Friday, September 16, 1993

Handout today:  project assignment
Reminder:  instructor out of town next MTuW but lectures meet as usual
-------------------------------------------------------------------------------
Proof of correctness of Peterson's algorithm

Mutual exclusion: Assume p1 has just passed test and about to enter
critical section.  At this point plbusy is true.  Can p2 enter its
critical section and violate mutual exclusion?  p1 has passed test
not (p2busy and turn == 2) so by DeMorgans, we know 
	(not p2busy)  or (turn != 2)
Case 1:  p2busy is false
	hence p2 cannot be currently trying to enter its Critical Section
	if it tries to enter Critical Section it must first clear the
		while(p1busy and turn == 1) ;
	test.  p1busy is currently true so p2 must wait

Case 2:  turn != 2  (in other words, turn == 1)
	implies that wherever p2 is at this time it will find
		(p1busy and turn == 1)
	to be true when it attempts to enter its Critical Section and
	hence will have to wait.

Progress and bounded waiting:
	process prevented from entering critical section only if stuck
		in while loop
	Say p1 is circling in while loop
	p2 is either (1) not trying to enter the critical section
	(2) waiting in its own while loop, or (3) repeatedly executing
	its entire loop very quickly.
Case 1: p1 detects that p2busy=false hence enters its own loop
Case 2: cannot happen because turn must either be 1 or 2, hence one or
	the other process is clear to go
Case 3: cannot happen because p2 will set turn to 1, permitting p1 to
	proceed and trapping itsenf in its own loop
-------------------------------------------------------------------------------
Next: semaphores
Help us generalize critical section problem solutions to more complex
problems involving more than just two processes