[Question] about Linux kernel learning

[Doubt] for details about Linux kernel learning-General Linux technology-Linux programming and kernel, refer to the following. I am a beginner in linux. I have encountered some questions that I don't understand. I don't know how many new users can help me solve them. thank you.
1. each process has its own kernel stack. in the vicinity of the task data structure, the driver module can be called by each process, where the variables defined in it are stored, so that every process can use the variables it defines?
2. some kernel functions have a _ identifier before them. what is the purpose of this function?