Commit | Line | Data |
---|---|---|
f405df5d PZ |
1 | #ifndef _LINUX_REFCOUNT_H |
2 | #define _LINUX_REFCOUNT_H | |
3 | ||
4 | /* | |
5 | * Variant of atomic_t specialized for reference counts. | |
6 | * | |
7 | * The interface matches the atomic_t interface (to aid in porting) but only | |
8 | * provides the few functions one should use for reference counting. | |
9 | * | |
10 | * It differs in that the counter saturates at UINT_MAX and will not move once | |
11 | * there. This avoids wrapping the counter and causing 'spurious' | |
12 | * use-after-free issues. | |
13 | * | |
14 | * Memory ordering rules are slightly relaxed wrt regular atomic_t functions | |
15 | * and provide only what is strictly required for refcounts. | |
16 | * | |
17 | * The increments are fully relaxed; these will not provide ordering. The | |
18 | * rationale is that whatever is used to obtain the object we're increasing the | |
19 | * reference count on will provide the ordering. For locked data structures, | |
20 | * its the lock acquire, for RCU/lockless data structures its the dependent | |
21 | * load. | |
22 | * | |
23 | * Do note that inc_not_zero() provides a control dependency which will order | |
24 | * future stores against the inc, this ensures we'll never modify the object | |
25 | * if we did not in fact acquire a reference. | |
26 | * | |
27 | * The decrements will provide release order, such that all the prior loads and | |
28 | * stores will be issued before, it also provides a control dependency, which | |
29 | * will order us against the subsequent free(). | |
30 | * | |
31 | * The control dependency is against the load of the cmpxchg (ll/sc) that | |
32 | * succeeded. This means the stores aren't fully ordered, but this is fine | |
33 | * because the 1->0 transition indicates no concurrency. | |
34 | * | |
35 | * Note that the allocator is responsible for ordering things between free() | |
36 | * and alloc(). | |
37 | * | |
38 | */ | |
39 | ||
40 | #include <linux/atomic.h> | |
41 | #include <linux/bug.h> | |
42 | #include <linux/mutex.h> | |
43 | #include <linux/spinlock.h> | |
44 | ||
45 | #ifdef CONFIG_DEBUG_REFCOUNT | |
46 | #define REFCOUNT_WARN(cond, str) WARN_ON(cond) | |
47 | #define __refcount_check __must_check | |
48 | #else | |
49 | #define REFCOUNT_WARN(cond, str) (void)(cond) | |
50 | #define __refcount_check | |
51 | #endif | |
52 | ||
53 | typedef struct refcount_struct { | |
54 | atomic_t refs; | |
55 | } refcount_t; | |
56 | ||
57 | #define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), } | |
58 | ||
59 | static inline void refcount_set(refcount_t *r, unsigned int n) | |
60 | { | |
61 | atomic_set(&r->refs, n); | |
62 | } | |
63 | ||
64 | static inline unsigned int refcount_read(const refcount_t *r) | |
65 | { | |
66 | return atomic_read(&r->refs); | |
67 | } | |
68 | ||
69 | static inline __refcount_check | |
70 | bool refcount_add_not_zero(unsigned int i, refcount_t *r) | |
71 | { | |
72 | unsigned int old, new, val = atomic_read(&r->refs); | |
73 | ||
74 | for (;;) { | |
75 | if (!val) | |
76 | return false; | |
77 | ||
78 | if (unlikely(val == UINT_MAX)) | |
79 | return true; | |
80 | ||
81 | new = val + i; | |
82 | if (new < val) | |
83 | new = UINT_MAX; | |
84 | old = atomic_cmpxchg_relaxed(&r->refs, val, new); | |
85 | if (old == val) | |
86 | break; | |
87 | ||
88 | val = old; | |
89 | } | |
90 | ||
91 | REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); | |
92 | ||
93 | return true; | |
94 | } | |
95 | ||
96 | static inline void refcount_add(unsigned int i, refcount_t *r) | |
97 | { | |
98 | REFCOUNT_WARN(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n"); | |
99 | } | |
100 | ||
101 | /* | |
102 | * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN. | |
103 | * | |
104 | * Provides no memory ordering, it is assumed the caller has guaranteed the | |
105 | * object memory to be stable (RCU, etc.). It does provide a control dependency | |
106 | * and thereby orders future stores. See the comment on top. | |
107 | */ | |
108 | static inline __refcount_check | |
109 | bool refcount_inc_not_zero(refcount_t *r) | |
110 | { | |
111 | unsigned int old, new, val = atomic_read(&r->refs); | |
112 | ||
113 | for (;;) { | |
114 | new = val + 1; | |
115 | ||
116 | if (!val) | |
117 | return false; | |
118 | ||
119 | if (unlikely(!new)) | |
120 | return true; | |
121 | ||
122 | old = atomic_cmpxchg_relaxed(&r->refs, val, new); | |
123 | if (old == val) | |
124 | break; | |
125 | ||
126 | val = old; | |
127 | } | |
128 | ||
129 | REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); | |
130 | ||
131 | return true; | |
132 | } | |
133 | ||
134 | /* | |
135 | * Similar to atomic_inc(), will saturate at UINT_MAX and WARN. | |
136 | * | |
137 | * Provides no memory ordering, it is assumed the caller already has a | |
138 | * reference on the object, will WARN when this is not so. | |
