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1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef _TOOLS_LINUX_REFCOUNT_H
3#define _TOOLS_LINUX_REFCOUNT_H
4
5/*
6 * Variant of atomic_t specialized for reference counts.
7 *
8 * The interface matches the atomic_t interface (to aid in porting) but only
9 * provides the few functions one should use for reference counting.
10 *
11 * It differs in that the counter saturates at UINT_MAX and will not move once
12 * there. This avoids wrapping the counter and causing 'spurious'
13 * use-after-free issues.
14 *
15 * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
16 * and provide only what is strictly required for refcounts.
17 *
18 * The increments are fully relaxed; these will not provide ordering. The
19 * rationale is that whatever is used to obtain the object we're increasing the
20 * reference count on will provide the ordering. For locked data structures,
21 * its the lock acquire, for RCU/lockless data structures its the dependent
22 * load.
23 *
24 * Do note that inc_not_zero() provides a control dependency which will order
25 * future stores against the inc, this ensures we'll never modify the object
26 * if we did not in fact acquire a reference.
27 *
28 * The decrements will provide release order, such that all the prior loads and
29 * stores will be issued before, it also provides a control dependency, which
30 * will order us against the subsequent free().
31 *
32 * The control dependency is against the load of the cmpxchg (ll/sc) that
33 * succeeded. This means the stores aren't fully ordered, but this is fine
34 * because the 1->0 transition indicates no concurrency.
35 *
36 * Note that the allocator is responsible for ordering things between free()
37 * and alloc().
38 *
39 */
40
41#include <linux/atomic.h>
42#include <linux/kernel.h>
43
44#ifdef NDEBUG
45#define REFCOUNT_WARN(cond, str) (void)(cond)
46#define __refcount_check
47#else
48#define REFCOUNT_WARN(cond, str) BUG_ON(cond)
49#define __refcount_check __must_check
50#endif
51
52typedef struct refcount_struct {
53 atomic_t refs;
54} refcount_t;
55
56#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
57
58static inline void refcount_set(refcount_t *r, unsigned int n)
59{
60 atomic_set(&r->refs, n);
61}
62
63static inline void refcount_set_release(refcount_t *r, unsigned int n)
64{
65 atomic_set(&r->refs, n);
66}
67
68static inline unsigned int refcount_read(const refcount_t *r)
69{
70 return atomic_read(&r->refs);
71}
72
73/*
74 * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
75 *
76 * Provides no memory ordering, it is assumed the caller has guaranteed the
77 * object memory to be stable (RCU, etc.). It does provide a control dependency
78 * and thereby orders future stores. See the comment on top.
79 */
80static inline __refcount_check
81bool refcount_inc_not_zero(refcount_t *r)
82{
83 unsigned int old, new, val = atomic_read(&r->refs);
84
85 for (;;) {
86 new = val + 1;
87
88 if (!val)
89 return false;
90
91 if (unlikely(!new))
92 return true;
93
94 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
95 if (old == val)
96 break;
97
98 val = old;
99 }
100
101 REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
102
103 return true;
104}
105
106/*
107 * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
108 *
109 * Provides no memory ordering, it is assumed the caller already has a
110 * reference on the object, will WARN when this is not so.
111 */
112static inline void refcount_inc(refcount_t *r)
113{
114 REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
115}
116
117/*
118 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
119 * decrement when saturated at UINT_MAX.
120 *
121 * Provides release memory ordering, such that prior loads and stores are done
122 * before, and provides a control dependency such that free() must come after.
123 * See the comment on top.
124 */
125static inline __refcount_check
126bool refcount_sub_and_test(unsigned int i, refcount_t *r)
127{
128 unsigned int old, new, val = atomic_read(&r->refs);
129
130 for (;;) {
131 if (unlikely(val == UINT_MAX))
132 return false;
133
134 new = val - i;
135 if (new > val) {
136 REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");
137 return false;
138 }
139
140 old = atomic_cmpxchg_release(&r->refs, val, new);
141 if (old == val)
142 break;
143
144 val = old;
145 }
146
147 return !new;
148}
149
150static inline __refcount_check
151bool refcount_dec_and_test(refcount_t *r)
152{
153 return refcount_sub_and_test(1, r);
154}
155
156
157#endif /* _ATOMIC_LINUX_REFCOUNT_H */