tdlight/tdutils/td/utils/HazardPointers.h

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//
// Copyright Aliaksei Levin (levlam@telegram.org), Arseny Smirnov (arseny30@gmail.com) 2014-2019
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#pragma once
#include "td/utils/common.h"
#include <array>
#include <atomic>
namespace td {
template <class T, int MaxPointersN = 1>
class HazardPointers {
public:
explicit HazardPointers(size_t threads_n) : threads_(threads_n) {
for (auto &data : threads_) {
for (auto &ptr : data.hazard) {
// workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64658
#if TD_GCC && GCC_VERSION <= 40902
ptr = nullptr;
#else
std::atomic_init(&ptr, static_cast<T *>(nullptr));
#endif
}
}
}
HazardPointers(const HazardPointers &other) = delete;
HazardPointers &operator=(const HazardPointers &other) = delete;
HazardPointers(HazardPointers &&other) = delete;
HazardPointers &operator=(HazardPointers &&other) = delete;
class Holder {
public:
T *protect(std::atomic<T *> &to_protect) {
return do_protect(hazard_ptr_, to_protect);
}
Holder(const Holder &other) = delete;
Holder &operator=(const Holder &other) = delete;
Holder(Holder &&other) = default; // TODO
Holder &operator=(Holder &&other) = delete;
~Holder() {
clear();
}
void clear() {
hazard_ptr_.store(nullptr, std::memory_order_release);
}
private:
friend class HazardPointers;
explicit Holder(std::atomic<T *> &ptr) : hazard_ptr_(ptr) {
}
std::atomic<T *> &hazard_ptr_;
};
Holder get_holder(size_t thread_id, size_t pos) {
return Holder(get_hazard_ptr(thread_id, pos));
}
void retire(size_t thread_id, T *ptr = nullptr) {
CHECK(thread_id < threads_.size());
auto &data = threads_[thread_id];
if (ptr) {
data.to_delete.push_back(unique_ptr<T>(ptr));
}
for (auto it = data.to_delete.begin(); it != data.to_delete.end();) {
if (!is_protected(it->get())) {
it->reset();
it = data.to_delete.erase(it);
} else {
++it;
}
}
}
// old inteface
T *protect(size_t thread_id, size_t pos, std::atomic<T *> &ptr) {
return do_protect(get_hazard_ptr(thread_id, pos), ptr);
}
void clear(size_t thread_id, size_t pos) {
do_clear(get_hazard_ptr(thread_id, pos));
}
size_t to_delete_size_unsafe() const {
size_t res = 0;
for (auto &thread : threads_) {
res += thread.to_delete.size();
}
return res;
}
private:
struct ThreadData {
std::array<std::atomic<T *>, MaxPointersN> hazard;
char pad[TD_CONCURRENCY_PAD - sizeof(std::array<std::atomic<T *>, MaxPointersN>)];
// stupid gc
std::vector<unique_ptr<T>> to_delete;
char pad2[TD_CONCURRENCY_PAD - sizeof(std::vector<unique_ptr<T>>)];
};
std::vector<ThreadData> threads_;
char pad2[TD_CONCURRENCY_PAD - sizeof(std::vector<ThreadData>)];
static T *do_protect(std::atomic<T *> &hazard_ptr, std::atomic<T *> &to_protect) {
T *saved = nullptr;
T *to_save;
while ((to_save = to_protect.load()) != saved) {
hazard_ptr.store(to_save);
saved = to_save;
}
return saved;
}
static void do_clear(std::atomic<T *> &hazard_ptr) {
hazard_ptr.store(nullptr, std::memory_order_release);
}
bool is_protected(T *ptr) {
for (auto &thread : threads_) {
for (auto &hazard_ptr : thread.hazard) {
if (hazard_ptr.load() == ptr) {
return true;
}
}
}
return false;
}
std::atomic<T *> &get_hazard_ptr(size_t thread_id, size_t pos) {
CHECK(thread_id < threads_.size());
return threads_[thread_id].hazard[pos];
}
};
} // namespace td