Advanced Python Programming

The effectiveness of locks

With locks, we can turn a shared resource in a concurrent program into a critical section, whose integrity of data is guaranteed to be protected. A critical section guarantees the mutual exclusion of a shared resource, and cannot be accessed concurrently by multiple processes or threads; this will prevent any protected data from being updated or altered with conflicting information, resulting from race conditions.

In the following diagram, Thread B is blocked from accessing the shared resource—the critical section, named var—by a mutex (mutual exclusion) lock, because Thread A is already accessing the resource: