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1 | /* | |
2 | * Variant of atomic_t specialized for reference counts. | |
3 | * | |
4 | * The interface matches the atomic_t interface (to aid in porting) but only | |
5 | * provides the few functions one should use for reference counting. | |
6 | * | |
7 | * It differs in that the counter saturates at UINT_MAX and will not move once | |
8 | * there. This avoids wrapping the counter and causing 'spurious' | |
9 | * use-after-free issues. | |
10 | * | |
11 | * Memory ordering rules are slightly relaxed wrt regular atomic_t functions | |
12 | * and provide only what is strictly required for refcounts. | |
13 | * | |
14 | * The increments are fully relaxed; these will not provide ordering. The | |
15 | * rationale is that whatever is used to obtain the object we're increasing the | |
16 | * reference count on will provide the ordering. For locked data structures, | |
17 | * its the lock acquire, for RCU/lockless data structures its the dependent | |
18 | * load. | |
19 | * | |
20 | * Do note that inc_not_zero() provides a control dependency which will order | |
21 | * future stores against the inc, this ensures we'll never modify the object | |
22 | * if we did not in fact acquire a reference. | |
23 | * | |
24 | * The decrements will provide release order, such that all the prior loads and | |
25 | * stores will be issued before, it also provides a control dependency, which | |
26 | * will order us against the subsequent free(). | |
27 | * | |
28 | * The control dependency is against the load of the cmpxchg (ll/sc) that | |
29 | * succeeded. This means the stores aren't fully ordered, but this is fine | |
30 | * because the 1->0 transition indicates no concurrency. | |
31 | * | |
32 | * Note that the allocator is responsible for ordering things between free() | |
33 | * and alloc(). | |
34 | * | |
35 | */ | |
36 | ||
37 | #include <linux/refcount.h> | |
38 | #include <linux/bug.h> | |
39 | ||
40 | #ifdef CONFIG_REFCOUNT_FULL | |
41 | ||
42 | /** | |
43 | * refcount_add_not_zero - add a value to a refcount unless it is 0 | |
44 | * @i: the value to add to the refcount | |
45 | * @r: the refcount | |
46 | * | |
47 | * Will saturate at UINT_MAX and WARN. | |
48 | * | |
49 | * Provides no memory ordering, it is assumed the caller has guaranteed the | |
50 | * object memory to be stable (RCU, etc.). It does provide a control dependency | |
51 | * and thereby orders future stores. See the comment on top. | |
52 | * | |
53 | * Use of this function is not recommended for the normal reference counting | |
54 | * use case in which references are taken and released one at a time. In these | |
55 | * cases, refcount_inc(), or one of its variants, should instead be used to | |
56 | * increment a reference count. | |
57 | * | |
58 | * Return: false if the passed refcount is 0, true otherwise | |
59 | */ | |
60 | bool refcount_add_not_zero(unsigned int i, refcount_t *r) | |
61 | { | |
62 | unsigned int new, val = atomic_read(&r->refs); | |
63 | ||
64 | do { | |
65 | if (!val) | |
66 | return false; | |
67 | ||
68 | if (unlikely(val == UINT_MAX)) | |
69 | return true; | |
70 | ||
71 | new = val + i; | |
72 | if (new < val) | |
73 | new = UINT_MAX; | |
74 | ||
75 | } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); | |
76 | ||
77 | WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); | |
78 | ||
79 | return true; | |
80 | } | |
81 | EXPORT_SYMBOL(refcount_add_not_zero); | |
82 | ||
83 | /** | |
84 | * refcount_add - add a value to a refcount | |
85 | * @i: the value to add to the refcount | |
86 | * @r: the refcount | |
87 | * | |
88 | * Similar to atomic_add(), but will saturate at UINT_MAX and WARN. | |
89 | * | |
90 | * Provides no memory ordering, it is assumed the caller has guaranteed the | |
91 | * object memory to be stable (RCU, etc.). It does provide a control dependency | |
92 | * and thereby orders future stores. See the comment on top. | |
93 | * | |
94 | * Use of this function is not recommended for the normal reference counting | |
95 | * use case in which references are taken and released one at a time. In these | |
96 | * cases, refcount_inc(), or one of its variants, should instead be used to | |
97 | * increment a reference count. | |
98 | */ | |
99 | void refcount_add(unsigned int i, refcount_t *r) | |
100 | { | |
101 | WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n"); | |
