The lab checkoff sheet for all students can be found right here.

Get Started

Before starting, go ahead and clone the lab5 folder, which contains the test framework for the EventBarrier class discussed in Problem 2.

git clone /usr/class/cs110/repos/lab5/shared lab5

Problem 1: Read-Write Locks

The following was a final exam question from a few years ago. The students were expected to write the code that follows.

The read-write lock (implemented by the rwlock class) is a mutex-like class with three public methods:

class rwlock {
 public:
  rwlock();
  void acquireAsReader();
  void acquireAsWriter();
  void release();

 private:
  // object state omitted
};

Any number of threads can acquire the lock as a reader without blocking one another. However, if a thread acquires the lock as a writer, then all other acquireAsReader and acquireAsWriter requests block until the writer releases the lock. Waiting for the write lock will block until all readers release the lock so that the writer is guaranteed exclusive access to the resource being protected. This is useful if, say, you use some shared data structure that only very periodically needs to be modified. All reads from the data structure require you to hold the reader lock (so as many threads as you want can read the data structure at once), but any writes require you to hold the writer lock (giving the writing thread exclusive access).

The implementation ensures that as soon as one thread tries to get the writer lock, all other threads trying to acquire the lock—either as a reader or a writer—block until that writer gets the locks and releases it. That means the state of the lock can be one of three things:

• Ready, meaning that no one is trying to get the write lock.
• Pending, meaning that someone is trying to get the write lock but is waiting for all the readers to finish.
• Writing, meaning that someone is writing.

The following solution (written by Jerry Cain) relies on two mutexes and two condition_variable_anys. Here is the full interface for the rwlock class:

class rwlock {
 public:
  rwlock(): numReaders(0), writeState(Ready) {}
  void acquireAsReader();
  void acquireAsWriter();
  void release();

 private:
  int numReaders;
  enum { Ready, Pending, Writing } writeState;
  mutex readLock, stateLock;
  condition_variable_any readCond, stateCond;
};

And here are the implementations of the three public methods:

void rwlock::acquireAsReader() {
  lock_guard<mutex> lgs(stateLock);
  stateCond.wait(stateLock, [this]{ return writeState == Ready; });
  lock_guard<mutex> lgr(readLock);
  numReaders++;
}

void rwlock::acquireAsWriter() {
  stateLock.lock();
  stateCond.wait(stateLock, [this]{ return writeState == Ready; });
  writeState = Pending;
  stateLock.unlock();
  lock_guard<mutex> lgr(readLock);
  readCond.wait(readLock, [this]{ return numReaders == 0; });
  writeState = Writing;
}

void rwlock::release() {
  stateLock.lock();
  if (writeState == Writing) {
    writeState = Ready;
    stateLock.unlock();
    stateCond.notify_all();
    return;
  }

  stateLock.unlock();
  lock_guard<mutex> lgr(readLock);
  numReaders--;
  if (numReaders == 0) readCond.notify_one();
}

Very carefully study the implementation of the three methods, and answer the questions that appear below. This lab problem is designed to force you to really internalize the condition_variable_any—one of the more difficult concepts in the entire threading segment of the course, and understand how it works.

• The implementation of acquireAsReader acquires the stateLock (via the lock_guard) before it does anything else, and it doesn’t release the stateLock until the method exits. Why can’t the implementation be this instead?

void rwlock::acquireAsReader() {
  stateLock.lock();
  stateCond.wait(stateLock, [this]{ return writeState == Ready; });
  stateLock.unlock();
  lock_guard<mutex> lgr(readLock);
  numReaders++;
}

• The implementation of acquireAsWriter acquires the stateLock before it does anything else and it releases the stateLock just before it acquires the readLock. Why can’t acquireAsWriter adopt the same approach as acquireAsReader and just hold onto stateLock until the method returns?

• Notice that we have a single release method instead of releaseAsReader and releaseAsWriter methods. How does the implementation know if the thread acquired the rwlock as a writer instead of a reader (assuming proper use of the class)?

• The implementation of release relies on notify_all in one place and notify_one in another. Why are those the correct versions of notify to call in each case?

• A thread that owns the lock as a reader might want to upgrade its ownership of the lock to that of a writer without releasing the lock first. Besides the fact that it’s a waste of time, what’s the advantage of not releasing the read lock before re-acquiring it as a writer, and how could the implementation of acquireAsWriter be updated so it can be called after acquireAsReader without an intervening release call?

Problem 2: Event Barriers

An event barrier allows a group of one or more threads—we call them consumers—to efficiently wait until an event occurs (i.e. the barrier is lifted by another thread, called the producer). The barrier is eventually restored by the producer, but only after consumers have detected the event, executed what they could only execute because the barrier was lifted, and notified the producer they’ve done what they need to do and moved past the barrier. In fact, consumers and producers efficiently block (in lift and past, respectively) until all consumers have moved past the barrier. We say an event is in progress while consumers are responding to and moving past it.

