lib/dec_and_lock.c

   1 #include <linux/export.h>
   2 #include <linux/spinlock.h>
   3 #include <linux/atomic.h>
   4
   5 /*
   6  * This is an implementation of the notion of "decrement a
   7  * reference count, and return locked if it decremented to zero".
   8  *
   9  * NOTE NOTE NOTE! This is _not_ equivalent to
  10  *
  11  *      if (atomic_dec_and_test(&atomic)) {
  12  *              spin_lock(&lock);
  13  *              return 1;
  14  *      }
  15  *      return 0;
  16  *
  17  * because the spin-lock and the decrement must be
  18  * "atomic".
  19  */
  20 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  21 {
  22         /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
  23         if (atomic_add_unless(atomic, -1, 1))
  24                 return 0;
  25
  26         /* Otherwise do it the slow way */
  27         spin_lock(lock);
  28         if (atomic_dec_and_test(atomic))
  29                 return 1;
  30         spin_unlock(lock);
  31         return 0;
  32 }
  33
  34 EXPORT_SYMBOL(_atomic_dec_and_lock);