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#pragma once
#include <util/generic/noncopyable.h>
#include <atomic>
#include <cstddef>
#include <utility>
//////////////////////////////
// lock free lifo stack
template <class T>
class TLockFreeStack: TNonCopyable {
struct TNode {
T Value;
std::atomic<TNode*> Next;
TNode() = default;
template <class U>
explicit TNode(U&& val)
: Value(std::forward<U>(val))
, Next(nullptr)
{
}
};
std::atomic<TNode*> Head = nullptr;
std::atomic<TNode*> FreePtr = nullptr;
std::atomic<size_t> DequeueCount = 0;
void TryToFreeMemory() {
TNode* current = FreePtr.load(std::memory_order_acquire);
if (!current) {
return;
}
if (DequeueCount.load() == 1) {
// node current is in free list, we are the last thread so try to cleanup
if (FreePtr.compare_exchange_strong(current, nullptr)) {
EraseList(current);
}
}
}
void EraseList(TNode* p) {
while (p) {
TNode* next = p->Next;
delete p;
p = next;
}
}
void EnqueueImpl(TNode* head, TNode* tail) {
auto headValue = Head.load(std::memory_order_acquire);
for (;;) {
tail->Next.store(headValue, std::memory_order_release);
// NB. See https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange
// The weak forms (1-2) of the functions are allowed to fail spuriously, that is,
// act as if *this != expected even if they are equal.
// When a compare-and-exchange is in a loop, the weak version will yield better
// performance on some platforms.
if (Head.compare_exchange_weak(headValue, head)) {
break;
}
}
}
template <class U>
void EnqueueImpl(U&& u) {
TNode* node = new TNode(std::forward<U>(u));
EnqueueImpl(node, node);
}
public:
TLockFreeStack() = default;
~TLockFreeStack() {
EraseList(Head.load());
EraseList(FreePtr.load());
}
void Enqueue(const T& t) {
EnqueueImpl(t);
}
void Enqueue(T&& t) {
EnqueueImpl(std::move(t));
}
template <typename TCollection>
void EnqueueAll(const TCollection& data) {
EnqueueAll(data.begin(), data.end());
}
template <typename TIter>
void EnqueueAll(TIter dataBegin, TIter dataEnd) {
if (dataBegin == dataEnd) {
return;
}
TIter i = dataBegin;
TNode* node = new TNode(*i);
TNode* tail = node;
for (++i; i != dataEnd; ++i) {
TNode* nextNode = node;
node = new TNode(*i);
node->Next.store(nextNode, std::memory_order_release);
}
EnqueueImpl(node, tail);
}
bool Dequeue(T* res) {
++DequeueCount;
for (TNode* current = Head.load(std::memory_order_acquire); current;) {
if (Head.compare_exchange_weak(current, current->Next.load(std::memory_order_acquire))) {
*res = std::move(current->Value);
// delete current; // ABA problem
// even more complex node deletion
TryToFreeMemory();
if (--DequeueCount == 0) {
// no other Dequeue()s, can safely reclaim memory
delete current;
} else {
// Dequeue()s in progress, put node to free list
for (TNode* freePtr = FreePtr.load(std::memory_order_acquire);;) {
current->Next.store(freePtr, std::memory_order_release);
if (FreePtr.compare_exchange_weak(freePtr, current)) {
break;
}
}
}
return true;
}
}
TryToFreeMemory();
--DequeueCount;
return false;
}
// add all elements to *res
// elements are returned in order of dequeue (top to bottom; see example in unittest)
template <typename TCollection>
void DequeueAll(TCollection* res) {
++DequeueCount;
for (TNode* current = Head.load(std::memory_order_acquire); current;) {
if (Head.compare_exchange_weak(current, nullptr)) {
for (TNode* x = current; x;) {
res->push_back(std::move(x->Value));
x = x->Next;
}
// EraseList(current); // ABA problem
// even more complex node deletion
TryToFreeMemory();
if (--DequeueCount == 0) {
// no other Dequeue()s, can safely reclaim memory
EraseList(current);
} else {
// Dequeue()s in progress, add nodes list to free list
TNode* currentLast = current;
while (currentLast->Next) {
currentLast = currentLast->Next;
}
for (TNode* freePtr = FreePtr.load(std::memory_order_acquire);;) {
currentLast->Next.store(freePtr, std::memory_order_release);
if (FreePtr.compare_exchange_weak(freePtr, current)) {
break;
}
}
}
return;
}
}
TryToFreeMemory();
--DequeueCount;
}
bool DequeueSingleConsumer(T* res) {
for (TNode* current = Head.load(std::memory_order_acquire); current;) {
if (Head.compare_exchange_weak(current, current->Next)) {
*res = std::move(current->Value);
delete current; // with single consumer thread ABA does not happen
return true;
}
}
return false;
}
// add all elements to *res
// elements are returned in order of dequeue (top to bottom; see example in unittest)
template <typename TCollection>
void DequeueAllSingleConsumer(TCollection* res) {
for (TNode* head = Head.load(std::memory_order_acquire); head;) {
if (Head.compare_exchange_weak(head, nullptr)) {
for (TNode* x = head; x;) {
res->push_back(std::move(x->Value));
x = x->Next;
}
EraseList(head); // with single consumer thread ABA does not happen
return;
}
}
}
bool IsEmpty() {
return Head.load() == nullptr; // without lock, so result is approximate
}
};
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