5.5.2 Locks

OpenMP 3.0 specifies that locks are no longer owned by threads, but by tasks. Once a lock is acquired, the current task owns it, and the same task must release it before task completion.

The critical construct, on the other hand, remains as a thread-based mutual exclusion mechanism.

The change in lock ownership requires extra care when using locks. The following program (it appears as Example A.43.1c in the OpenMP Specification version 3.0) is conforming in OpenMP 2.5 because the thread that releases the lock lck in the parallel region is the same thread that acquired the lock in the sequential part of the program (the master thread of a parallel region and the initial thread are the same). However, it is not conforming in OpenMP 3.0, because the task region that releases the lock lck is different from the task region that acquires the lock.

Example 5–2 Example Using Locks: Non-Conformingn in OpenMP 3.0

#include <stdlib.h>
#include <stdio.h>
#include <omp.h>

int main()
{
  int x;
  omp_lock_t lck;

  omp_init_lock (&lck);
  omp_set_lock (&lck);
  x = 0;

  #pragma omp parallel shared (x)
  {
    #pragma omp master
    {
      x = x + 1;
      omp_unset_lock (&lck);
    }
  }
  omp_destroy_lock (&lck);
}