Process synchronization #

Race condition #

Critical section and mutual exclusion #

In the above code examples, the counter++ and counter-- are considered a critical section.

How to implement mutual exclusion #

Note: This solution isn’t fully correct.

producer:
    while (lock == 0)
        lock = 1
        put
        lock = 0

consumer:
    while (lock == 0)
        lock = 1
        fetch
        lock = 0

The problem with this code is that if the OS does a context switch during a critical section, it can create an error.

One solution is to disable interrupts.

Sync hardware, spin locks and compare-and-swap #

Mutex locks #

Mutex locks allow the OS to provide a mutual exclusion as a service.

Semaphores #

A semaphore is simply a non-negative interger that has two valid operations.