linux/lib/dec_and_lock.c
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   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 */
  20int _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
  34EXPORT_SYMBOL(_atomic_dec_and_lock);
  35
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