A Simple Centralized Barrier

struct bar_type {int counter; struct lock_type lock; int flag = 0;} bar_name;

BARRIER (bar_name, p) {

LOCK(bar_name.lock);

if (bar_name.counter == 0)

bar_name.flag = 0; /* reset flag if first to reach*/

mycount = bar_name.counter++; /* mycount is private */

UNLOCK(bar_name.lock);

if (mycount == p) { /* last to arrive */

bar_name.counter = 0; /* reset for next barrier */

bar_name.flag = 1; /* release waiters */

}

else while (bar_name.flag == 0) {}; /* busy wait for release */

}

• Shared counter maintains number of processes that have arrived

  • increment when arrive (lock), check until reaches numprocs
  • Problem?

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