Is there any difference between "mutex" and "atomic operation"?

Question

I'm learning Operating System now, and I'm quite confused with the two concepts - mutex and atomic operation. In my understanding, they are the same, but my OS instructor gave us such a question,

Suppose a multi-processor operating system kernel tracks the number of processes created by each user. This operating system kernel maintains a counter variable for each user that it increments every time it creates a new process for a user and decrements every time a process from that user terminates. Furthermore, this operating system runs on a processor that provides atomic fetch-and-increment and fetch-and-decrement instructions. Should the operating system update the counter using the atomic increment and decrement instructions, or should it update the counter in a critical section protected by a mutex?

This question indicates that mutex and atomic operation are two things. Could anyone help me with it?

Answer 1

An atomic operation is one that cannot be subdivided into smaller parts. As such, it will never be halfway done, so you can guarantee that it will always be observed in a consistent state. For example, modern hardware implements atomic compare-and-swap operations.

A mutex (short for mutual exclusion) excludes other processes or threads from executing the same section of code (the critical section). Basically, it ensures that at most one thread is executing a given section of code. A mutex is also called a lock.

Underneath the hood, locks must be implemented using hardware somehow, and the implementation must make use of the atomicity guarantees of the underlying hardware.

Most nontrivial operations cannot be made atomic, so you must either use a lock to block other threads from operating while the critical section executes, or else you must carefully design a lock-free algorithm that ensures that all the critical state-changing operations can be safely implemented using atomic operations.

This is a very deep subject, and there is a large body of literature on all these topics. The Wikipedia links I've given are a good starting point, but since you're taking a class on operating systems right now, it might be best for you to ask your professor to provide good resources for learning and understanding this stuff.