Puzzle: Write synchronization code for oxygen and hydrogen molecules that enforces these constraints.

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2 Problem 2: H 2 O Building Problem There are two kinds of threads, oxygen and hydrogen. In order to assemble these threads into water molecules, we have to create a barrier that makes each thread wait until a complete molecule is ready to proceed. As each thread passes the barrier, it should invoke bond. You must guarantee that all the threads from one molecule invoke bond before any of the threads from the next molecule do. In other words: If an oxygen thread arrives at the barrier when no hydrogen threads are present, it has to wait for two hydrogen threads. If a hydrogen thread arrives at the barrier when no other threads are present, it has to wait for an oxygen thread and another hydrogen thread. We don t have to worry about matching the threads up explicitly; that is, the threads do not necessarily know which other threads they are paired up with. The key is just that threads pass the barrier in complete sets; thus, if we examine the sequence of threads that invoke bond and divide them into groups of three, each group should contain one oxygen and two hydrogen threads. Puzzle: Write synchronization code for oxygen and hydrogen molecules that enforces these constraints.

3 Problem 3: Dining Savages Problem A tribe of savages eats communal dinners from a large pot that can hold M servings of stewed missionary. When a savage wants to eat, he helps himself from the pot, unless it is empty. If the port is empty, the savage wakes up the cook and then waits until the cook has refilled the pot. Any number of savage threads run the following code: Unsynchronized savage code 1. while true: 2. getservingfrompot() 3. eat() And one cook thread runs this code: Unsynchronized cook code 1. while true: 2. putservingsinpot(m) The synchronization constraints are: Savages cannot invoke getservingfrompot if the pot is empty. The cook can invoke putservingsinpot only if the pot is empty. Please add code for the savages and the cook that satisfies the synchronization constraints.

4 Solution1 for the barbershop problem: customers = 0 mutex = Semaphore(1) customer = Semaphore(0) barber = Semaphore(0) <Customer> 1 mutex.wait() 2 if customers == n+1: 3 mutex.signal() 4 leave() 5 customers += 1 6 mutex.signal() 7 8 customer.signal() 9 barber.wait() 10 gethaircut() mutex.wait() 13 customers -= 1 14 mutex.signal() <Barber> 1 customer.wait() 2 barber.signal() 3 cuthair()

5 Solution2 for the barbershop problem: Semaphore Customers = 0 Semaphore Barber = 0 Semaphore accessseats (mutex) = 1 int NumberOfFreeSeats = N //total number of seats <Customer> while(true) { //runs in an infinite loop SemWait(accessSeats) //tries to get access to the chairs if ( NumberOfFreeSeats > 0 ) { //if there are any free seats NumberOfFreeSeats-- //sitting down on a chair SemSignal(Customers) //notify the barber, who's waiting that there is a customer SemSignal(accessSeats) //don't need to lock the chairs anymore SemWait(Barber) //now it's this customer's turn, but wait if the barber is busy //here the customer is having his hair cut } else { //there are no free seats //tough luck SemSignal(accessSeats) //but don't forget to release the lock on the seats //customer leaves without a haircut } } <Barber> while(true) { //runs in an infinite loop //tries to acquire a customer - if none is available he goes to sleep SemWait(Customers) //at this time he has been awakened - want to modify the number of available seats SemWait(accessSeats) NumberOfFreeSeats++ //one chair gets free SemSignal(Barber) //the barber is ready to cut SemSignal(accessSeats) //we don't need the lock on the chairs anymore //here the barber is cutting hair }

6 Solution for Building H 2 O Problem: mutex = Semaphore(1) oxygen = 0 hydrogen = 0 oxyqueue = Semaphore(0) hydroqueue = Semaphore(0) <Oxygen> 1 mutex.wait() 2 oxygen += 1 3 if hydrogen >= 2: 4 hydroqueue.signal() 5 hydroqueue.signal() 6 hydrogen -= 2 7 oxyqueue.signal() 8 oxygen -= 1 9 else 10 mutex.signal() oxyqueue.wait() 13 bond() 14 mutex.signal() <Hydrogen> 1 mutex.wait() 2 hydrogen += 1 3 if hydrogen >= 2 and oxygen >= 1: 4 hydroqueue.signal() 5 hydroqueue.signal() 6 hydrogen -= 2 7 oxyqueue.signal() 8 oxygen -= 1 9 else 10 mutex.signal() hydroqueue.wait() 13 bond()

7 Solution for the dining savages problem: servings = 0 mutex = Semaphore(1) emptypot = Semaphore(0) fullpot = Semaphore(0) <Cook> 1 while true: 2 emptypot.wait() 3 putservingsinpot(m) 4 fullpot.signal() <Savage> 1 while true: 2 mutex.wait() 3 if servings == 0: 4 emptypot.signal() 5 fullpot.wait() 6 servings = M 7 servings -= 1 8 getservingfrompot() 9 mutex.signal() eat()