The EventBarrier implements this idea via a constructor and three zero-argument methods called wait, lift, and past. The EventBarrier requires no external synchronization, and maintains enough internal state to track the number of waiting consumers and whether an event is in progress. If a consumer arrives at the barrier while an event is in progress, wait returns immediately without blocking.

The following test program and sample run illustrate how the EventBarrier works. The backstory for the sample run: weary travelers approach a castle only to be blocked by a castle gate. The travelers wait until the gatekeeper lifts the gate, allowing the travelers to pass through. The gatekeeper only lowers the gate after all travelers have passed through the gate, and the travelers only proceed toward the castle once all have passed through the gate.

// A list of the names of the "traveler threads" and its length
static const string kTravelerNames[] = { "Peter", "Paul", "Mary", "Manny", "Moe", "Jack" };
static const int kTravelerNamesLength = sizeof(kTravelerNames) / sizeof(kTravelerNames[0]);


/* The gatekeeper sleeps for a random amount of time, then opens the castle gate
 * by lifting the event barrier.  That will block until all travelers have passed
 * through, at which point this function returns.
 */
static void gatekeeper(EventBarrier& castleGate) {
  sleep(random() % 5 + 7);
  cout << oslock << "Gatekeeper opens the gate." << endl << osunlock;
  castleGate.lift();
  cout << oslock << "Gatekeeper closes the gate, knowing all have passed." 
       << endl << osunlock;
}

/* A traveler sleeps for some amount of time to simulate approaching the
 * the castle gate.  Then it waits for the castle gate to open.  Once the 
 * gate is opened, the traveler sleeps for some amount of time to simulate 
 * passing through the gate.  Then, it waits for all others to pass through
 * before returning.
 */
static void traveler(const string& name, EventBarrier& castleGate) {
  cout << oslock << name << " walks toward the castle gate." << endl << osunlock;
  sleep(random() % 3 + 3);
  cout << oslock << name << " arrives at the castle gate, must wait." << endl << osunlock;
  castleGate.wait();
  cout << oslock << name << " detects castle gate lifted, starts entering." << endl << osunlock;
  sleep(random() % 3 + 2);
  cout << oslock << name << " has passed through the castle gate." << endl << osunlock;
  castleGate.past();
  cout << oslock << name << " carries on, knowing all others have passed." << endl << osunlock;
}

int main(int argc, char *argv[]) {
  // An EventBarrier to model the gate.  The "barrier" represents the gate.
  EventBarrier castleGate;

  // Spawn the traveler and gatekeeper threads
  thread travelers[kTravelerNamesLength];
  for (size_t i = 0; i < kTravelerNamesLength; i++) {
    travelers[i] = thread(traveler, kTravelerNames[i], ref(castleGate));
  }
  thread gatekeeperThread(gatekeeper, ref(castleGate));

  // Wait for all threads to finish
  for (thread& traveler : travelers) traveler.join();
  gatekeeperThread.join();

  return 0;
}

$ ./ebtest
Peter walks toward the castle gate.
Paul walks toward the castle gate.
Mary walks toward the castle gate.
Manny walks toward the castle gate.
Moe walks toward the castle gate.
Jack walks toward the castle gate.
Mary arrives at the castle gate, must wait.
Peter arrives at the castle gate, must wait.
Paul arrives at the castle gate, must wait.
Manny arrives at the castle gate, must wait.
Jack arrives at the castle gate, must wait.
Moe arrives at the castle gate, must wait.
Gatekeeper opens the gate.
Mary detects castle gate lifted, starts entering.
Peter detects castle gate lifted, starts entering.
Paul detects castle gate lifted, starts entering.
Manny detects castle gate lifted, starts entering.
Jack detects castle gate lifted, starts entering.
Moe detects castle gate lifted, starts entering.
Mary has passed through the castle gate.
Peter has passed through the castle gate.
Paul has passed through the castle gate.
Jack has passed through the castle gate.
Manny has passed through the castle gate.
Moe has passed through the castle gate.
Moe carries on, knowing all others have passed.
Gatekeeper closes the gate, knowing all have passed.
Mary carries on, knowing all others have passed.
Peter carries on, knowing all others have passed.
Paul carries on, knowing all others have passed.
Jack carries on, knowing all others have passed.
Manny carries on, knowing all others have passed.

Your lab5 folder includes event-barrier.h, event-barrier.cc, and ebtest.cc, and typing make should generate an executable called ebtest that you can run to ensure that the EventBarrier class you'll flesh out in event-barrier.h and .cc are working properly. The one exercise in this lab that has you do any coding is this one, as it expects you complete the implementation stub you've been supplied with.