blob: 53d5ca265ea08e5a57de42041744a033f36362d1 (
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
|
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// barrier.h
// -----------------------------------------------------------------------------
#ifndef ABSL_SYNCHRONIZATION_BARRIER_H_
#define ABSL_SYNCHRONIZATION_BARRIER_H_
#include "absl/base/thread_annotations.h"
#include "absl/synchronization/mutex.h"
namespace absl {
inline namespace lts_2019_08_08 {
// Barrier
//
// This class creates a barrier which blocks threads until a prespecified
// threshold of threads (`num_threads`) utilizes the barrier. A thread utilizes
// the `Barrier` by calling `Block()` on the barrier, which will block that
// thread; no call to `Block()` will return until `num_threads` threads have
// called it.
//
// Exactly one call to `Block()` will return `true`, which is then responsible
// for destroying the barrier; because stack allocation will cause the barrier
// to be deleted when it is out of scope, barriers should not be stack
// allocated.
//
// Example:
//
// // Main thread creates a `Barrier`:
// barrier = new Barrier(num_threads);
//
// // Each participating thread could then call:
// if (barrier->Block()) delete barrier; // Exactly one call to `Block()`
// // returns `true`; that call
// // deletes the barrier.
class Barrier {
public:
// `num_threads` is the number of threads that will participate in the barrier
explicit Barrier(int num_threads)
: num_to_block_(num_threads), num_to_exit_(num_threads) {}
Barrier(const Barrier&) = delete;
Barrier& operator=(const Barrier&) = delete;
// Barrier::Block()
//
// Blocks the current thread, and returns only when the `num_threads`
// threshold of threads utilizing this barrier has been reached. `Block()`
// returns `true` for precisely one caller, which may then destroy the
// barrier.
//
// Memory ordering: For any threads X and Y, any action taken by X
// before X calls `Block()` will be visible to Y after Y returns from
// `Block()`.
bool Block();
private:
Mutex lock_;
int num_to_block_ GUARDED_BY(lock_);
int num_to_exit_ GUARDED_BY(lock_);
};
} // inline namespace lts_2019_08_08
} // namespace absl
#endif // ABSL_SYNCHRONIZATION_BARRIER_H_
|
