LXR linux/lib/dec_and

   1// SPDX-License-Identifier: GPL-2.0
   2#include <linux/export.h>
   3#include <linux/spinlock.h>
   4#include <linux/atomic.h>
   5
   6/*
   7 * This is an implementation of the notion of "decrement a
   8 * reference count, and return locked if it decremented to zero".
   9 *
  10 * NOTE NOTE NOTE! This is _not_ equivalent to
  11 *
  12 *      if (atomic_dec_and_test(&atomic)) {
  13 *              spin_lock(&lock);
  14 *              return 1;
  15 *      }
  16 *      return 0;
  17 *
  18 * because the spin-lock and the decrement must be
  19 * "atomic".
  20 */
  21int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
  22{
  23        /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
  24        if (atomic_add_unless(atomic, -1, 1))
  25                return 0;
  26
  27        /* Otherwise do it the slow way */
  28        spin_lock(lock);
  29        if (atomic_dec_and_test(atomic))
  30                return 1;
  31        spin_unlock(lock);
  32        return 0;
  33}
  34
  35EXPORT_SYMBOL(_atomic_dec_and_lock);
  36
  37int _atomic_dec_and_lock_irqsave(atomic_t *atomic, spinlock_t *lock,
  38                                 unsigned long *flags)
  39{
  40        /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
  41        if (atomic_add_unless(atomic, -1, 1))
  42                return 0;
  43
  44        /* Otherwise do it the slow way */
  45        spin_lock_irqsave(lock, *flags);
  46        if (atomic_dec_and_test(atomic))
  47                return 1;
  48        spin_unlock_irqrestore(lock, *flags);
  49        return 0;
  50}
  51EXPORT_SYMBOL(_atomic_dec_and_lock_irqsave);
  52