C++ mutlthreading synchronization issue

I have an issue with my code ... my below code is createing databases(database here is a kd tree) and indexing one image per database. i have two classes LastDatabaseTndexingPolicy and another forwardingDatabaseaccessor.cpp .

I am calling the GetDatabaseToAccess() function from forwardingDatabaseAccessor.cpp class .GetDatabaseToAccess() function is present in LastDatabaseTndexingPolicy class and it returns the database created as a pointer and using that database pointer we call another function which actually indexes the image to the database .

Now my issue is i am not able to have multiple threads act on the following functions as DatabaseAccessor_ptr db which is in the following file is coupled with two functions and however i put locks in the LastDatabaseTndexingPolicy file as below i end up getting synchronization issue ..........

Hence now i am using a single lock in forwardingDatabaseAccessor.cpp and serializing the code,.can anyone suggest me how can i change my design to parallelize this code .........

In ForwardingDatabaseAccessor.cpp we are calling function from LastDatabaseTndexingPolicy as shown below:-

DatabaseAccessor_ptr db is something which needs to be synchroinized. I tried createing 256 databases with one image each and when i run this code i ended up creating 175 databses and though i was restricting in code with locks that every database has only one image i ended up having two images in single database ..... ideally i had to get only one image per database but i got two images in few of them hence instead of 256 database this code created 175 or so databases.

indexing::IndexReturnValue ForwardDatabaseAccessor::index(cv::Mat image,
const std::string& imageName, features::Camera::Type indexCameraType,
features::Camera::Type recogCameraType) {
DatabaseAccessor_ptr db = this->IndexingPolicy_ptr->GetDBToIndex();

return db->index(image, imageName, indexCameraType, recogCameraType);
}

In file LastDatabaseTndexingPolicy we have this function GetDatabaseToIndex as follows:----

DatabaseAccessor_ptr LastDatabaseIndexingPolicy::GetDBToIndex()
{
int numDBs;
std::string db_Name;
size_t totNoOfIndexedImagesInDB;
DatabaseAccessor_ptr db;
std::vector<std::string>::iterator endIter;
{
READING_LOCK(lock, adbMutex);
numDBs = this->associateDBs->GetNumberOfAssociateDBs();
endIter = this->associateDBs->end();
endIter--;

}
if (numDBs == 0) {

std::string newDBName = this->CreateAssociateDBs(numDBs);
{
WRITING_LOCK(lock, dbMutex);
db = dbGroup.getDatabase(newDBName);
return db;
}

}

else {

WRITING_LOCK_ADV(writeLock, dbMutex);
totNoOfIndexedImagesInDB = dbGroup.getDatabase(*endIter)->size();
if (totNoOfIndexedImagesInDB >= (this->maxNumberOfImagesDBCFG))
{
writeLock.unlock();
std::string newDBName = this->CreateAssociateDBs(numDBs);
{
WRITING_LOCK(lock, dbMutex);
db = dbGroup.getDatabase(newDBName);
return db;
}
}
else
{

db = dbGroup.getDatabase(*endIter);
return db;
writeLock.unlock();

}
return db;
}
}

std::string IndexToLastDB::CreateAssociateDBs(int numDB) {

std::ostringstream convert;
convert << (numDB + 1);
std::string newDBName = this->dbName + "_server_fwd_" + convert.str();
std::string db_properties = "type=ripe";
LOG(info,dbName) << "Name of the new database: " << newDBName;
{
WRITING_LOCK(lock, dbMutex);
dbGroup.createDatabase(newDBName, db_properties);
}

{
WRITING_LOCK(lock, adbMutex);
this->associateDBs->AddAssociateDB(newDBName);
}
{
WRITING_LOCK(fileLock, fileMutex);
util::serialization::save<indexing::forward::AssociateDBCollection>(
*this->associateDBs, dbFile.string(), serializationFormat);
}

return newDBName;

}