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Nicholas Ormrod
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/*
* Copyright 2015 Facebook, Inc.
*
* 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 FOLLY_EXPERIMENTAL_EVENTCOUNT_H_
#define FOLLY_EXPERIMENTAL_EVENTCOUNT_H_
#include <unistd.h>
#include <syscall.h>
#include <linux/futex.h>
#include <sys/time.h>
#include <cassert>
#include <climits>
#include <atomic>
#include <thread>
#include <folly/Bits.h>
#include <folly/Likely.h>
namespace folly {
namespace detail {
inline int futex(int* uaddr, int op, int val, const timespec* timeout,
int* uaddr2, int val3) noexcept {
return syscall(SYS_futex, uaddr, op, val, timeout, uaddr2, val3);
}
} // namespace detail
/**
* 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 {
public:
EventCount() noexcept : val_(0) { }
class Key {
friend class EventCount;
explicit Key(uint32_t e) noexcept : epoch_(e) { }
uint32_t epoch_;
};
void notify() noexcept;
void notifyAll() noexcept;
Key prepareWait() noexcept;
void cancelWait() noexcept;
void wait(Key key) noexcept;
/**
* Wait for condition() to become true. Will clean up appropriately if
* condition() throws, and then rethrow.
*/
template <class Condition>
void await(Condition condition);
private:
void doNotify(int n) noexcept;
EventCount(const EventCount&) = delete;
EventCount(EventCount&&) = delete;
EventCount& operator=(const EventCount&) = delete;
EventCount& operator=(EventCount&&) = delete;
// This requires 64-bit
static_assert(sizeof(int) == 4, "bad platform");
static_assert(sizeof(uint32_t) == 4, "bad platform");
static_assert(sizeof(uint64_t) == 8, "bad platform");
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
static constexpr size_t kEpochOffset = 1;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
static constexpr size_t kEpochOffset = 0; // in units of sizeof(int)
#else
# error Your machine uses a weird endianness!
#endif
// val_ stores the epoch in the most significant 32 bits and the
// waiter count in the least significant 32 bits.
std::atomic<uint64_t> val_;
static constexpr uint64_t kAddWaiter = uint64_t(1);
static constexpr uint64_t kSubWaiter = uint64_t(-1);
static constexpr size_t kEpochShift = 32;
static constexpr uint64_t kAddEpoch = uint64_t(1) << kEpochShift;
static constexpr uint64_t kWaiterMask = kAddEpoch - 1;
};
inline void EventCount::notify() noexcept {
doNotify(1);
}
inline void EventCount::notifyAll() noexcept {
doNotify(INT_MAX);
}
inline void EventCount::doNotify(int n) noexcept {
uint64_t prev = val_.fetch_add(kAddEpoch, std::memory_order_acq_rel);
if (UNLIKELY(prev & kWaiterMask)) {
detail::futex(reinterpret_cast<int*>(&val_) + kEpochOffset,
FUTEX_WAKE, n, nullptr, nullptr, 0);
}
}
inline EventCount::Key EventCount::prepareWait() noexcept {
uint64_t prev = val_.fetch_add(kAddWaiter, std::memory_order_acq_rel);
return Key(prev >> kEpochShift);
}
inline void EventCount::cancelWait() noexcept {
// memory_order_relaxed would suffice for correctness, but the faster
// #waiters gets to 0, the less likely it is that we'll do spurious wakeups
// (and thus system calls).
uint64_t prev = val_.fetch_add(kSubWaiter, std::memory_order_seq_cst);
assert((prev & kWaiterMask) != 0);
}
inline void EventCount::wait(Key key) noexcept {
while ((val_.load(std::memory_order_acquire) >> kEpochShift) == key.epoch_) {
detail::futex(reinterpret_cast<int*>(&val_) + kEpochOffset,
FUTEX_WAIT, key.epoch_, nullptr, nullptr, 0);
}
// memory_order_relaxed would suffice for correctness, but the faster
// #waiters gets to 0, the less likely it is that we'll do spurious wakeups
// (and thus system calls)
uint64_t prev = val_.fetch_add(kSubWaiter, std::memory_order_seq_cst);
assert((prev & kWaiterMask) != 0);
}
template <class Condition>
void EventCount::await(Condition condition) {
if (condition()) return; // fast path
// condition() is the only thing that may throw, everything else is
// noexcept, so we can hoist the try/catch block outside of the loop
try {
for (;;) {
auto key = prepareWait();
if (condition()) {
cancelWait();
break;
} else {
wait(key);
}
}
} catch (...) {
cancelWait();
throw;
}
}
} // namespace folly
#endif /* FOLLY_EXPERIMENTAL_EVENTCOUNT_H_ */
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