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#pragma once
#include <util/generic/noncopyable.h>
#include <util/system/atomic.h>
//////////////////////////////
// lock free lifo stack
template <class T>
class TLockFreeStack: TNonCopyable {
struct TNode {
T Value;
TNode* Next;
TNode() = default;
template <class U>
explicit TNode(U&& val)
: Value(std::forward<U>(val))
, Next(nullptr)
{
}
};
TNode* Head;
TNode* FreePtr;
TAtomic DequeueCount;
void TryToFreeMemory() {
TNode* current = AtomicGet(FreePtr);
if (!current)
return;
if (AtomicAdd(DequeueCount, 0) == 1) {
// node current is in free list, we are the last thread so try to cleanup
if (AtomicCas(&FreePtr, (TNode*)nullptr, current))
EraseList(current);
}
}
void EraseList(TNode* volatile p) {
while (p) {
TNode* next = p->Next;
delete p;
p = next;
}
}
void EnqueueImpl(TNode* volatile head, TNode* volatile tail) {
for (;;) {
tail->Next = AtomicGet(Head);
if (AtomicCas(&Head, head, tail->Next))
break;
}
}
template <class U>
void EnqueueImpl(U&& u) {
TNode* volatile node = new TNode(std::forward<U>(u));
EnqueueImpl(node, node);
}
public:
TLockFreeStack()
: Head(nullptr)
, FreePtr(nullptr)
, DequeueCount(0)
{
}
~TLockFreeStack() {
EraseList(Head);
EraseList(FreePtr);
}
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* volatile node = new TNode(*i);
TNode* volatile tail = node;
for (++i; i != dataEnd; ++i) {
TNode* nextNode = node;
node = new TNode(*i);
node->Next = nextNode;
}
EnqueueImpl(node, tail);
}
bool Dequeue(T* res) {
AtomicAdd(DequeueCount, 1);
for (TNode* current = AtomicGet(Head); current; current = AtomicGet(Head)) {
if (AtomicCas(&Head, AtomicGet(current->Next), current)) {
*res = std::move(current->Value);
// delete current; // ABA problem
// even more complex node deletion
TryToFreeMemory();
if (AtomicAdd(DequeueCount, -1) == 0) {
// no other Dequeue()s, can safely reclaim memory
delete current;
} else {
// Dequeue()s in progress, put node to free list
for (;;) {
AtomicSet(current->Next, AtomicGet(FreePtr));
if (AtomicCas(&FreePtr, current, current->Next))
break;
}
}
return true;
}
}
TryToFreeMemory();
AtomicAdd(DequeueCount, -1);
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) {
AtomicAdd(DequeueCount, 1);
for (TNode* current = AtomicGet(Head); current; current = AtomicGet(Head)) {
if (AtomicCas(&Head, (TNode*)nullptr, current)) {
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 (AtomicAdd(DequeueCount, -1) == 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 (;;) {
AtomicSet(currentLast->Next, AtomicGet(FreePtr));
if (AtomicCas(&FreePtr, current, currentLast->Next))
break;
}
}
return;
}
}
TryToFreeMemory();
AtomicAdd(DequeueCount, -1);
}
bool DequeueSingleConsumer(T* res) {
for (TNode* current = AtomicGet(Head); current; current = AtomicGet(Head)) {
if (AtomicCas(&Head, current->Next, current)) {
*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* current = AtomicGet(Head); current; current = AtomicGet(Head)) {
if (AtomicCas(&Head, (TNode*)nullptr, current)) {
for (TNode* x = current; x;) {
res->push_back(std::move(x->Value));
x = x->Next;
}
EraseList(current); // with single consumer thread ABA does not happen
return;
}
}
}
bool IsEmpty() {
AtomicAdd(DequeueCount, 0); // mem barrier
return AtomicGet(Head) == nullptr; // without lock, so result is approximate
}
};
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