Notes / University Notes / Operating System / Chapter 6: Process Synchronization

Chapter 6: Process Synchronization

Created: 2025-12-16Updated: 2025-12-16

Why concurrent access to shared data goes wrong, and the progression of fixes — software locks that need only memory, hardware atomics, then semaphores.

background: concurrent access to shared data may result in data inconsistency
consumer-producer problem: counter itself will have synchronization issues
Race Condition: the outcome depends on which process finished first, which is uncontrollable
Solution: 1) non-preemptive scheduling: can be used in kernel mode only. 2) critical section
Critical Section
- N processes are competing to use shared data
- each process has a code segment, called critical section (CS), in which the shared data is accessed
- ensure mutual exclusion: only one process can be in its CS at a time
Critical Section Requirements
- Mutual Exclusion: only one process in CS at a time
- Progress: No deadlock --- It ensures the system doesn't freeze. If the CS is empty, someone must be allowed to enter.
- Bounded Waiting: No starvation --- It ensures fairness. An individual process cannot be forced to wait forever while others cut in line.
Entry Section: code to request permission to enter CS
Exit Section: code to release CS

Software Solutions

Peterson's Solution (2 processes only)

background: concurrent access to shared data may result in data inconsistency
consumer-producer problem: counter itself will have synchronization issues
Race Condition: the outcome depends on which process finished first, which is uncontrollable
Solution: 1) non-preemptive scheduling: can be used in kernel mode only. 2) critical section
Critical Section
- N processes are competing to use shared data
- each process has a code segment, called critical section (CS), in which the shared data is accessed
- ensure mutual exclusion: only one process can be in its CS at a time
Critical Section Requirements
- Mutual Exclusion: only one process in CS at a time
- Progress: No deadlock --- It ensures the system doesn't freeze. If the CS is empty, someone must be allowed to enter.
- Bounded Waiting: No starvation --- It ensures fairness. An individual process cannot be forced to wait forever while others cut in line.
Entry Section: code to request permission to enter CS
Exit Section: code to release CS