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 
52 typedef struct refcount_struct {
53 	atomic_t refs;
54 } refcount_t;
55 
56 #define REFCOUNT_INIT(n)	{ .refs = ATOMIC_INIT(n), }
57 
refcount_set(refcount_t * r,unsigned int n)58 static inline void refcount_set(refcount_t *r, unsigned int n)
59 {
60 	atomic_set(&r->refs, n);
61 }
62 
refcount_set_release(refcount_t * r,unsigned int n)63 static inline void refcount_set_release(refcount_t *r, unsigned int n)
64 {
65 	atomic_set(&r->refs, n);
66 }
67 
refcount_read(const refcount_t * r)68 static 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  */
80 static inline __refcount_check
refcount_inc_not_zero(refcount_t * r)81 bool 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  */
refcount_inc(refcount_t * r)112 static 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  */
125 static inline __refcount_check
refcount_sub_and_test(unsigned int i,refcount_t * r)126 bool 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 
150 static inline __refcount_check
refcount_dec_and_test(refcount_t * r)151 bool 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 */
158