EventCount.h

// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) 2016 Dmitry Vyukov <dvyukov@google.com>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

#ifndef EIGEN_CXX11_THREADPOOL_EVENTCOUNT_H_
#define EIGEN_CXX11_THREADPOOL_EVENTCOUNT_H_

namespace Eigen {

// EventCount allows to wait for arbitrary predicates in non-blocking
// algorithms. Think of condition variable, but wait predicate does not need to
// be protected by a mutex. Usage:
// Waiting thread does:
//
//   if (predicate)
//     return act();
//   EventCount::Waiter& w = waiters[my_index];
//   ec.Prewait(&w);
//   if (predicate) {
//     ec.CancelWait(&w);
//     return act();
//   }
//   ec.CommitWait(&w);
//
// Notifying thread does:
//
//   predicate = true;
//   ec.Notify(true);
//
// Notify is cheap if there are no waiting threads. Prewait/CommitWait are not
// cheap, but they are executed only if the preceeding predicate check has
// failed.
//
// Algorihtm outline:
// There are two main variables: predicate (managed by user) and state_.
// Operation closely resembles Dekker mutual algorithm:
// https://en.wikipedia.org/wiki/Dekker%27s_algorithm
// Waiting thread sets state_ then checks predicate, Notifying thread sets
// predicate then checks state_. Due to seq_cst fences in between these
// operations it is guaranteed than either waiter will see predicate change
// and won't block, or notifying thread will see state_ change and will unblock
// the waiter, or both. But it can't happen that both threads don't see each
// other changes, which would lead to deadlock.
class EventCount {
 public:
  class Waiter;

  EventCount(MaxSizeVector<Waiter>& waiters) : waiters_(waiters) {
    eigen_assert(waiters.size() < (1 << kWaiterBits) - 1);
    // Initialize epoch to something close to overflow to test overflow.
    state_ = kStackMask | (kEpochMask - kEpochInc * waiters.size() * 2);
  }

  ~EventCount() {
    // Ensure there are no waiters.
    eigen_assert((state_.load() & (kStackMask | kWaiterMask)) == kStackMask);
  }

  // Prewait prepares for waiting.
  // After calling this function the thread must re-check the wait predicate
  // and call either CancelWait or CommitWait passing the same Waiter object.
  void Prewait(Waiter* w) {
    w->epoch = state_.fetch_add(kWaiterInc, std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_seq_cst);
  }

  // CommitWait commits waiting.
  void CommitWait(Waiter* w) {
    w->state = Waiter::kNotSignaled;
    // Modification epoch of this waiter.
    uint64_t epoch =
        (w->epoch & kEpochMask) +
        (((w->epoch & kWaiterMask) >> kWaiterShift) << kEpochShift);
    uint64_t state = state_.load(std::memory_order_seq_cst);
    for (;;) {
      if (int64_t((state & kEpochMask) - epoch) < 0) {
        // The preceeding waiter has not decided on its fate. Wait until it
        // calls either CancelWait or CommitWait, or is notified.
        EIGEN_THREAD_YIELD();
        state = state_.load(std::memory_order_seq_cst);
        continue;
      }
      // We've already been notified.
      if (int64_t((state & kEpochMask) - epoch) > 0) return;
      // Remove this thread from prewait counter and add it to the waiter list.
      eigen_assert((state & kWaiterMask) != 0);
      uint64_t newstate = state - kWaiterInc + kEpochInc;
      newstate = (newstate & ~kStackMask) | (w - &waiters_[0]);
      if ((state & kStackMask) == kStackMask)
        w->next.store(nullptr, std::memory_order_relaxed);
      else
        w->next.store(&waiters_[state & kStackMask], std::memory_order_relaxed);
      if (state_.compare_exchange_weak(state, newstate,
                                       std::memory_order_release))
        break;
    }
    Park(w);
  }

  // CancelWait cancels effects of the previous Prewait call.
  void CancelWait(Waiter* w) {
    uint64_t epoch =
        (w->epoch & kEpochMask) +
        (((w->epoch & kWaiterMask) >> kWaiterShift) << kEpochShift);
    uint64_t state = state_.load(std::memory_order_relaxed);
    for (;;) {
      if (int64_t((state & kEpochMask) - epoch) < 0) {
        // The preceeding waiter has not decided on its fate. Wait until it
        // calls either CancelWait or CommitWait, or is notified.
        EIGEN_THREAD_YIELD();
        state = state_.load(std::memory_order_relaxed);
        continue;
      }
      // We've already been notified.
      if (int64_t((state & kEpochMask) - epoch) > 0) return;
      // Remove this thread from prewait counter.
      eigen_assert((state & kWaiterMask) != 0);
      if (state_.compare_exchange_weak(state, state - kWaiterInc + kEpochInc,
                                       std::memory_order_relaxed))
        return;
    }
  }

