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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#include "generationhandler.h"
#include <cassert>
namespace vespalib {
GenerationHandler::GenerationHold::GenerationHold() noexcept
: _refCount(1),
_generation(0),
_next(0)
{ }
GenerationHandler::GenerationHold::~GenerationHold() {
assert(getRefCount() == 0);
}
void
GenerationHandler::GenerationHold::setValid() noexcept {
auto old = _refCount.fetch_sub(1, std::memory_order_release);
assert(!valid(old));
}
bool
GenerationHandler::GenerationHold::setInvalid() noexcept {
uint32_t refs = 0;
if (_refCount.compare_exchange_strong(refs, 1,
std::memory_order_acq_rel,
std::memory_order_relaxed))
{
return true;
} else {
assert(valid(refs));
return false;
}
}
GenerationHandler::GenerationHold *
GenerationHandler::GenerationHold::acquire() noexcept {
if (valid(_refCount.fetch_add(2, std::memory_order_acq_rel))) {
return this;
} else {
release();
return nullptr;
}
}
GenerationHandler::GenerationHold *
GenerationHandler::GenerationHold::copy(GenerationHold *self) noexcept {
if (self == nullptr) {
return nullptr;
} else {
uint32_t oldRefCount = self->_refCount.fetch_add(2, std::memory_order_relaxed);
(void) oldRefCount;
assert(valid(oldRefCount));
return self;
}
}
GenerationHandler::Guard &
GenerationHandler::Guard::operator=(const Guard & rhs) noexcept
{
if (&rhs != this) {
cleanup();
_hold = GenerationHold::copy(rhs._hold);
}
return *this;
}
GenerationHandler::Guard &
GenerationHandler::Guard::operator=(Guard &&rhs) noexcept
{
if (&rhs != this) {
cleanup();
_hold = rhs._hold;
rhs._hold = nullptr;
}
return *this;
}
void
GenerationHandler::update_oldest_used_generation()
{
for (;;) {
if (_first == _last.load(std::memory_order_relaxed))
break; // No elements can be freed
if (!_first->setInvalid()) {
break; // First element still in use
}
GenerationHold *toFree = _first;
assert(toFree->_next != nullptr);
_first = toFree->_next;
toFree->_next = _free;
_free = toFree;
}
_oldest_used_generation.store(_first->_generation, std::memory_order_relaxed);
}
GenerationHandler::GenerationHandler()
: _generation(0),
_oldest_used_generation(0),
_last(nullptr),
_first(nullptr),
_free(nullptr),
_numHolds(0u)
{
_last = _first = new GenerationHold;
++_numHolds;
_first->_generation.store(getCurrentGeneration(), std::memory_order_relaxed);
_first->setValid();
}
GenerationHandler::~GenerationHandler(void)
{
update_oldest_used_generation();
assert(_first == _last.load(std::memory_order_relaxed));
while (_free != nullptr) {
GenerationHold *toFree = _free;
_free = toFree->_next;
--_numHolds;
delete toFree;
}
assert(_numHolds == 1);
delete _first;
}
GenerationHandler::Guard
GenerationHandler::takeGuard() const
{
Guard guard(_last.load(std::memory_order_acquire));
for (;;) {
// Must check valid() after increasing refcount
if (guard.valid())
break; // Might still be marked invalid, that's OK
/*
* Clashed with writer freeing entry. Must abandon current
* guard and try again.
*/
guard = Guard(_last.load(std::memory_order_acquire));
}
// Guard has been valid after bumping refCount
return guard;
}
void
GenerationHandler::incGeneration()
{
generation_t ngen = getNextGeneration();
auto last = _last.load(std::memory_order_relaxed);
if (last->getRefCountAcqRel() == 0) {
// Last generation is unused, morph it to new generation. This is
// the typical case when no readers are present.
set_generation(ngen);
last->_generation.store(ngen, std::memory_order_relaxed);
update_oldest_used_generation();
return;
}
GenerationHold *nhold = nullptr;
if (_free == nullptr) {
nhold = new GenerationHold;
++_numHolds;
} else {
nhold = _free;
_free = nhold->_next;
}
nhold->_generation.store(ngen, std::memory_order_relaxed);
nhold->_next = nullptr;
nhold->setValid();
last->_next = nhold;
set_generation(ngen);
_last.store(nhold, std::memory_order_release);
update_oldest_used_generation();
}
uint32_t
GenerationHandler::getGenerationRefCount(generation_t gen) const
{
if (static_cast<sgeneration_t>(gen - getCurrentGeneration()) > 0)
return 0u;
if (static_cast<sgeneration_t>(get_oldest_used_generation() - gen) > 0)
return 0u;
for (GenerationHold *hold = _first; hold != nullptr; hold = hold->_next) {
if (hold->_generation.load(std::memory_order_relaxed) == gen)
return hold->getRefCount();
}
return 0u;
}
uint64_t
GenerationHandler::getGenerationRefCount(void) const
{
uint64_t ret = 0;
for (GenerationHold *hold = _first; hold != nullptr; hold = hold->_next) {
ret += hold->getRefCount();
}
return ret;
}
}
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