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Peter Zijlstra29dee3c2017-02-10 16:27:52 +01001/*
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
David Windsorbd174162017-03-10 10:34:12 -050040/**
41 * refcount_add_not_zero - add a value to a refcount unless it is 0
42 * @i: the value to add to the refcount
43 * @r: the refcount
44 *
45 * Will saturate at UINT_MAX and WARN.
46 *
47 * Provides no memory ordering, it is assumed the caller has guaranteed the
48 * object memory to be stable (RCU, etc.). It does provide a control dependency
49 * and thereby orders future stores. See the comment on top.
50 *
51 * Use of this function is not recommended for the normal reference counting
52 * use case in which references are taken and released one at a time. In these
53 * cases, refcount_inc(), or one of its variants, should instead be used to
54 * increment a reference count.
55 *
56 * Return: false if the passed refcount is 0, true otherwise
57 */
Peter Zijlstra29dee3c2017-02-10 16:27:52 +010058bool refcount_add_not_zero(unsigned int i, refcount_t *r)
59{
60 unsigned int old, new, val = atomic_read(&r->refs);
61
62 for (;;) {
63 if (!val)
64 return false;
65
66 if (unlikely(val == UINT_MAX))
67 return true;
68
69 new = val + i;
70 if (new < val)
71 new = UINT_MAX;
72 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
73 if (old == val)
74 break;
75
76 val = old;
77 }
78
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +010079 WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +010080
81 return true;
82}
83EXPORT_SYMBOL_GPL(refcount_add_not_zero);
84
David Windsorbd174162017-03-10 10:34:12 -050085/**
86 * refcount_add - add a value to a refcount
87 * @i: the value to add to the refcount
88 * @r: the refcount
89 *
90 * Similar to atomic_add(), but will saturate at UINT_MAX and WARN.
91 *
92 * Provides no memory ordering, it is assumed the caller has guaranteed the
93 * object memory to be stable (RCU, etc.). It does provide a control dependency
94 * and thereby orders future stores. See the comment on top.
95 *
96 * Use of this function is not recommended for the normal reference counting
97 * use case in which references are taken and released one at a time. In these
98 * cases, refcount_inc(), or one of its variants, should instead be used to
99 * increment a reference count.
100 */
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100101void refcount_add(unsigned int i, refcount_t *r)
102{
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +0100103 WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100104}
105EXPORT_SYMBOL_GPL(refcount_add);
106
David Windsorbd174162017-03-10 10:34:12 -0500107/**
108 * refcount_inc_not_zero - increment a refcount unless it is 0
109 * @r: the refcount to increment
110 *
111 * Similar to atomic_inc_not_zero(), but will saturate at UINT_MAX and WARN.
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100112 *
113 * Provides no memory ordering, it is assumed the caller has guaranteed the
114 * object memory to be stable (RCU, etc.). It does provide a control dependency
115 * and thereby orders future stores. See the comment on top.
David Windsorbd174162017-03-10 10:34:12 -0500116 *
117 * Return: true if the increment was successful, false otherwise
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100118 */
119bool refcount_inc_not_zero(refcount_t *r)
120{
121 unsigned int old, new, val = atomic_read(&r->refs);
122
123 for (;;) {
124 new = val + 1;
125
126 if (!val)
127 return false;
128
129 if (unlikely(!new))
130 return true;
131
132 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
133 if (old == val)
134 break;
135
136 val = old;
137 }
138
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +0100139 WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100140
141 return true;
142}
143EXPORT_SYMBOL_GPL(refcount_inc_not_zero);
144
David Windsorbd174162017-03-10 10:34:12 -0500145/**
146 * refcount_inc - increment a refcount
147 * @r: the refcount to increment
148 *
149 * Similar to atomic_inc(), but will saturate at UINT_MAX and WARN.
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100150 *
151 * Provides no memory ordering, it is assumed the caller already has a
David Windsorbd174162017-03-10 10:34:12 -0500152 * reference on the object.
153 *
154 * Will WARN if the refcount is 0, as this represents a possible use-after-free
155 * condition.
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100156 */
157void refcount_inc(refcount_t *r)
158{
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +0100159 WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100160}
161EXPORT_SYMBOL_GPL(refcount_inc);
162
David Windsorbd174162017-03-10 10:34:12 -0500163/**
164 * refcount_sub_and_test - subtract from a refcount and test if it is 0
165 * @i: amount to subtract from the refcount
166 * @r: the refcount
167 *
168 * Similar to atomic_dec_and_test(), but it will WARN, return false and
169 * ultimately leak on underflow and will fail to decrement when saturated
170 * at UINT_MAX.
171 *
172 * Provides release memory ordering, such that prior loads and stores are done
173 * before, and provides a control dependency such that free() must come after.
174 * See the comment on top.
175 *
176 * Use of this function is not recommended for the normal reference counting
177 * use case in which references are taken and released one at a time. In these
178 * cases, refcount_dec(), or one of its variants, should instead be used to
179 * decrement a reference count.
