The critical section problem

1
The critical section problem

• Reason why
• Mechanisms
• Typical problems

2
Why are critical sections used

• Protecting temporary use of shared resources
• Data structures
• Typically the master data structure (print queue
  index) is the one shared
• Individual jobs are entered and then linked into
  the queue
• Entering job needs no protection linking does
• Devices
• Example scanner needs protection because entry
  operation is human and slow
• Printer is held by system anyway
• File system
• Files and the file system structure are global
• These global structures are cached for speed
• Only the data blocks themselves actually move
  with every operation

3
Definition of the critical section problem

• A theoretical description, but fits the real
  world
• Any correct solution must satisfy
• Mutual exclusion is enforced
• Only processes wanting access participate in the
  decision
• No process has to wait if the critical section is
  free
• No process has to wait more than a finite number
  of steps
• Assumption processes stay in the critical
  section only a finite time
• Approaches
• Software only
• Special instructions
• Interprocess communication mechanisms central
  controlling process for any given resource

4
Various software methods

• Two-process purely programmed algorithms
• Dijkstras algorithm
• First valid solution (1965)
• Petersons algorithm
• Distinctly simpler (1968) but no real advantage
• Problems with both
• Two-process, not n-process
• Busy-waiting
• Global variables
• Special instructions
• Exchange requires static variable inside the
  function
• TestAndSet requires static variable inside the
  function

5
General code for a critical section
Global variable definitions
While (1) wait to enter C. S. critical
section notify leaving C. S. remainder
section
Process I Local Variables Process I Code
Process II Local Variables Process II Code
6
Critical section mechanisms

• General classes in common use
• Libraries for use in standard languages
• Libc process-handling system calls
• Pthread libraries
• System V IPC
• Semaphores
• Message queues
• Shared memory
• Language-based
• Monitors
• Momitor is a class with mutual exclusion handling
• Java threads
• Extends thread, synchronized methods
• Ada tasks

7
The basic mechanisms

• Semaphores and related ideas
• Conventional binary semaphore
• Integer-valued semaphore
• Reader/writer-oriented semaphores
• Message-based
• Message queues
• mailboxes
• Pipes, named and unnamed
• Bidirectional network connections