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 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 */
75static inline __refcount_check
76bool 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 */
107static 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 */
120static inline __refcount_check
121bool 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
145static inline __refcount_check
146bool 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 */
153