folly/folly/synchronization/EventCount.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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
 *
 *     http://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.
 */

#pragma once

#include <atomic>
#include <climits>
#include <thread>

#include <glog/logging.h>

#include <folly/Likely.h>
#include <folly/detail/Futex.h>
#include <folly/lang/Bits.h>
#include <folly/portability/SysTime.h>
#include <folly/portability/Unistd.h>

namespace folly {

/**
 * Event count: a condition variable for lock free algorithms.
 *
 * See http://www.1024cores.net/home/lock-free-algorithms/eventcounts for
 * details.
 *
 * Event counts allow you to convert a non-blocking lock-free / wait-free
 * algorithm into a blocking one, by isolating the blocking logic.  You call
 * prepareWait() before checking your condition and then either cancelWait()
 * or wait() depending on whether the condition was true.  When another
 * thread makes the condition true, it must call notify() / notifyAll() just
 * like a regular condition variable.
 *
 * If "<" denotes the happens-before relationship, consider 2 threads (T1 and
 * T2) and 3 events:
 * - E1: T1 returns from prepareWait
 * - E2: T1 calls wait
 *   (obviously E1 < E2, intra-thread)
 * - E3: T2 calls notifyAll
 *
 * If E1 < E3, then E2's wait will complete (and T1 will either wake up,
 * or not block at all)
 *
 * This means that you can use an EventCount in the following manner:
 *
 * Waiter:
 *   if (!condition()) {  // handle fast path first
 *     for (;;) {
 *       auto key = eventCount.prepareWait();
 *       if (condition()) {
 *         eventCount.cancelWait();
 *         break;
 *       } else {
 *         eventCount.wait(key);
 *       }
 *     }
 *  }
 *
 *  (This pattern is encapsulated in await())
 *
 * Poster:
 *   make_condition_true();
 *   eventCount.notifyAll();
 *
 * Note that, just like with regular condition variables, the waiter needs to
 * be tolerant of spurious wakeups and needs to recheck the condition after
 * being woken up.  Also, as there is no mutual exclusion implied, "checking"
 * the condition likely means attempting an operation on an underlying
 * data structure (push into a lock-free queue, etc) and returning true on
 * success and false on failure.
 */
class EventCount { … };

inline void EventCount::notify() noexcept { … }

inline void EventCount::notifyAll() noexcept { … }

inline void EventCount::doNotify(int n) noexcept { … }

inline EventCount::Key EventCount::prepareWait() noexcept { … }

inline void EventCount::cancelWait() noexcept { … }

inline void EventCount::wait(Key key) noexcept { … }

template <class Condition>
void EventCount::await(Condition condition) { … }

} // namespace folly