// 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 preceding predicate check has
// failed.
//
// Algorithm 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)
: state_(kStackMask), waiters_(waiters) {
eigen_plain_assert(waiters.size() < (1 << kWaiterBits) - 1);
}
~EventCount() {
// Ensure there are no waiters.
eigen_plain_assert(state_.load() == kStackMask);
}
// Prewait prepares for waiting.
// After calling Prewait, the thread must re-check the wait predicate
// and then call either CancelWait or CommitWait.
void Prewait() {
uint64_t state = state_.load(std::memory_order_relaxed);
for (;;) {
CheckState(state);
uint64_t newstate = state + kWaiterInc;
CheckState(newstate);
if (state_.compare_exchange_weak(state, newstate,
std::memory_order_seq_cst))
return;
}
}
// CommitWait commits waiting after Prewait.
void CommitWait(Waiter* w) {
eigen_plain_assert((w->epoch & ~kEpochMask) == 0);
w->state = Waiter::kNotSignaled;
const uint64_t me = (w - &waiters_[0]) | w->epoch;
uint64_t state = state_.load(std::memory_order_seq_cst);
for (;;) {
CheckState(state,
true);
uint64_t newstate;
if ((state & kSignalMask) != 0) {
// Consume the signal and return immidiately.
newstate = state - kWaiterInc - kSignalInc;
}
else {
// Remove this thread from pre-wait counter and add to the waiter stack.
newstate = ((state & kWaiterMask) - kWaiterInc) | me;
w->next.store(state & (kStackMask | kEpochMask),
std::memory_order_relaxed);
}
CheckState(newstate);
if (state_.compare_exchange_weak(state, newstate,
std::memory_order_acq_rel)) {
if ((state & kSignalMask) == 0) {
w->epoch += kEpochInc;
Park(w);
}
return;
}
}
}
// CancelWait cancels effects of the previous Prewait call.
void CancelWait() {
uint64_t state = state_.load(std::memory_order_relaxed);
for (;;) {
CheckState(state,
true);
uint64_t newstate = state - kWaiterInc;
// We don't know if the thread was also notified or not,
// so we should not consume a signal unconditionaly.
// Only if number of waiters is equal to number of signals,
// we know that the thread was notified and we must take away the signal.
if (((state & kWaiterMask) >> kWaiterShift) ==
((state & kSignalMask) >> kSignalShift))
newstate -= kSignalInc;
CheckState(newstate);
if (state_.compare_exchange_weak(state, newstate,
std::memory_order_acq_rel))
return;
}
}
// Notify wakes one or all waiting threads.
// Must be called after changing the associated wait predicate.
void Notify(
bool notifyAll) {
std::atomic_thread_fence(std::memory_order_seq_cst);
uint64_t state = state_.load(std::memory_order_acquire);
for (;;) {
CheckState(state);
const uint64_t waiters = (state & kWaiterMask) >> kWaiterShift;
const uint64_t signals = (state & kSignalMask) >> kSignalShift;
// Easy case: no waiters.
if ((state & kStackMask) == kStackMask && waiters == signals)
return;
uint64_t newstate;
if (notifyAll) {
// Empty wait stack and set signal to number of pre-wait threads.
newstate =
(state & kWaiterMask) | (waiters << kSignalShift) | kStackMask;
}
else if (signals < waiters) {
// There is a thread in pre-wait state, unblock it.
newstate = state + kSignalInc;
}
else {
// Pop a waiter from list and unpark it.
Waiter* w = &waiters_[state & kStackMask];
uint64_t next = w->next.load(std::memory_order_relaxed);
newstate = (state & (kWaiterMask | kSignalMask)) | next;
}
CheckState(newstate);
if (state_.compare_exchange_weak(state, newstate,
std::memory_order_acq_rel)) {
if (!notifyAll && (signals < waiters))
return;
// unblocked pre-wait thread
if ((state & kStackMask) == kStackMask)
return;
Waiter* w = &waiters_[state & kStackMask];
if (!notifyAll) w->next.store(kStackMask, 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<uint64_t> next;
std::mutex mu;
std::condition_variable cv;
uint64_t epoch = 0;
unsigned state = kNotSignaled;
enum {
kNotSignaled,
kWaiting,
kSignaled,
};
};
private:
// State_ layout:
// - low kWaiterBits is a stack of waiters committed wait
// (indexes in waiters_ array are used as stack elements,
// kStackMask means empty stack).
// - next kWaiterBits is count of waiters in prewait state.
// - next kWaiterBits is count of pending signals.
// - remaining bits are ABA counter for the stack.
// (stored in Waiter node and incremented on push).
static const uint64_t kWaiterBits = 14;
static const uint64_t kStackMask = (1ull << kWaiterBits) - 1;
static const uint64_t kWaiterShift = kWaiterBits;
static const uint64_t kWaiterMask = ((1ull << kWaiterBits) - 1)
<< kWaiterShift;
static const uint64_t kWaiterInc = 1ull << kWaiterShift;
static const uint64_t kSignalShift = 2 * kWaiterBits;
static const uint64_t kSignalMask = ((1ull << kWaiterBits) - 1)
<< kSignalShift;
static const uint64_t kSignalInc = 1ull << kSignalShift;
static const uint64_t kEpochShift = 3 * kWaiterBits;
static const uint64_t kEpochBits = 64 - kEpochShift;
static const uint64_t kEpochMask = ((1ull << kEpochBits) - 1) << kEpochShift;
static const uint64_t kEpochInc = 1ull << kEpochShift;
std::atomic<uint64_t> state_;
MaxSizeVector<Waiter>& waiters_;
static void CheckState(uint64_t state,
bool waiter =
false) {
static_assert(kEpochBits >= 20,
"not enough bits to prevent ABA problem");
const uint64_t waiters = (state & kWaiterMask) >> kWaiterShift;
const uint64_t signals = (state & kSignalMask) >> kSignalShift;
eigen_plain_assert(waiters >= signals);
eigen_plain_assert(waiters < (1 << kWaiterBits) - 1);
eigen_plain_assert(!waiter || waiters > 0);
(
void)waiters;
(
void)signals;
}
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* w) {
for (Waiter* next; w; w = next) {
uint64_t wnext = w->next.load(std::memory_order_relaxed) & kStackMask;
next = wnext == kStackMask ? nullptr : &waiters_[wnext];
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_
