dec_and_lock.c

Go to the documentation of this file.

#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/atomic.h>

/*
 * This is an implementation of the notion of "decrement a
 * reference count, and return locked if it decremented to zero".
 *
 * NOTE NOTE NOTE! This is _not_ equivalent to
 *
 *  if (atomic_dec_and_test(&atomic)) {
 *      spin_lock(&lock);
 *      return 1;
 *  }
 *  return 0;
 *
 * because the spin-lock and the decrement must be
 * "atomic".
 */
int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
{
    /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
    if (atomic_add_unless(atomic, -1, 1))
        return 0;

    /* Otherwise do it the slow way */
    spin_lock(lock);
    if (atomic_dec_and_test(atomic))
        return 1;
    spin_unlock(lock);
    return 0;
}

EXPORT_SYMBOL(_atomic_dec_and_lock);