1 #include <linux/module.h>
2 #include <linux/spinlock.h>
3 #include <asm/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 */
20 int _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
34 EXPORT_SYMBOL(_atomic_dec_and_lock);
35
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