102 | } | |
103 | EXPORT_SYMBOL(refcount_add); | |
104 | ||
105 | /** | |
106 | * refcount_inc_not_zero - increment a refcount unless it is 0 | |
107 | * @r: the refcount to increment | |
108 | * | |
109 | * Similar to atomic_inc_not_zero(), but will saturate at UINT_MAX and WARN. | |
110 | * | |
111 | * Provides no memory ordering, it is assumed the caller has guaranteed the | |
112 | * object memory to be stable (RCU, etc.). It does provide a control dependency | |
113 | * and thereby orders future stores. See the comment on top. | |
114 | * | |
115 | * Return: true if the increment was successful, false otherwise | |
116 | */ | |
117 | bool refcount_inc_not_zero(refcount_t *r) | |
118 | { | |
119 | unsigned int new, val = atomic_read(&r->refs); | |
120 | ||
121 | do { | |
122 | new = val + 1; | |
123 | ||
124 | if (!val) | |
125 | return false; | |
126 | ||
127 | if (unlikely(!new)) | |
128 | return true; | |
129 | ||
130 | } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); | |
131 | ||
132 | WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); | |
133 | ||
134 | return true; | |
135 | } | |
136 | EXPORT_SYMBOL(refcount_inc_not_zero); | |
137 | ||
138 | /** | |
139 | * refcount_inc - increment a refcount | |
140 | * @r: the refcount to increment | |
141 | * | |
142 | * Similar to atomic_inc(), but will saturate at UINT_MAX and WARN. | |
143 | * | |
144 | * Provides no memory ordering, it is assumed the caller already has a | |
145 | * reference on the object. | |
146 | * | |
147 | * Will WARN if the refcount is 0, as this represents a possible use-after-free | |
148 | * condition. | |
149 | */ | |
150 | void refcount_inc(refcount_t *r) | |
151 | { | |
152 | WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n"); | |
153 | } | |
154 | EXPORT_SYMBOL(refcount_inc); | |
155 | ||
156 | /** | |
157 | * refcount_sub_and_test - subtract from a refcount and test if it is 0 | |
158 | * @i: amount to subtract from the refcount | |
159 | * @r: the refcount | |
160 | * | |
161 | * Similar to atomic_dec_and_test(), but it will WARN, return false and | |
162 | * ultimately leak on underflow and will fail to decrement when saturated | |
163 | * at UINT_MAX. | |
164 | * | |
165 | * Provides release memory ordering, such that prior loads and stores are done | |
166 | * before, and provides a control dependency such that free() must come after. | |
167 | * See the comment on top. | |
168 | * | |
169 | * Use of this function is not recommended for the normal reference counting | |
170 | * use case in which references are taken and released one at a time. In these | |
171 | * cases, refcount_dec(), or one of its variants, should instead be used to | |
172 | * decrement a reference count. | |
173 | * | |
174 | * Return: true if the resulting refcount is 0, false otherwise | |
175 | */ | |
176 | bool refcount_sub_and_test(unsigned int i, refcount_t *r) | |
177 | { | |
178 | unsigned int new, val = atomic_read(&r->refs); | |
179 | ||
180 | do { | |
181 | if (unlikely(val == UINT_MAX)) | |
182 | return false; | |
183 | ||
184 | new = val - i; | |
185 | if (new > val) { | |
186 | WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n"); | |
187 | return false; | |
188 | } | |
189 | ||
190 | } while (!atomic_try_cmpxchg_release(&r->refs, &val, new)); | |
191 | ||
192 | return !new; | |
193 | } | |
194 | EXPORT_SYMBOL(refcount_sub_and_test); | |
195 | ||
196 | /** | |
197 | * refcount_dec_and_test - decrement a refcount and test if it is 0 | |
198 | * @r: the refcount | |
199 | * | |
200 | * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to | |
201 | * decrement when saturated at UINT_MAX. | |
202 | * | |
203 | * Provides release memory ordering, such that prior loads and stores are done | |
204 | * before, and provides a control dependency such that free() must come after. | |
205 | * See the comment on top. | |
206 | * | |
207 | * Return: true if the resulting refcount is 0, false otherwise | |
208 | */ | |
209 | bool refcount_dec_and_test(refcount_t *r) | |
210 | { | |
211 | return refcount_sub_and_test(1, r); | |
212 | } | |
213 | EXPORT_SYMBOL(refcount_dec_and_test); | |
214 | ||
215 | /** | |
216 | * refcount_dec - decrement a refcount | |
217 | * @r: the refcount | |
218 | * | |
219 | * Similar to atomic_dec(), it will WARN on underflow and fail to decrement | |
220 | * when saturated at UINT_MAX. | |
221 | * | |
