Multiple readers and single writer lock

multi-reader-single-writer-lock

Mutex counter_mutex;
Condition read_phase, write_phase;
int resource_counter = 0;

// READERS
Lock(counter_mutex) {
  while(resource_counter == -1)
    Wait(counter_mutex, read_phase);
  resource_counter++;
} // unlock
// ... read data ...
Lock(counter_mutex) {
  resource_counter--;
  if(resource_counter == 0)
    Signal(write_phase);
} // unlock

// WRITER
Lock(counter_mutex) {
  while(resource_counter != 0)
    Wait(counter_mutex, write_phase);
  resource_counter = -1;
} // unlock
// ... write data ...
Lock(counter_mutex) {
  resource_counter = 0;
} // unlock
// avoid spurious wake-ups
Broadcast(read_phase);
Signal(write_phase);