1/*
2 * Copyright 2018-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/Portability.h>
18
19#if FOLLY_HAS_COROUTINES
20
21#include <folly/experimental/coro/Baton.h>
22
23#include <cassert>
24
25using namespace folly::coro;
26
27Baton::~Baton() {
28 // Should not be any waiting coroutines when the baton is destruced.
29 // Caller should ensure the baton is posted before destructing.
30 assert(
31 state_.load(std::memory_order_relaxed) == static_cast<void*>(this) ||
32 state_.load(std::memory_order_relaxed) == nullptr);
33}
34
35void Baton::post() noexcept {
36 void* signalledState = static_cast<void*>(this);
37 void* oldValue = state_.exchange(signalledState, std::memory_order_acq_rel);
38 if (oldValue != signalledState && oldValue != nullptr) {
39 // We are the first thread to set the state to signalled and there is
40 // a waiting coroutine. We are responsible for resuming it.
41 WaitOperation* awaiter = static_cast<WaitOperation*>(oldValue);
42 awaiter->awaitingCoroutine_.resume();
43 }
44}
45
46bool Baton::waitImpl(WaitOperation* awaiter) const noexcept {
47 void* oldValue = nullptr;
48 if (!state_.compare_exchange_strong(
49 oldValue,
50 static_cast<void*>(awaiter),
51 std::memory_order_release,
52 std::memory_order_acquire)) {
53 // If the compare-exchange fails it should be because the baton was
54 // set to the signalled state. If this not the case then this could
55 // indicate that there are two awaiting coroutines.
56 assert(oldValue == static_cast<const void*>(this));
57 return false;
58 }
59 return true;
60}
61
62#endif // FOLLY_HAS_COROUTINES
63