Parallel Computer Architecture

354 CHAPTER 5 Shared Memory Multiprocessors

on the flag associated with the barrier; if so, it writes the flag to release the p - 1

waiting processes. A simple attempt at a barrier algorithm may therefore look like

struct bar_type {

int counter;

struct lock_type lock;

int flag = 0;

} barename;

BARRIER (bar_name, p)

{

LOCK(bar^name·lock);

if (bar_name.counter == 0)

bar_name.flag =

0 ;

mycount = bar_name.counter++;

UNLOCK (barjame

. lock)

;

if (mycount == p) {

bar__name

.

counter = 0;

bar_name.flag =

1 ;

}

else

while (bar_name.flag ==0) {};

Preset flag if first to

reach*/

I* my count is a private variable*/

/Hast to arrive*/

l*reset counter for ...