C++ :: Creating Databases And Indexing One Image Per Database - Multithreading Synchronization

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,. 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();

[Code] .....