so the object
    /// must be sendable to other threads.
    ///
    /// We intentionally didn't require `T: Send`, because Rust's type systems usually cannot prove
    /// `T: Send` for typical use cases. For example, consider the following code snippet, which
    /// exemplifies the typical use case of deferring the deallocation of a shared reference:
    ///
    /// ```ignore
    /// let shared = Owned::new(7i32).into_shared(guard);
    /// guard.defer_destroy(shared); // `Shared` is not `Send`!
    /// ```
    ///
    /// While `Shared` is not `Send`, it's safe for another thread to call the destructor, because
    /// it's called only after the grace period and `shared` is no longer shared with other
    /// threads. But we don't expect type systems to prove this.
    ///
    /// # Examples
    ///
    /// When a heap-allocated object in a data structure becomes unreachable, it has to be
    /// deallocated. However, the current thread and other threads may be still holding references
    /// on the stack to that same object. Therefore it cannot be deallocated before those references
    /// get dropped. This method can defer deallocation until all those threads get unpinned and
    /// consequently drop all their references on the stack.
    ///
    /// ```
    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
    /// use std::sync::atomic::Ordering::SeqCst;
    ///
    /// let a = Atomic::new("foo");
    ///
    /// // Now suppose that `a` is shared among multiple threads and concurrently
    /// // accessed and modified...
    ///
    /// // Pin the current thread.
    /// let guard = &epoch::pin();
    ///
    /// // Steal the object currently stored in `a` and swap it with another one.
    /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard);
    ///
    /// if !p.is_null() {
    ///     // The object `p` is pointing to is now unreachable.
    ///     // Defer its deallocation until all currently pinned threads get unpinned.
    ///     unsafe {
    ///         guard.defer_destroy(p);
    ///     }
    /// }
    /// ```
    pub unsafe fn defer_destroy<T>(&self, ptr: Shared<'_, T>) {
        self.defer_unchecked(move || ptr.into_owned());
    }

    /// Clears up the thread-local cache of deferred functions by executing them or moving into the
    /// global cache.
    ///
    /// Call this method after deferring execution of a function if you want to get it executed as
    /// soon as possible. Flushing will make sure it is residing in in the global cache, so that
    /// any thread has a chance of taking the function and executing it.
    ///
    /// If this method is called from an [`unprotected`] guard, it is a no-op (nothing happens).
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_epoch as epoch;
    ///
    /// let guard = &epoch::pin();
    /// guard.defer(move || {
    ///     println!("This better be printed as soon as possible!");
    /// });
    /// guard.flush();
    /// ```
    pub fn flush(&self) {
        if let Some(local) = unsafe { self.local.as_ref() } {
            local.flush(self);
        }
    }

    /// Unpins and then immediately re-pins the thread.
    ///
    /// This method is useful when you don't want delay the advancement of the global epoch by
    /// holding an old epoch. For safety, you should not maintain any guard-based reference across
    /// the call (the latter is enforced by `&mut self`). The thread will only be repinned if this
    /// is the only active guard for the current thread.
    ///
    /// If this method is called from an [`unprotected`] guard, then the call will be just no-op.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_epoch::{self as epoch, Atomic};
    /// use std::sync::atomic::Ordering::SeqCst;
    ///
    /// let a = Atomic::new(777);
    /// let mut guard = epoch::pin();
    /// {
    ///     let p = a.load(SeqCst, &guard);
    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
    /// }
    /// guard.repin();
    /// {
    ///     let p = a.load(SeqCst, &guard);
    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
    /// }
    /// ```
    pub fn repin(&mut self) {
        if let Some(local) = unsafe { self.local.as_ref() } {
            local.repin();
        }
    }

    /// Temporarily unpins the thread, executes the given function and then re-pins the thread.
    ///
    /// This method is useful when you need to perform a long-running operation (e.g. sleeping)
    /// and don't need to maintain any guard-based reference across the call (the latter is enforced
    /// by `&mut self`). The thread will only be unpinned if this is the only active guard for the
    /// current thread.
    ///
    /// If this method is called from an [`unprotected`] guard, then the passed function is called
    /// directly without unpinning the thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_epoch::{self as epoch, Atomic};
    /// use std::sync::atomic::Ordering::SeqCst;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let a = Atomic::new(777);
    /// let mut guard = epoch::pin();
    /// {
    ///     let p = a.load(SeqCst, &guard);
    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
    /// }
    /// guard.repin_after(|| thread::sleep(Duration::from_millis(50)));
    /// {
    ///     let p = a.load(SeqCst, &guard);
    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
    /// }
    /// ```
    pub fn repin_after<F, R>(&mut self, f: F) -> R
    where
        F: FnOnce() -> R,
    {
        if let Some(local) = unsafe { self.local.as_ref() } {
            // We need to acquire a handle here to ensure the Local doesn't
            // disappear from under us.
            local.acquire_handle();
            local.unpin();
        }

