#include <linux/export.h>
  #include <linux/spinlock.h>
  #include <linux/atomic.h>
  
  /*
   * This is an implementation of the notion of "decrement a
   * reference count, and return locked if it decremented to zero".
   *
   * NOTE NOTE NOTE! This is _not_ equivalent to
   *
   *	if (atomic_dec_and_test(&atomic)) {
   *		spin_lock(&lock);
   *		return 1;
   *	}
   *	return 0;
   *
   * because the spin-lock and the decrement must be
   * "atomic".
   */
  int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  {
  	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
  	if (atomic_add_unless(atomic, -1, 1))
  		return 0;
  
  	/* Otherwise do it the slow way */
  	spin_lock(lock);
  	if (atomic_dec_and_test(atomic))
  		return 1;
  	spin_unlock(lock);
  	return 0;
  }
  
  EXPORT_SYMBOL(_atomic_dec_and_lock);