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