src/Yarn/ConditionVariable.hpp

// Copyright 2019 The SwiftShader Authors. All Rights Reserved.
//
// 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.

#ifndef yarn_condition_variable_hpp
#define yarn_condition_variable_hpp

#include "Containers.hpp"
#include "Debug.hpp"
#include "Scheduler.hpp"

#include <atomic>
#include <mutex>

namespace yarn {

// ConditionVariable is a synchronization primitive that can be used to block
// one or more fibers or threads, until another fiber or thread modifies a
// shared variable (the condition) and notifies the ConditionVariable.
//
// If the ConditionVariable is blocked on a thread with a Scheduler bound, the
// thread will work on other tasks until the ConditionVariable is unblocked.
class ConditionVariable
{
public:
    // Notifies and potentially unblocks one waiting fiber or thread.
    inline void notify_one();

    // Notifies and potentially unblocks all waiting fibers and/or threads.
    inline void notify_all();

    // Blocks the current fiber or thread until the predicate is satisfied
    // and the ConditionVariable is notified.
    template <typename Predicate>
    inline void wait(std::unique_lock<std::mutex>& lock, Predicate pred);

private:
    std::mutex mutex;
    containers::vector<Scheduler::Fiber*, 4> waiting;
    std::condition_variable condition;
    std::atomic<int> numWaiting = { 0 };
    std::atomic<int> numWaitingOnCondition = { 0 };
};

void ConditionVariable::notify_one()
{
    if (numWaiting == 0) { return; }
    std::unique_lock<std::mutex> lock(mutex);
    if (waiting.size() > 0)
    {
        auto fiber = waiting.back();
        waiting.pop_back();
        fiber->schedule();
    }
    lock.unlock();
    if (numWaitingOnCondition > 0) { condition.notify_one(); }
}

void ConditionVariable::notify_all()
{
    if (numWaiting == 0) { return; }
    std::unique_lock<std::mutex> lock(mutex);
    while (waiting.size() > 0)
    {
        auto fiber = waiting.back();
        waiting.pop_back();
        fiber->schedule();
    }
    lock.unlock();
    if (numWaitingOnCondition > 0) { condition.notify_all(); }
}

template <typename Predicate>
void ConditionVariable::wait(std::unique_lock<std::mutex>& dataLock, Predicate pred)
{
    if (pred())
    {
        return;
    }
    numWaiting++;
    if (auto fiber = Scheduler::Fiber::current())
    {
        // Currently executing on a scheduler fiber.
        // Yield to let other tasks run that can unblock this fiber.
        while (!pred())
        {
            mutex.lock();
            waiting.push_back(fiber);
            mutex.unlock();

            dataLock.unlock();
            fiber->yield();
            dataLock.lock();
        }
    }
    else
    {
        // Currently running outside of the scheduler.
        // Delegate to the std::condition_variable.
        numWaitingOnCondition++;
        condition.wait(dataLock, pred);
        numWaitingOnCondition--;
    }
    numWaiting--;
}

} // namespace yarn

#endif // yarn_condition_variable_hpp