Operating Systems Chapter 6: Synchronization Tools

Chapter 6: Synchronization Tools

• This chapter explores tools used for synchronization in operating systems.
• It addresses the challenges of concurrent processes accessing and modifying shared data, which can lead to inconsistencies.
• Key concepts include the critical-section problem, race conditions, and solutions like Peterson's solution, mutex locks, semaphores, and monitors.
• Hardware solutions such as memory barriers, compare-and-swap, and atomic variables are also discussed.
• The chapter details the requirements for solving the critical-section problem, including mutual exclusion, progress, and bounded waiting.

Background

• Concurrent processes may be interrupted at any point during their execution.
• Concurrent access to shared data can result in data inconsistencies if not managed properly.
• Synchronization mechanisms are needed to ensure orderly execution of cooperating processes, maintaining data consistency.
• The Bounded Buffer problem, illustrated in Chapter 4, presents a scenario where concurrently updated counters lead to race conditions.

Race Condition

• A race condition arises when multiple processes access and modify shared resources concurrently in an unpredictable way.
• This unpredictability can lead to erroneous results.
• This is demonstrated using the fork() system call, where access to a shared kernel variable like next_available_pid can lead to the same PID being assigned to different processes.

Critical Section Problem

• The critical section problem involves managing the access of multiple processes to a shared resource, ensuring that only one process accesses the shared resource at a time.
• Each process has a critical section of code that must be protected from concurrent access by other processes.
• A protocol needs to be designed to enforce mutual exclusion, prevent progress issues, and limit waiting for accessing the critical section indefinitely.

Critical Section

• Process structure outline to prevent conflicting access; the critical section, the entry section, the exit section, and remainder section.
• Explicit description for entry, section and exit section to manage resource access.

Requirements for Critical Section Solutions

• Mutual Exclusion: Only one process can be in the critical section at any time.
• Progress: If no process is in the critical section and some processes want to enter, the selection of the next process cannot be blocked indefinitely.
• Bounded Waiting: A bound must exist on the number of times other processes are allowed to enter their critical sections before a requesting process is granted access.
• No Assumptions on Relative Process Speeds: The design must work regardless of the execution speeds of the different processes.

Interrupt-Based Solution

• Disabling interrupts in the entry and exit sections of the critical section to prevent concurrency can cause problems.
• The approach is unsuitable for long critical sections or multi-CPU systems, as some processes may starve.
• Issues arise if the critical section is long; some processes may be delayed indefinitely.

Software Solution 1

• A two-process solution using a shared variable turn to track whose turn it is to access the critical section.
• This solution assumes atomic load and store operations, which may not always be guaranteed across different architectures.

Peterson's Solution

• A two-process solution using shared variables turn and flag to manage access to the critical section.
• Requires atomic load and store operations.

Algorithm for Processes P₀ and P₁

• Detailed algorithms for each process.
• Demonstrates how the turn and flag variables cooperate to guarantee mutual exclusion.

Correctness of Solutions

• Peterson's solution ensures mutual exclusion for two processes.
• Discussion on progress and bounded waiting requirements.

Peterson's Solution and Modern Architectures

• Peterson's solution might fail due to instruction reordering in modern processors.
• Reordering can create inconsistencies or unexpected results on multi-threaded systems.
• Memory barriers are needed to ensure proper operation across different architectures

Modern Architecture Example

• Example that demonstrates possible instruction reordering issues on modern architectures.
• Explanation that showcases reordering effects affecting expected outcome.

Peterson's Solution Revisited

• The effects of instruction reordering may lead to processes entering the critical section simultaneously (violation of mutual exclusion).
• Memory barriers are crucial for ensuring correct operation on modern architectures with reordering potentials.

This quiz focuses on Chapter 6, which explores synchronization tools used in operating systems. It discusses concepts like critical-section problems, race conditions, and various solutions including mutex locks and semaphores. Test your understanding of these essential mechanisms for managing concurrent processes and ensuring data consistency.