lib/dec_and_lock.c

   1 #include <linux/module.h>
   2 #include <linux/spinlock.h>
   3 #include <asm/atomic.h>
   4 #include <asm/system.h>
   5
   6 #ifdef __HAVE_ARCH_CMPXCHG
   7 /*
   8  * This is an implementation of the notion of "decrement a
   9  * reference count, and return locked if it decremented to zero".
  10  *
  11  * This implementation can be used on any architecture that
  12  * has a cmpxchg, and where atomic->value is an int holding
  13  * the value of the atomic (i.e. the high bits aren't used
  14  * for a lock or anything like that).
  15  */
  16 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  17 {
  18         int counter;
  19         int newcount;
  20
  21         for (;;) {
  22                 counter = atomic_read(atomic);
  23                 newcount = counter - 1;
  24                 if (!newcount)
  25                         break;          /* do it the slow way */
  26
  27                 newcount = cmpxchg(&atomic->counter, counter, newcount);
  28                 if (newcount == counter)
  29                         return 0;
  30         }
  31
  32         spin_lock(lock);
  33         if (atomic_dec_and_test(atomic))
  34                 return 1;
  35         spin_unlock(lock);
  36         return 0;
  37 }
  38 #else
  39 /*
  40  * This is an architecture-neutral, but slow,
  41  * implementation of the notion of "decrement
  42  * a reference count, and return locked if it
  43  * decremented to zero".
  44  *
  45  * NOTE NOTE NOTE! This is _not_ equivalent to
  46  *
  47  *      if (atomic_dec_and_test(&atomic)) {
  48  *              spin_lock(&lock);
  49  *              return 1;
  50  *      }
  51  *      return 0;
  52  *
  53  * because the spin-lock and the decrement must be
  54  * "atomic".
  55  *
  56  * This slow version gets the spinlock unconditionally,
  57  * and releases it if it isn't needed. Architectures
  58  * are encouraged to come up with better approaches,
  59  * this is trivially done efficiently using a load-locked
  60  * store-conditional approach, for example.
  61  */
  62 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  63 {
  64         spin_lock(lock);
  65         if (atomic_dec_and_test(atomic))
  66                 return 1;
  67         spin_unlock(lock);
  68         return 0;
  69 }
  70 #endif
  71
  72 EXPORT_SYMBOL(_atomic_dec_and_lock);