Chapter 5: Process Synchronization

Race Condition

Occurs when multiple processes access and manipulate the same data concurrently, and the outcome depends on the order in which the accesses take place.

Critical Section

Part of a process where the process accesses shared resources, such as shared data, and potentially modifies them

Only one process should execute in its critical section at a time to avoid data inconsistency

Critical Section Solution

Each process must:

1. Request permission to enter its critical section in the entry section.

2. Execute the critical section.

3. Follow the critical section with the exit section.

4. Then execute the remainder section.

Peterson’s Solution

Software-based protocol for mutual exclusion in concurrent programming for two processes

It uses two shared variables: int turn to indicate whose turn it is and Boolean flag[2] to indicate if a process is ready to enter its critical section

Semaphores

Synchronization tool that does not require busy waiting and can be used to control access to a common resource in concurrent systems

It supports two standard operations: wait() (decrement) and signal() (increment).

Deadlock

Condition where two or more processes are waiting indefinitely for an event that can only be caused by one of the waiting processes

Starvation: Occurs when a process is perpetually denied the resources it needs to proceed because other processes are continuously given priority

Priority inversion: Situation where a lower-priority process holds a lock needed by a higher-priority process, causing the higher-priority process to wait indefinitely

Mutual Exclusion (Mutex)

Synchronization primitive that provides exclusive access to a shared resource or critical section

Ensures that only one thread or process can access the critical section at a time, preventing race conditions

Ex: Semaphore, Read-Write, Spinlock

Monitor

High-level abstraction for synchronization that allows only one process to be active within the monitor at any time

It can use condition variables to suspend and resume processes

Condition Variables for Monitors

Condition variables in monitors are used to suspend and resume processes.

x.wait() suspends the process invoking it, while x.signal() resumes one of the processes that invoked x.wait()