lib/dec_and_lock.c

   1 #include <linux/module.h>
   2 #include <linux/spinlock.h>
   3 #include <asm/atomic.h>
   4
   5 /*
   6  * This is an architecture-neutral, but slow,
   7  * implementation of the notion of "decrement
   8  * a reference count, and return locked if it
   9  * decremented to zero".
  10  *
  11  * NOTE NOTE NOTE! This is _not_ equivalent to
  12  *
  13  *      if (atomic_dec_and_test(&atomic)) {
  14  *              spin_lock(&lock);
  15  *              return 1;
  16  *      }
  17  *      return 0;
  18  *
  19  * because the spin-lock and the decrement must be
  20  * "atomic".
  21  *
  22  * This slow version gets the spinlock unconditionally,
  23  * and releases it if it isn't needed. Architectures
  24  * are encouraged to come up with better approaches,
  25  * this is trivially done efficiently using a load-locked
  26  * store-conditional approach, for example.
  27  */
  28 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  29 {
  30         spin_lock(lock);
  31         if (atomic_dec_and_test(atomic))
  32                 return 1;
  33         spin_unlock(lock);
  34         return 0;
  35 }
  36
  37 EXPORT_SYMBOL(_atomic_dec_and_lock);