Readers and Writers
Andrews Chapter 4

Problem Definition

Two kinds of threads (processes) wish to access a common data item or items. Readers only need to read the data and writers can read and write the data. Multiple readers can access the data simultaneously but writers must have exclusive access to the data to write it.

Reader/Writer Solutions

• Mutual Exclusion Problem
• Just use semaphores to give mutual exclusion to all threads (guarantees safety)
• But this restricts performance because readers can not access simultaneously
• Outline of readers and writers solution (in Andrews notation)
• remember that <...> is an atomic region

int nr =0;   # number of active readers
sem rw =1    # lock for readers and writer exclusion

process Reader [i = 1 to M] {
  while (true) {
    ...
    <nr =nr + 1;
      if  (nr == 1) P(rw);      # if first reader you get the lock
    >
    read the database;
    < nr = nr-1;
      if (nr == 0) V(rw);      # if last reader, release the lock
    >
  }
}

process Writer [ j = 1 to N] {
  while (true) {
    ...
    P(rw);
    write the database;
    V(rw);
  }
}

• Readers and Writers Using Condition Synchronization
• implementing the atomic region
• Need to introduce two critical regions to update "nr", the number of readers

int nr = 0;    # number of active readers
sem rw =1;  #lock for access to the data
sem mutexR = 1;       #lock for readers access to "nr"

process Reader [i = 1 to m] {
  while (true) {
  ...
    P(mutexR);
      nr = nr = nr + 1;
      if (nr == 1 ) P(rw);  # if first reader, you get the lock
    V(mutexR);
    read the database;
    P(mutexR);
      nr = nr - 1;
      if (nr == 0) V(rw);   # if last reader, give up the lock
    V(mutexR);
  }
}

process Writer [j = 1 to n] {
  while (true) {
    ...
    P(rw);      # only one writer gets access to database
    write the database;
    V(rw);
  }
}