sharedreaderlock.cpp
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/* Copyright (C) 2019
*
* This file is part of the osdev components suite
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License as published by the
* Free Software Foundation; either version 2, or (at your option) any
* later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#include "sharedreaderlock.h"
// #include "mlogic/common/invalidoperationexception.h"
#include "lockguard.h"
#include "mqttstream.h"
// #include "mlogic/common/logger/loggerprovider.h"
using namespace osdev::components::mqtt;
// What happens if a read lock owner locks again
// - another read lock : map the threads owning read locks and update a count.
// - a write lock : upgrade the read lock to an exclusive write lock (including all recursive locks).
//
// The same for a write lock owner.
// - another write lock : map the threads owning a write lock and update the count.
// - a read lock : treat the lock as an exclusive write lock
//
// What happens if a thread that never locks does an unlock.
// - when no locks are active : nothing happens
// - during active read locks : nothing happens
// - during active write lock : nothing happens
//
// What happens when multiple threads want a write lock.
// - Only one writer can be active at any time. Other writers will have to wait until the active writer has unlocked.
//
namespace {
/**
* @param lockMap The map to check for active locks.
* @return true when the lockMap contains an active lock, false otherwise.
*/
bool hasActiveLock(const std::map<std::thread::id, LockData>& lockMap)
{
for (const auto& lockPair : lockMap) {
if (lockPair.second.active()) {
return true;
}
}
return false;
}
} // namespace
SharedReaderLock::SharedReaderLock()
: m_mutex()
, m_readLockMap()
, m_writeLockMap()
, m_readersCV()
, m_writersCV()
{
}
SharedReaderLock::~SharedReaderLock()
{
OSDEV_COMPONENTS_LOCKGUARD(m_mutex);
if (!m_readLockMap.empty() || !m_writeLockMap.empty())
{
// toLogFile ("SharedReaderLock", "Cannot destroy this lock because threads are still registered. Readers : %1, Writers : %2", m_readLockMap, m_writeLockMap);
// (InvalidOperationException, "Cannot destroy SharedReaderLock");
}
}
void SharedReaderLock::lockShared()
{
OSDEV_COMPONENTS_UNIQUELOCK_CREATE(m_mutex);
if (m_writeLockMap.end() != m_writeLockMap.find(std::this_thread::get_id()))
{ // If the thread owns a write lock than update the write lock count
m_writeLockMap[std::this_thread::get_id()].increase();
return;
}
auto resultPair = m_readLockMap.insert(std::make_pair(std::this_thread::get_id(), LockData{}));
if (!resultPair.second)
{ // thread has a read lock already...
resultPair.first->second.increase(); // add recursive reader lock
return; // Already owner of the shared lock, so proceed.
}
m_readersCV.wait(OSDEV_COMPONENTS_UNIQUELOCK(m_mutex), [this] { return m_writeLockMap.empty(); });
// Reread the iterator because it might have been invalidated.
auto it = m_readLockMap.find(std::this_thread::get_id());
if (m_readLockMap.end() == it)
{
// normally Throw(InvalidOperationException, "Thread id must be registered in the shared reader map at this point");
}
it->second.increase();
// the reader is now registered and is a shared owner of the SharedReaderLock
}
void SharedReaderLock::lockExclusive()
{
OSDEV_COMPONENTS_UNIQUELOCK_CREATE(m_mutex);
// First check if the thread already owns a read lock.
auto readLockIter = m_readLockMap.find(std::this_thread::get_id());
if (m_readLockMap.end() != readLockIter) {
auto resultPair = m_writeLockMap.insert(std::make_pair(readLockIter->first, LockData::initialize(readLockIter->second)));
if (!resultPair.second)
{
// throw (InvalidOperationException, "Thread id cannot be registered in the write map at this point");
}
// toLogFile ("SharedReaderLock", "Upgrade to exclusive lock");
m_readLockMap.erase(readLockIter);
// This is an upgrade of the read lock to an exclusive write lock.
// proceed to the wait call on m_writersCV.
}
else
{
auto resultPair = m_writeLockMap.insert(std::make_pair(std::this_thread::get_id(), LockData{}));
if (!resultPair.second)
{ // thread has a write lock already...
resultPair.first->second.increase(); // add recursive write lock
return; // Already owner of the exclusive lock, so proceed.
}
}
m_writersCV.wait(OSDEV_COMPONENTS_UNIQUELOCK(m_mutex), [this] { return !hasActiveLock(m_readLockMap) && !hasActiveLock(m_writeLockMap); });
// Reread the iterator because it might have been invalidated.
auto it = m_writeLockMap.find(std::this_thread::get_id());
if (m_writeLockMap.end() == it)
{
// throw (InvalidOperationException, "Thread id must be registered in the writer map at this point");
}
it->second.increase();
// the thread is now registered and is an exclusive owner of the SharedReaderLock
}
void SharedReaderLock::unlock()
{
OSDEV_COMPONENTS_UNIQUELOCK_CREATE(m_mutex);
auto readLockIter = m_readLockMap.find(std::this_thread::get_id());
auto writeLockIter = m_writeLockMap.find(std::this_thread::get_id());
if (m_writeLockMap.end() != writeLockIter && m_readLockMap.end() != readLockIter)
{
return; // thread does not have a lock. Do nothing.
}
if (m_readLockMap.end() != readLockIter)
{
if (!readLockIter->second.decrease())
{ // The lock is not active anymore so remove it
m_readLockMap.erase(readLockIter);
if (!hasActiveLock(m_readLockMap) && !m_writeLockMap.empty())
{ // no active read lock anymore and a writer that waits for an exclusive lock.
// toLogFileDebug("SharedReaderLock", "notify_one writersCV");
OSDEV_COMPONENTS_UNIQUELOCK_UNLOCK(m_mutex);
m_writersCV.notify_one();
}
}
return;
}
if (m_writeLockMap.end() != writeLockIter)
{
if (!writeLockIter->second.decrease())
{
// remove the exclusive write lock
m_writeLockMap.erase(writeLockIter);
if (m_writeLockMap.empty())
{
// toLogFile ("SharedReaderLock", "notify_all readersCV");
OSDEV_COMPONENTS_UNIQUELOCK_UNLOCK(m_mutex);
m_readersCV.notify_all();
}
else
{
// toLogFile ("SharedReaderLock", "notify_one writersCV");
OSDEV_COMPONENTS_UNIQUELOCK_UNLOCK(m_mutex);
m_writersCV.notify_one();
}
}
return;
}
}