Readers Writers: Simple Version

roomEmpty = S(1)
mutex = S(1) // 
nReader = 0 // Tracks the number of readers

writer

while (True) {
  wait(roomEmpty);
  // modify data
  signal(roomEmpty);
}

reader

while (True) {
  wait(mutex); // Prevent race condition between other readers, next section is critical section.
  // CS BEGIN

  nReader++; // Indicate you are try to enter critical section
  if (nReader == 1) { // If you are the first reader,
    wait(roomEmpty); // Lock the resource from writers
  }
  
  // CS END
  signal(mutex);
  
  // <Read data>

  wait(mutex);
  // CS BEGIN

  nReader--; // Once you are done reading,
  if (nReader == 0) { // If you are last reader, you can exit
    signal(roomEmpty); // Unlock if you are the last reader
  }
  
  // CS END
  signal(mutex);
}

Shortcoming: starvation

If many readers access the data structure, the writer ends up getting locked out.

References

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