// 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_