139 | */ | |
140 | static inline void refcount_inc(refcount_t *r) | |
141 | { | |
142 | REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n"); | |
143 | } | |
144 | ||
145 | /* | |
146 | * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to | |
147 | * decrement when saturated at UINT_MAX. | |
148 | * | |
149 | * Provides release memory ordering, such that prior loads and stores are done | |
150 | * before, and provides a control dependency such that free() must come after. | |
151 | * See the comment on top. | |
152 | */ | |
153 | static inline __refcount_check | |
154 | bool refcount_sub_and_test(unsigned int i, refcount_t *r) | |
155 | { | |
156 | unsigned int old, new, val = atomic_read(&r->refs); | |
157 | ||
158 | for (;;) { | |
159 | if (unlikely(val == UINT_MAX)) | |
160 | return false; | |
161 | ||
162 | new = val - i; | |
163 | if (new > val) { | |
164 | REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n"); | |
165 | return false; | |
166 | } | |
167 | ||
168 | old = atomic_cmpxchg_release(&r->refs, val, new); | |
169 | if (old == val) | |
170 | break; | |
171 | ||
172 | val = old; | |
173 | } | |
174 | ||
175 | return !new; | |
176 | } | |
177 | ||
178 | static inline __refcount_check | |
179 | bool refcount_dec_and_test(refcount_t *r) | |
180 | { | |
181 | return refcount_sub_and_test(1, r); | |
182 | } | |
183 | ||
184 | /* | |
185 | * Similar to atomic_dec(), it will WARN on underflow and fail to decrement | |
186 | * when saturated at UINT_MAX. | |
187 | * | |
188 | * Provides release memory ordering, such that prior loads and stores are done | |
189 | * before. | |
190 | */ | |
191 | static inline | |
192 | void refcount_dec(refcount_t *r) | |
193 | { | |
194 | REFCOUNT_WARN(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n"); | |
195 | } | |
196 | ||
197 | /* | |
198 | * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the | |
199 | * success thereof. | |
200 | * | |
201 | * Like all decrement operations, it provides release memory order and provides | |
202 | * a control dependency. | |
203 | * | |
204 | * It can be used like a try-delete operator; this explicit case is provided | |
205 | * and not cmpxchg in generic, because that would allow implementing unsafe | |
206 | * operations. | |
207 | */ | |
208 | static inline __refcount_check | |
209 | bool refcount_dec_if_one(refcount_t *r) | |
210 | { | |
211 | return atomic_cmpxchg_release(&r->refs, 1, 0) == 1; | |
212 | } | |
213 | ||
214 | /* | |
215 | * No atomic_t counterpart, it decrements unless the value is 1, in which case | |
216 | * it will return false. | |
217 | * | |
218 | * Was often done like: atomic_add_unless(&var, -1, 1) | |
219 | */ | |
220 | static inline __refcount_check | |
221 | bool refcount_dec_not_one(refcount_t *r) | |
222 | { | |
223 | unsigned int old, new, val = atomic_read(&r->refs); | |
224 | ||
225 | for (;;) { | |
226 | if (unlikely(val == UINT_MAX)) | |
227 | return true; | |
228 | ||
229 | if (val == 1) | |
230 | return false; | |
231 | ||
232 | new = val - 1; | |
233 | if (new > val) { | |
234 | REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n"); | |
235 | return true; | |
236 | } | |
237 | ||
238 | old = atomic_cmpxchg_release(&r->refs, val, new); | |
239 | if (old == val) | |
240 | break; | |
241 | ||
242 | val = old; | |
243 | } | |
244 | ||
245 | return true; | |
246 | } | |
247 | ||
248 | /* | |
249 | * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail | |
250 | * to decrement when saturated at UINT_MAX. | |
251 | * | |
252 | * Provides release memory ordering, such that prior loads and stores are done | |
253 | * before, and provides a control dependency such that free() must come after. | |
254 | * See the comment on top. | |
255 | */ | |
256 | static inline __refcount_check | |
257 | bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) | |
258 | { | |
259 | if (refcount_dec_not_one(r)) | |
260 | return false; | |
261 | ||
262 | mutex_lock(lock); | |
263 | if (!refcount_dec_and_test(r)) { | |
264 | mutex_unlock(lock); | |
265 | return false; | |
266 | } | |
267 | ||
268 | return true; | |
269 | } | |
270 | ||
271 | /* | |
272 | * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to | |
273 | * decrement when saturated at UINT_MAX. | |
274 | * | |
275 | * Provides release memory ordering, such that prior loads and stores are done | |
276 | * before, and provides a control dependency such that free() must come after. | |
277 | * See the comment on top. | |
278 | */ | |
279 | static inline __refcount_check | |
280 | bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) | |
281 | { | |
282 | if (refcount_dec_not_one(r)) | |
283 | return false; | |
284 | ||
285 | spin_lock(lock); | |
286 | if (!refcount_dec_and_test(r)) { | |
287 | spin_unlock(lock); | |
288 | return false; | |
289 | } | |
290 | ||
291 | return true; | |
292 | } | |
293 | ||
294 | #endif /* _LINUX_REFCOUNT_H */ |