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