2.6.2 A Program that Verifies Array-Value Types

A group of threads call check_bad_array() concurrently to check whether any element of array data_array is corrupt. Each thread checks a different section of the array. If a thread finds that an element is corrupt, it sets the value of a global shared variable is_bad to true.

20  volatile int is_bad = 0;
 ...

 100  /* 
 101   * Each thread checks its assigned portion of data_array, and sets 
 102   * the global flag is_bad to 1 once it finds a bad data element.
 103   */
 104  void check_bad_array(volatile data_t *data_array, unsigned int thread_id)    
 105  {
 106     int i;
 107     for (i=my_start(thread_id); i<my_end(thread_id); i++) {
 108          if (is_bad) 
 109              return;
 110          else {
 111              if (is_bad_element(data_array[i])) { 
 112                  is_bad = 1;
 113                  return;
 114              }
 115          }
 116     }
 117  }

There is a data-race between the read of is_bad on line 108 and the write to is_bad on line 112. However, the data-race does not affect the correctness of the final result.

The initial value of is_bad is zero. When the threads update is_bad, they assign it the value one. That is, the threads store one in the same bit in the same byte of memory for is_bad. On current architectures, it is safe to assume that those stores are atomic. Therefore, when is_bad is read by a thread, the value read will either be zero or one. If a thread checks is_bad (line 108) before it has been assigned the value one, then it continues executing the for loop. If, in the meantime, another thread has assigned the value one to is_bad (line 112), that does not change the final result. It just means that the thread executed the for loop longer than necessary.