Monitors, Condition Variables, and Readers-Writers

1
Monitors, Condition Variables, and Readers-Writers
2
Motivation for Monitors

• Semaphores are a big step from the low-level
  loads and stores
• However, semaphores are used for both mutual
  exclusion and synchronization
• The idea of monitors is to separate these two
  concerns

3
Monitor Defined

• Monitor a lock condition variables
• Lock provides mutual exclusion to shared data
• Condition variable provides a queue for waiting
  threads inside a critical section
• Enabling synchronization between processes.

4
Locks

• A lock provides two operations
• LockAcquire()
• Wait until the lock is free, then grab it
• LockRelease()
• Unlock, wake up anyone waiting to Acquire
• A lock is initially free

5
More on Locks

• Always acquire a lock before accessing a shared
  data structure
• Always release the lock after finishing with the
  shared data structure

6
An Example of Using Locks

• AddToQueue()
• Lock.Acquire()
• // put 1 item to the queue
• Lock.Release()
• RemoveFromQueue()
• Lock.Acquire()
• // if something on the queue
• // remove 1 item form the queue
• Lock.Release()
• return item

7
Condition Variables

• We need additional mechanisms to wait inside
  locked regions
• However, holding the lock while waiting prevents
  other threads from entering the locked region
• Condition variables make it possible to sleep
  inside a critical section
• Atomically release the lock go to sleep

8
Condition Variables

• Each condition variable
• Consists of a queue of threads
• Provides three operations
• Wait()
• Atomically release the lock and go to sleep
• Reacquire lock on return
• Signal()
• Wake up one waiting thread, if any
• Broadcast()
• Wake up all waiting threads

9
Condition Variables

• Note
• The three operations can only be used inside
  locked regions

10
An Example of Using Condition Variables

• AddToQueue()
• lock.Acquire()
• // put 1 item to the queue
• condition.Signal(lock)
• lock.Release()
• RemoveFromQueue()
• lock.Acquire()
• while nothing on queue
• condition.Wait(lock)
• lock.Release()
• return item

11
Hoare vs. Mesa Monitors

• Hoare Monitors (used in most textbooks)
• Signal() transfers the CPU directly to a waiting
  thread

12
Mesa vs. Hoare Monitors

• Mesa Monitors (used in most real operating
  systems)
• Signal() only puts a waiting thread on the
  schedulers ready queue
• By the time the waken thread gets the CPU, the
  waiting condition may no longer be true and needs
  to be retested

13
Readers-Writers Problem

• Commonly seen in database applications
• Readers never modify the database
• Writers read and modify the database
• We want one writer at a time, but many readers at
  the same time

14
Constraints

• A reader should wait when a writer is accessing
  or waiting for the database
• Condition okToRead
• A write should wait when there is a reader or
  writer accessing the database
• Condition okToWrite
• A reader or a writer should wait when someone is
  modifying global states
• Lock lock

15
Basic Structure of the Solution

• Reader
• Wait until no writers
• Access database
• Wake up waiting writers
• Writer
• Wait until no active readers or writers
• Access database
• Wake up waiting readers or writers

16
Developing the Solution

• Global states
• activeReaders 0
• activeWriters 0
• waitingReaders 0
• waitingWriters 0
• Condition okToRead NULL
• Condition okToWrite NULL
• Lock lock FREE

17
Put in Locks and Some Global States

• Reader()
• lock.Acquire()
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• lock.Release()

• Writer()
• lock.Acquire()
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• lock.Release()

18
Add the Wait Condition for Reader

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• lock.Release()

• Writer()
• lock.Acquire()
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• lock.Release()

19
Add the Wait Condition for Writer

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• lock.Release()

20
Add the Signal Condition for Reader

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• lock.Release()

21
Add the Signal Condition for Writer

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

22
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

23
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

24
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

25
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

26
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

27
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

28
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

29
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

30
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

31
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

32
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

33
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

34
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

35
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

36
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

37
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

38
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

39
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

40
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

41
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

42
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

43
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

44
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

45
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

46
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

47
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

48
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

49
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

50
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

51
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

52
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

53
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

54
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

55
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

56
Code Demonstration

• Reader()
• lock.Acquire()
• while (activeWriters gt 0
• waitingWriters gt 0)
• waitingReaders
• okToRead.Wait(lock)
• --waitingReaders
• activeReaders
• lock.Release()
• // access database
• lock.Acquire()
• --activeReaders
• if (activeReaders 0
• waitingWriters gt 0)
• okToWrite.Signal(lock)
• lock.Release()

• Writer()
• lock.Acquire()
• while (activeWriters gt 0
• activeReaders gt 0)
• waitingWriters
• okToWrite.Wait(lock)
• --waitingWriters
• activeWriters
• lock.Release()
• // access database
• lock.Acquire()
• --activeWriters
• if (waitingWriters gt 0)
• okToWrite.Signal(lock)
• else if (waitingReaders gt 0)
• okToRead.Broadcast(lock)
• lock.Release()

57
Semaphores vs. Monitors

• Can we implement monitors with semaphores?
• Wait()
• P(s)
• Signal()
• V(s)

58
Semaphores vs. Monitors

• Not quite
• Condition variables only work inside a lock
• Using semaphores inside a lock may deadlock

59
Semaphores vs. Monitors

• Condition variables have no history, but
  semaphores have
• Signal() with an empty queue does nothing
• A subsequent Wait() will wait
• V() increments semaphore
• A subsequent P() will not wait