A Working Centralized Barrier

• Consecutively entering the same barrier doesn’t work

  • Must prevent process from entering until all have left previous instance
  • Could use another counter, but increases latency and contention

• Sense reversal: wait for flag to take different value consecutive times

  • Toggle this value only when all processes reach

BARRIER (bar_name, p) {

local_sense = !(local_sense); /* toggle private sense variable */

LOCK(bar_name.lock);

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

if (bar_name.counter == p)

UNLOCK(bar_name.lock);

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

else

{ UNLOCK(bar_name.lock);

while (bar_name.flag != local_sense) {}; }

}

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