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Greg Kroah-Hartmanb2441312017-11-01 15:07:57 +01001// SPDX-License-Identifier: GPL-2.0
Paul Gortmaker8bc3bcc2011-11-16 21:29:17 -05002#include <linux/export.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -07003#include <linux/spinlock.h>
Arun Sharma600634972011-07-26 16:09:06 -07004#include <linux/atomic.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -07005
David S. Miller4db2ce02005-09-14 21:47:01 -07006/*
7 * This is an implementation of the notion of "decrement a
8 * reference count, and return locked if it decremented to zero".
9 *
Linus Torvalds1da177e2005-04-16 15:20:36 -070010 * 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".
Linus Torvalds1da177e2005-04-16 15:20:36 -070020 */
Linus Torvalds1da177e2005-04-16 15:20:36 -070021int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
22{
Nick Piggina57004e12006-01-08 01:02:19 -080023 /* 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;
Jan Blunck417dcdf2009-06-16 15:33:33 -070026
Nick Piggina57004e12006-01-08 01:02:19 -080027 /* Otherwise do it the slow way */
Linus Torvalds1da177e2005-04-16 15:20:36 -070028 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);