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