        // Ensure the Guard is re-pinned even if the function panics
        defer! {
            if let Some(local) = unsafe { self.local.as_ref() } {
                mem::forget(local.pin());
                local.release_handle();
            }
        }

        f()
    }

    /// Returns the `Collector` associated with this guard.
    ///
    /// This method is useful when you need to ensure that all guards used with
    /// a data structure come from the same collector.
    ///
    /// If this method is called from an [`unprotected`] guard, then `None` is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_epoch as epoch;
    ///
    /// let guard1 = epoch::pin();
    /// let guard2 = epoch::pin();
    /// assert!(guard1.collector() == guard2.collector());
    /// ```
    pub fn collector(&self) -> Option<&Collector> {
        unsafe { self.local.as_ref().map(|local| local.collector()) }
    }
}

impl Drop for Guard {
    #[inline]
    fn drop(&mut self) {
        if let Some(local) = unsafe { self.local.as_ref() } {
            local.unpin();
        }
    }
}

impl fmt::Debug for Guard {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("Guard { .. }")
    }
}

/// Returns a reference to a dummy guard that allows unprotected access to [`Atomic`]s.
///
/// This guard should be used in special occasions only. Note that it doesn't actually keep any
/// thread pinned - it's just a fake guard that allows loading from [`Atomic`]s unsafely.
///
/// Note that calling [`defer`] with a dummy guard will not defer the function - it will just
/// execute the function immediately.
///
/// If necessary, it's possible to create more dummy guards by cloning: `unprotected().clone()`.
///
/// # Safety
///
/// Loading and dereferencing data from an [`Atomic`] using this guard is safe only if the
/// [`Atomic`] is not being concurrently modified by other threads.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::Relaxed;
///
/// let a = Atomic::new(7);
///
/// unsafe {
///     // Load `a` without pinning the current thread.
///     a.load(Relaxed, epoch::unprotected());
///
///     // It's possible to create more dummy guards by calling `clone()`.
///     let dummy = &epoch::unprotected().clone();
///
///     dummy.defer(move || {
///         println!("This gets executed immediately.");
///     });
///
///     // Dropping `dummy` doesn't affect the current thread - it's just a noop.
/// }
/// ```
///
/// The most common use of this function is when constructing or destructing a data structure.
///
/// For example, we can use a dummy guard in the destructor of a Treiber stack because at that
/// point no other thread could concurrently modify the [`Atomic`]s we are accessing.
///
/// If we were to actually pin the current thread during destruction, that would just unnecessarily
/// delay garbage collection and incur some performance cost, so in cases like these `unprotected`
/// is very helpful.
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::mem::ManuallyDrop;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// struct Stack<T> {
///     head: Atomic<Node<T>>,
/// }
///
/// struct Node<T> {
///     data: ManuallyDrop<T>,
///     next: Atomic<Node<T>>,
/// }
///
/// impl<T> Drop for Stack<T> {
///     fn drop(&mut self) {
///         unsafe {
///             // Unprotected load.
///             let mut node = self.head.load(Relaxed, epoch::unprotected());
///
///             while let Some(n) = node.as_ref() {
///                 // Unprotected load.
///                 let next = n.next.load(Relaxed, epoch::unprotected());
///
///                 // Take ownership of the node, then drop its data and deallocate it.
///                 let mut o = node.into_owned();
///                 ManuallyDrop::drop(&mut o.data);
///                 drop(o);
///
///                 node = next;
///             }
///         }
///     }
/// }
/// ```
///
/// [`Atomic`]: super::Atomic
/// [`defer`]: Guard::defer
#[inline]
pub unsafe fn unprotected() -> &'static Guard {
    // An unprotected guard is just a `Guard` with its field `local` set to null.
    // We make a newtype over `Guard` because `Guard` isn't `Sync`, so can't be directly stored in
    // a `static`
    struct GuardWrapper(Guard);
    unsafe impl Sync for GuardWrapper {}
    static UNPROTECTED: GuardWrapper = GuardWrapper(Guard {
        local: core::ptr::null(),
    });
    &UNPROTECTED.0
}