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b2441318 | 1 | /* SPDX-License-Identifier: GPL-2.0 */ |
73a9bf95 ACM |
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 | ||
52 | typedef struct refcount_struct { | |
53 | atomic_t refs; | |
54 | } refcount_t; | |
55 | ||
56 | #define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), } | |
57 | ||
58 | static inline void refcount_set(refcount_t *r, unsigned int n) | |
59 | { | |
60 | atomic_set(&r->refs, n); | |
61 | } | |
62 | ||
63 | static inline unsigned int refcount_read(const refcount_t *r) | |
64 | { | |
65 | return atomic_read(&r->refs); | |
66 | } | |
67 | ||
68 | /* | |
69 | * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN. | |
70 | * | |
71 | * Provides no memory ordering, it is assumed the caller has guaranteed the | |
72 | * object memory to be stable (RCU, etc.). It does provide a control dependency | |
73 | * and thereby orders future stores. See the comment on top. | |
74 | */ | |
75 | static inline __refcount_check | |
76 | bool refcount_inc_not_zero(refcount_t *r) | |
77 | { | |
78 | unsigned int old, new, val = atomic_read(&r->refs); | |
79 | ||
80 | for (;;) { | |
81 | new = val + 1; | |
82 | ||
83 | if (!val) | |
84 | return false; | |
85 | ||
86 | if (unlikely(!new)) | |
87 | return true; | |
88 | ||
89 | old = atomic_cmpxchg_relaxed(&r->refs, val, new); | |
90 | if (old == val) | |
91 | break; | |
92 | ||
93 | val = old; | |
94 | } | |
95 | ||
96 | REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); | |
97 | ||
98 | return true; | |
99 | } | |
100 | ||
101 | /* | |
102 | * Similar to atomic_inc(), will saturate at UINT_MAX and WARN. | |
103 | * | |
104 | * Provides no memory ordering, it is assumed the caller already has a | |
105 | * reference on the object, will WARN when this is not so. | |
106 | */ | |
107 | static inline void refcount_inc(refcount_t *r) | |
108 | { | |
109 | REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n"); | |
110 | } | |
111 | ||
112 | /* | |
113 | * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to | |
114 | * decrement when saturated at UINT_MAX. | |
115 | * | |
116 | * Provides release memory ordering, such that prior loads and stores are done | |
117 | * before, and provides a control dependency such that free() must come after. | |
118 | * See the comment on top. | |
119 | */ | |
120 | static inline __refcount_check | |
121 | bool refcount_sub_and_test(unsigned int i, refcount_t *r) | |
122 | { | |
123 | unsigned int old, new, val = atomic_read(&r->refs); | |
124 | ||
125 | for (;;) { | |
126 | if (unlikely(val == UINT_MAX)) | |
127 | return false; | |
128 | ||
129 | new = val - i; | |
130 | if (new > val) { | |
131 | REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n"); | |
132 | return false; | |
133 | } | |
134 | ||
135 | old = atomic_cmpxchg_release(&r->refs, val, new); | |
136 | if (old == val) | |
137 | break; | |
138 | ||
139 | val = old; | |
140 | } | |
141 | ||
142 | return !new; | |
143 | } | |
144 | ||
145 | static inline __refcount_check | |
146 | bool refcount_dec_and_test(refcount_t *r) | |
147 | { | |
148 | return refcount_sub_and_test(1, r); | |
149 | } | |
150 | ||
151 | ||
152 | #endif /* _ATOMIC_LINUX_REFCOUNT_H */ |