  // Notify wakes one or all waiting threads.
  // Must be called after changing the associated wait predicate.
  void Notify(bool all) {
    std::atomic_thread_fence(std::memory_order_seq_cst);
    uint64_t state = state_.load(std::memory_order_acquire);
    for (;;) {
      // Easy case: no waiters.
      if ((state & kStackMask) == kStackMask && (state & kWaiterMask) == 0)
        return;
      uint64_t waiters = (state & kWaiterMask) >> kWaiterShift;
      uint64_t newstate;
      if (all) {
        // Reset prewait counter and empty wait list.
        newstate = (state & kEpochMask) + (kEpochInc * waiters) + kStackMask;
      } else if (waiters) {
        // There is a thread in pre-wait state, unblock it.
        newstate = state + kEpochInc - kWaiterInc;
      } else {
        // Pop a waiter from list and unpark it.
        Waiter* w = &waiters_[state & kStackMask];
        Waiter* wnext = w->next.load(std::memory_order_relaxed);
        uint64_t next = kStackMask;
        if (wnext != nullptr) next = wnext - &waiters_[0];
        // Note: we don't add kEpochInc here. ABA problem on the lock-free stack
        // can't happen because a waiter is re-pushed onto the stack only after
        // it was in the pre-wait state which inevitably leads to epoch
        // increment.
        newstate = (state & kEpochMask) + next;
      }
      if (state_.compare_exchange_weak(state, newstate,
                                       std::memory_order_acquire)) {
        if (!all && waiters) return;  // unblocked pre-wait thread
        if ((state & kStackMask) == kStackMask) return;
        Waiter* w = &waiters_[state & kStackMask];
        if (!all) w->next.store(nullptr, std::memory_order_relaxed);
        Unpark(w);
        return;
      }
    }
  }

  class Waiter {
    friend class EventCount;
    // Align to 128 byte boundary to prevent false sharing with other Waiter objects in the same vector.
    EIGEN_ALIGN_TO_BOUNDARY(128) std::atomic<Waiter*> next;
    std::mutex mu;
    std::condition_variable cv;
    uint64_t epoch;
    unsigned state;
    enum {
      kNotSignaled,
      kWaiting,
      kSignaled,
    };
  };

 private:
  // State_ layout:
  // - low kStackBits is a stack of waiters committed wait.
  // - next kWaiterBits is count of waiters in prewait state.
  // - next kEpochBits is modification counter.
  static const uint64_t kStackBits = 16;
  static const uint64_t kStackMask = (1ull << kStackBits) - 1;
  static const uint64_t kWaiterBits = 16;
  static const uint64_t kWaiterShift = 16;
  static const uint64_t kWaiterMask = ((1ull << kWaiterBits) - 1)
                                      << kWaiterShift;
  static const uint64_t kWaiterInc = 1ull << kWaiterBits;
  static const uint64_t kEpochBits = 32;
  static const uint64_t kEpochShift = 32;
  static const uint64_t kEpochMask = ((1ull << kEpochBits) - 1) << kEpochShift;
  static const uint64_t kEpochInc = 1ull << kEpochShift;
  std::atomic<uint64_t> state_;
  MaxSizeVector<Waiter>& waiters_;

  void Park(Waiter* w) {
    std::unique_lock<std::mutex> lock(w->mu);
    while (w->state != Waiter::kSignaled) {
      w->state = Waiter::kWaiting;
      w->cv.wait(lock);
    }
  }

  void Unpark(Waiter* waiters) {
    Waiter* next = nullptr;
    for (Waiter* w = waiters; w; w = next) {
      next = w->next.load(std::memory_order_relaxed);
      unsigned state;
      {
        std::unique_lock<std::mutex> lock(w->mu);
        state = w->state;
        w->state = Waiter::kSignaled;
      }
      // Avoid notifying if it wasn't waiting.
      if (state == Waiter::kWaiting) w->cv.notify_one();
    }
  }

  EventCount(const EventCount&) = delete;
  void operator=(const EventCount&) = delete;
};

}  // namespace Eigen

#endif  // EIGEN_CXX11_THREADPOOL_EVENTCOUNT_H_