180 *
181 * Return: true if the resulting refcount is 0, false otherwise
182 */
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100183bool refcount_sub_and_test(unsigned int i, refcount_t *r)
184{
185 unsigned int old, new, val = atomic_read(&r->refs);
186
187 for (;;) {
188 if (unlikely(val == UINT_MAX))
189 return false;
190
191 new = val - i;
192 if (new > val) {
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +0100193 WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100194 return false;
195 }
196
197 old = atomic_cmpxchg_release(&r->refs, val, new);
198 if (old == val)
199 break;
200
201 val = old;
202 }
203
204 return !new;
205}
206EXPORT_SYMBOL_GPL(refcount_sub_and_test);
207
David Windsorbd174162017-03-10 10:34:12 -0500208/**
209 * refcount_dec_and_test - decrement a refcount and test if it is 0
210 * @r: the refcount
211 *
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100212 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
213 * decrement when saturated at UINT_MAX.
214 *
215 * Provides release memory ordering, such that prior loads and stores are done
216 * before, and provides a control dependency such that free() must come after.
217 * See the comment on top.
David Windsorbd174162017-03-10 10:34:12 -0500218 *
219 * Return: true if the resulting refcount is 0, false otherwise
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100220 */
221bool refcount_dec_and_test(refcount_t *r)
222{
223 return refcount_sub_and_test(1, r);
224}
225EXPORT_SYMBOL_GPL(refcount_dec_and_test);
226
David Windsorbd174162017-03-10 10:34:12 -0500227/**
228 * refcount_dec - decrement a refcount
229 * @r: the refcount
230 *
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100231 * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
232 * when saturated at UINT_MAX.
233 *
234 * Provides release memory ordering, such that prior loads and stores are done
235 * before.
236 */
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100237void refcount_dec(refcount_t *r)
238{
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +0100239 WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100240}
241EXPORT_SYMBOL_GPL(refcount_dec);
242
David Windsorbd174162017-03-10 10:34:12 -0500243/**
244 * refcount_dec_if_one - decrement a refcount if it is 1
245 * @r: the refcount
246 *
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100247 * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
248 * success thereof.
249 *
250 * Like all decrement operations, it provides release memory order and provides
251 * a control dependency.
252 *
253 * It can be used like a try-delete operator; this explicit case is provided
254 * and not cmpxchg in generic, because that would allow implementing unsafe
255 * operations.
David Windsorbd174162017-03-10 10:34:12 -0500256 *
257 * Return: true if the resulting refcount is 0, false otherwise
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100258 */
259bool refcount_dec_if_one(refcount_t *r)
260{
261 return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
262}
263EXPORT_SYMBOL_GPL(refcount_dec_if_one);
264
David Windsorbd174162017-03-10 10:34:12 -0500265/**
266 * refcount_dec_not_one - decrement a refcount if it is not 1
267 * @r: the refcount
268 *
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100269 * No atomic_t counterpart, it decrements unless the value is 1, in which case
270 * it will return false.
271 *
272 * Was often done like: atomic_add_unless(&var, -1, 1)
David Windsorbd174162017-03-10 10:34:12 -0500273 *
274 * Return: true if the decrement operation was successful, false otherwise
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100275 */
276bool refcount_dec_not_one(refcount_t *r)
277{
278 unsigned int old, new, val = atomic_read(&r->refs);
279
280 for (;;) {
281 if (unlikely(val == UINT_MAX))
282 return true;
283
284 if (val == 1)
285 return false;
286
287 new = val - 1;
288 if (new > val) {
Ingo Molnar9dcfe2c2017-03-01 09:25:55 +0100289 WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100290 return true;
291 }
292
293 old = atomic_cmpxchg_release(&r->refs, val, new);
294 if (old == val)
295 break;
296
297 val = old;
298 }
299
300 return true;
301}
302EXPORT_SYMBOL_GPL(refcount_dec_not_one);
303
David Windsorbd174162017-03-10 10:34:12 -0500304/**
305 * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
306 * refcount to 0
307 * @r: the refcount
308 * @lock: the mutex to be locked
309 *
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100310 * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
311 * to decrement when saturated at UINT_MAX.
312 *
313 * Provides release memory ordering, such that prior loads and stores are done
314 * before, and provides a control dependency such that free() must come after.
315 * See the comment on top.
David Windsorbd174162017-03-10 10:34:12 -0500316 *
317 * Return: true and hold mutex if able to decrement refcount to 0, false
318 * otherwise
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100319 */
320bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
321{
322 if (refcount_dec_not_one(r))
323 return false;
324
325 mutex_lock(lock);
326 if (!refcount_dec_and_test(r)) {
327 mutex_unlock(lock);
328 return false;
329 }
330
331 return true;
332}
333EXPORT_SYMBOL_GPL(refcount_dec_and_mutex_lock);
334
David Windsorbd174162017-03-10 10:34:12 -0500335/**
336 * refcount_dec_and_lock - return holding spinlock if able to decrement
337 * refcount to 0
338 * @r: the refcount
339 * @lock: the spinlock to be locked
340 *
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100341 * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
342 * decrement when saturated at UINT_MAX.
343 *
344 * Provides release memory ordering, such that prior loads and stores are done
345 * before, and provides a control dependency such that free() must come after.
346 * See the comment on top.
David Windsorbd174162017-03-10 10:34:12 -0500347 *
348 * Return: true and hold spinlock if able to decrement refcount to 0, false
349 * otherwise
Peter Zijlstra29dee3c2017-02-10 16:27:52 +0100350 */
351bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
352{
353 if (refcount_dec_not_one(r))
354 return false;
355
356 spin_lock(lock);
357 if (!refcount_dec_and_test(r)) {
358 spin_unlock(lock);
359 return false;
360 }
361
362 return true;
363}
364EXPORT_SYMBOL_GPL(refcount_dec_and_lock);
365