GIL stands for Global Interpreter Lock. What this means is when you link your program against libpython.so and use it, you really have the entire Python interpreter in your code. The reason this exists is to make concurrent applications really easy. In Python you can have two threads reading/writing to the same location and Python automatically handles all of this for you; unlike say in Java, where you need to specify that everything is under the GIL in Python. There are two things to consider when talking about the GIL and what it does—instruction atomicity and read/write lock.
Remember that Cython necessarily generates the C code to make it look similar to any Python module that you can import. So, what's happening under the hood is that it will generate all the code to acquire lock on the GIL so that it can manipulate Python objects at runtime. Let's consider two types of execution. Firstly, you have the C stack where it executes atomically as...
