1. Overview Assignment 8: hints Assignment 7: solution Deadlocks
See sample solution on the web

2. A8 Ex1 – Barrier Objects synchronize processes
“checkpoint”: continue only when all processes are there
checkpoint
checkpoint
checkpoint
Barrier = OBJECT Init(count); Sync;
PROCESS:
FOR i := 0 TO Max DO
DoPhase(i);
barrier.Sync
END

3. Barrier, Example P1 P2 P3 // code before the barrier
Barrier.synchronize()
// code after the barrier

4. A8 Ex1 - Task
Write a barrier implementation in the (reasonable) language of your choice.
The Barrier object should have the following methods:
Initialize: to specify how many process are to be synchronized by this barrier.
Synchronize: wait until all processes reached the barrier.

5. A8 Ex2 – Deadlocks Coffman conditions Solutions
Mutual exclusion in resource usage
Holding a resource and waiting
No preemption of resources possible
Circular wait
Solutions
use protocol that ensures the system will never deadlock
detect deadlock and recover
ignore (...and reboot)

6. Deadlocks Transactions in a bank Deadlock avoidance:
move $ from x to y
concurrent system
acquire lock on both accounts, then perform transfer
Deadlock avoidance:
mutual exclusion (accounts are locked)
no preemption (once locked, unlock only after transfer)
hold and wait (first account locked, waiting for second one)
must avoid circular wait..... HOW?

7. Deadlocks: example Example: p1: lock(x, y); p2: lock(y, x); Scenario:
p1 locks x
p2 locks y
p1 tries to lock y and waits
p2 tries to lock x and waits
DEADLOCK

8. A8 Ex3 – Baboons Problem Part a): avoid deadlocks using semaphores.
more than one baboon can cross (but only in one direction)
if two (or more) baboons cross in different directions we have a deadlock.
Part a): avoid deadlocks using semaphores.
Part b): avoid starvation.