tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1#include <linux/module.h>
2#include <linux/spinlock.h>
3#include <asm/atomic.h>
4#include <asm/system.h>
5
6#ifdef __HAVE_ARCH_CMPXCHG
7/*
8 * This is an implementation of the notion of "decrement a
9 * reference count, and return locked if it decremented to zero".
10 *
11 * This implementation can be used on any architecture that
12 * has a cmpxchg, and where atomic->value is an int holding
13 * the value of the atomic (i.e. the high bits aren't used
14 * for a lock or anything like that).
15 */
16int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
17{
18 int counter;
19 int newcount;
20
21 for (;;) {
22 counter = atomic_read(atomic);
23 newcount = counter - 1;
24 if (!newcount)
25 break; /* do it the slow way */
26
27 newcount = cmpxchg(&atomic->counter, counter, newcount);
28 if (newcount == counter)
29 return 0;
30 }
31
32 spin_lock(lock);
33 if (atomic_dec_and_test(atomic))
34 return 1;
35 spin_unlock(lock);
36 return 0;
37}
38#else
39/*
40 * This is an architecture-neutral, but slow,
41 * implementation of the notion of "decrement
42 * a reference count, and return locked if it
43 * decremented to zero".
44 *
45 * NOTE NOTE NOTE! This is _not_ equivalent to
46 *
47 * if (atomic_dec_and_test(&atomic)) {
48 * spin_lock(&lock);
49 * return 1;
50 * }
51 * return 0;
52 *
53 * because the spin-lock and the decrement must be
54 * "atomic".
55 *
56 * This slow version gets the spinlock unconditionally,
57 * and releases it if it isn't needed. Architectures
58 * are encouraged to come up with better approaches,
59 * this is trivially done efficiently using a load-locked
60 * store-conditional approach, for example.
61 */
62int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
63{
64 spin_lock(lock);
65 if (atomic_dec_and_test(atomic))
66 return 1;
67 spin_unlock(lock);
68 return 0;
69}
70#endif
71
72EXPORT_SYMBOL(_atomic_dec_and_lock);