[OpenMP][Tests] fix data race in a OpenMP runtime test

[protze.joachim]

Summary

Aggressive optimization might translate

while(block);

to

while(tid!=0);

The atomic read/write adds the necessary flushes, so that the worker threads will finally see the update.

[AndreyChurbanov]

LGTM

openmp/runtime/test/tasking/omp_fill_taskqueue.c

for (i = 0; i < NUM_TASKS; i++) {
       {
         int tid;
         tid = omp_get_thread_num();
         if (tid == 0) {
           // As soon as the master thread starts executing task we should unlock
           // all tasks, and detect the test failure if it has not been done yet.
           if (failed < 0)
             failed = throttling ? enqueued == NUM_TASKS : enqueued < NUM_TASKS;
+#pragma omp atomic write
           block = 0;
         }
-        while (block)
-          ;
+        int wait = 0;
+        do {
+#pragma omp atomic read
+          wait = block;
+        } while (wait);
       }
     }
     block = 0;
   }
 
   return failed;
 }