CS 3733 Operating Systems

Lecture 18 [10/27/97]: Semaphores

Reading: PUP Section 8.2)

Example:
Does the following code solve the critical section problem? (Tanenbaum page 36)

PROCESS 1:                        PROCESS 2:
while(TRUE) {                     while(TRUE) {
   while (turn != 0) ;               while (turn != 1) ;
     ...critical section...             ...critical section...
   turn = 1;                         turn = 0;
     ...non-critical section...         ...non-critical section...
}                                }

Peterson's Solution (slightly modified from text)
The shared integer turn can be either 0 or 1 initially. The shared integer array flag[2] has both entries initialized to 0.

int turn;
int flag[2];
 
Process 0:                      Process 1:
 
flag[0]=1;                      flag[1]=1;
turn = 1;                       turn = 0;
while (flag[1]==1 & (turn=1));  while (flag[0]==1 & (turn=0));
   ...critical section...           ...critical section...
flag[0] = 0;                    flag[1] = 0;

Semaphore: an integer variable P which can only be accessed through two atomic operations: wait (P) and signal (V). (Note: Tanenbaum calls these DOWN and UP.)

An atomic operation is one that can't be interrupted so once a process starts the operation, it is guaranteed that no one will mess it up. (As discussed in class last lecture, the notion of can't be interrupted is subtle.)

Implementation of semaphores with busy waiting

wait(S):                                signal(S):
    while (S <= 0) ;                        S = S + 1;
    S = S - 1;

Use Test-and-Set to implement (Pup Exercise 8.11 and Example 8.11)

Semaphores can be used for n-process synchronization:

    wait(mutex)
       critical section
    signal(mutex)
       remainder section

How should S be initialized?
What if it is initialized to 0? --- No one gets in.
It should be initialized to 1.

Revision Date: 10/27/97