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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 | */ | |
21 | int _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 | ||
35 | EXPORT_SYMBOL(_atomic_dec_and_lock); |