projects/linux·blob v4.13-rc2 · refs

351 lines · 10.2 KiB

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 */
60bool 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}
81EXPORT_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 */
99void 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}
103EXPORT_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 */
117bool 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}
136EXPORT_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 */
150void refcount_inc(refcount_t *r)
151{
152 WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
153}
154EXPORT_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 */
176bool 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}
194EXPORT_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 */
209bool refcount_dec_and_test(refcount_t *r)
210{
211 return refcount_sub_and_test(1, r);
212}
213EXPORT_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 */
225void refcount_dec(refcount_t *r)
226{
227 WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
228}
229EXPORT_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 */
248bool refcount_dec_if_one(refcount_t *r)
249{
250 int val = 1;
251
252 return atomic_try_cmpxchg_release(&r->refs, &val, 0);
253}
254EXPORT_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 */
267bool 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}
288EXPORT_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 */
306bool 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}
319EXPORT_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 */
337bool 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}
350EXPORT_SYMBOL(refcount_dec_and_lock);
351