222 | * Provides release memory ordering, such that prior loads and stores are done | |
223 | * before. | |
224 | */ | |
225 | void refcount_dec(refcount_t *r) | |
226 | { | |
227 | WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n"); | |
228 | } | |
229 | EXPORT_SYMBOL(refcount_dec); | |
230 | #endif /* CONFIG_REFCOUNT_FULL */ | |
231 | ||
232 | /** | |
233 | * refcount_dec_if_one - decrement a refcount if it is 1 | |
234 | * @r: the refcount | |
235 | * | |
236 | * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the | |
237 | * success thereof. | |
238 | * | |
239 | * Like all decrement operations, it provides release memory order and provides | |
240 | * a control dependency. | |
241 | * | |
242 | * It can be used like a try-delete operator; this explicit case is provided | |
243 | * and not cmpxchg in generic, because that would allow implementing unsafe | |
244 | * operations. | |
245 | * | |
246 | * Return: true if the resulting refcount is 0, false otherwise | |
247 | */ | |
248 | bool refcount_dec_if_one(refcount_t *r) | |
249 | { | |
250 | int val = 1; | |
251 | ||
252 | return atomic_try_cmpxchg_release(&r->refs, &val, 0); | |
253 | } | |
254 | EXPORT_SYMBOL(refcount_dec_if_one); | |
255 | ||
256 | /** | |
257 | * refcount_dec_not_one - decrement a refcount if it is not 1 | |
258 | * @r: the refcount | |
259 | * | |
260 | * No atomic_t counterpart, it decrements unless the value is 1, in which case | |
261 | * it will return false. | |
262 | * | |
263 | * Was often done like: atomic_add_unless(&var, -1, 1) | |
264 | * | |
265 | * Return: true if the decrement operation was successful, false otherwise | |
266 | */ | |
267 | bool refcount_dec_not_one(refcount_t *r) | |
268 | { | |
269 | unsigned int new, val = atomic_read(&r->refs); | |
270 | ||
271 | do { | |
272 | if (unlikely(val == UINT_MAX)) | |
273 | return true; | |
274 | ||
275 | if (val == 1) | |
276 | return false; | |
277 | ||
278 | new = val - 1; | |
279 | if (new > val) { | |
280 | WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n"); | |
281 | return true; | |
282 | } | |
283 | ||
284 | } while (!atomic_try_cmpxchg_release(&r->refs, &val, new)); | |
285 | ||
286 | return true; | |
287 | } | |
288 | EXPORT_SYMBOL(refcount_dec_not_one); | |
289 | ||
290 | /** | |
291 | * refcount_dec_and_mutex_lock - return holding mutex if able to decrement | |
292 | * refcount to 0 | |
293 | * @r: the refcount | |
294 | * @lock: the mutex to be locked | |
295 | * | |
296 | * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail | |
297 | * to decrement when saturated at UINT_MAX. | |
298 | * | |
299 | * Provides release memory ordering, such that prior loads and stores are done | |
300 | * before, and provides a control dependency such that free() must come after. | |
301 | * See the comment on top. | |
302 | * | |
303 | * Return: true and hold mutex if able to decrement refcount to 0, false | |
304 | * otherwise | |
305 | */ | |
306 | bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) | |
307 | { | |
308 | if (refcount_dec_not_one(r)) | |
309 | return false; | |
310 | ||
311 | mutex_lock(lock); | |
312 | if (!refcount_dec_and_test(r)) { | |
313 | mutex_unlock(lock); | |
314 | return false; | |
315 | } | |
316 | ||
317 | return true; | |
318 | } | |
319 | EXPORT_SYMBOL(refcount_dec_and_mutex_lock); | |
320 | ||
321 | /** | |
322 | * refcount_dec_and_lock - return holding spinlock if able to decrement | |
323 | * refcount to 0 | |
324 | * @r: the refcount | |
325 | * @lock: the spinlock to be locked | |
326 | * | |
327 | * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to | |
328 | * decrement when saturated at UINT_MAX. | |
329 | * | |
330 | * Provides release memory ordering, such that prior loads and stores are done | |
331 | * before, and provides a control dependency such that free() must come after. | |
332 | * See the comment on top. | |
333 | * | |
334 | * Return: true and hold spinlock if able to decrement refcount to 0, false | |
335 | * otherwise | |
336 | */ | |
337 | bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) | |
338 | { | |
339 | if (refcount_dec_not_one(r)) | |
340 | return false; | |
341 | ||
342 | spin_lock(lock); | |
343 | if (!refcount_dec_and_test(r)) { | |
344 | spin_unlock(lock); | |
345 | return false; | |
346 | } | |
347 | ||
348 | return true; | |
349 | } | |
350 | EXPORT_SYMBOL(refcount_dec_and_lock); | |
351 |