1/*
2 * Copyright 2015-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#include <folly/futures/Barrier.h>
18#include <folly/lang/Exception.h>
19
20namespace folly {
21namespace futures {
22
23Barrier::Barrier(uint32_t n)
24 : size_(n), controlBlock_(allocateControlBlock()) {}
25
26Barrier::~Barrier() {
27 auto block = controlBlock_.load(std::memory_order_relaxed);
28 auto prev = block->valueAndReaderCount.load(std::memory_order_relaxed);
29 DCHECK_EQ(prev >> kReaderShift, 0u);
30 auto val = prev & kValueMask;
31 auto p = promises(block);
32
33 for (uint32_t i = 0; i < val; ++i) {
34 p[i].setException(
35 folly::make_exception_wrapper<std::runtime_error>("Barrier destroyed"));
36 }
37
38 freeControlBlock(controlBlock_);
39}
40
41auto Barrier::allocateControlBlock() -> ControlBlock* {
42 auto storage = malloc(controlBlockSize(size_));
43 if (!storage) {
44 throw_exception<std::bad_alloc>();
45 }
46 auto block = ::new (storage) ControlBlock();
47
48 auto p = promises(block);
49 uint32_t i = 0;
50 try {
51 for (i = 0; i < size_; ++i) {
52 new (p + i) BoolPromise();
53 }
54 } catch (...) {
55 for (; i != 0; --i) {
56 p[i - 1].~BoolPromise();
57 }
58 throw;
59 }
60
61 return block;
62}
63
64void Barrier::freeControlBlock(ControlBlock* block) {
65 auto p = promises(block);
66 for (uint32_t i = size_; i != 0; --i) {
67 p[i - 1].~BoolPromise();
68 }
69 free(block);
70}
71
72folly::Future<bool> Barrier::wait() {
73 // Load the current control block first. As we know there is at least
74 // one thread in the current epoch (us), this means that the value is
75 // < size_, so controlBlock_ can't change until we bump the value below.
76 auto block = controlBlock_.load(std::memory_order_acquire);
77 auto p = promises(block);
78
79 // Bump the value and record ourselves as reader.
80 // This ensures that block stays allocated, as the reader count is > 0.
81 auto prev = block->valueAndReaderCount.fetch_add(
82 kReader + 1, std::memory_order_acquire);
83
84 auto prevValue = static_cast<uint32_t>(prev & kValueMask);
85 DCHECK_LT(prevValue, size_);
86 auto future = p[prevValue].getFuture();
87
88 if (prevValue + 1 == size_) {
89 // Need to reset the barrier before fulfilling any futures. This is
90 // when the epoch is flipped to the next.
91 controlBlock_.store(allocateControlBlock(), std::memory_order_release);
92
93 p[0].setValue(true);
94 for (uint32_t i = 1; i < size_; ++i) {
95 p[i].setValue(false);
96 }
97 }
98
99 // Free the control block if we're the last reader at max value.
100 prev =
101 block->valueAndReaderCount.fetch_sub(kReader, std::memory_order_acq_rel);
102 if (prev == (kReader | uint64_t(size_))) {
103 freeControlBlock(block);
104 }
105
106 return future;
107}
108
109} // namespace futures
110} // namespace folly
111