1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Variant of atomic_t specialized for reference counts.
4 *
5 * The interface matches the atomic_t interface (to aid in porting) but only
6 * provides the few functions one should use for reference counting.
7 *
8 * It differs in that the counter saturates at UINT_MAX and will not move once
9 * there. This avoids wrapping the counter and causing 'spurious'
10 * use-after-free issues.
11 *
12 * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
13 * and provide only what is strictly required for refcounts.
14 *
15 * The increments are fully relaxed; these will not provide ordering. The
16 * rationale is that whatever is used to obtain the object we're increasing the
17 * reference count on will provide the ordering. For locked data structures,
18 * its the lock acquire, for RCU/lockless data structures its the dependent
19 * load.
20 *
21 * Do note that inc_not_zero() provides a control dependency which will order
22 * future stores against the inc, this ensures we'll never modify the object
23 * if we did not in fact acquire a reference.
24 *
25 * The decrements will provide release order, such that all the prior loads and
26 * stores will be issued before, it also provides a control dependency, which
27 * will order us against the subsequent free().
28 *
29 * The control dependency is against the load of the cmpxchg (ll/sc) that
30 * succeeded. This means the stores aren't fully ordered, but this is fine
31 * because the 1->0 transition indicates no concurrency.
32 *
33 * Note that the allocator is responsible for ordering things between free()
34 * and alloc().
35 *
36 * The decrements dec_and_test() and sub_and_test() also provide acquire
37 * ordering on success.
38 *
39 */
40
41 #include <linux/mutex.h>
42 #include <linux/refcount.h>
43 #include <linux/spinlock.h>
44 #include <linux/bug.h>
45
46 /**
47 * refcount_add_not_zero_checked - add a value to a refcount unless it is 0
48 * @i: the value to add to the refcount
49 * @r: the refcount
50 *
51 * Will saturate at UINT_MAX and WARN.
52 *
53 * Provides no memory ordering, it is assumed the caller has guaranteed the
54 * object memory to be stable (RCU, etc.). It does provide a control dependency
55 * and thereby orders future stores. See the comment on top.
56 *
57 * Use of this function is not recommended for the normal reference counting
58 * use case in which references are taken and released one at a time. In these
59 * cases, refcount_inc(), or one of its variants, should instead be used to
60 * increment a reference count.
61 *
62 * Return: false if the passed refcount is 0, true otherwise
63 */
64 bool refcount_add_not_zero_checked(unsigned int i, refcount_t *r)
65 {
66 unsigned int new, val = atomic_read(&r->refs);
67
68 do {
69 if (!val)
70 return false;
71
72 if (unlikely(val == UINT_MAX))
73 return true;
74
75 new = val + i;
76 if (new < val)
77 new = UINT_MAX;
78
79 } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
80
81 WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
82
83 return true;
84 }
85 EXPORT_SYMBOL(refcount_add_not_zero_checked);
86
87 /**
88 * refcount_add_checked - add a value to a refcount
89 * @i: the value to add to the refcount
90 * @r: the refcount
91 *
92 * Similar to atomic_add(), but will saturate at UINT_MAX and WARN.
93 *
94 * Provides no memory ordering, it is assumed the caller has guaranteed the
95 * object memory to be stable (RCU, etc.). It does provide a control dependency
96 * and thereby orders future stores. See the comment on top.
97 *
98 * Use of this function is not recommended for the normal reference counting
99 * use case in which references are taken and released one at a time. In these
100 * cases, refcount_inc(), or one of its variants, should instead be used to
101 * increment a reference count.
102 */
103 void refcount_add_checked(unsigned int i, refcount_t *r)
104 {
105 WARN_ONCE(!refcount_add_not_zero_checked(i, r), "refcount_t: addition on 0; use-after-free.\n");
106 }
107 EXPORT_SYMBOL(refcount_add_checked);
108
109 /**
110 * refcount_inc_not_zero_checked - increment a refcount unless it is 0
111 * @r: the refcount to increment
112 *
113 * Similar to atomic_inc_not_zero(), but will saturate at UINT_MAX and WARN.
114 *
115 * Provides no memory ordering, it is assumed the caller has guaranteed the
116 * object memory to be stable (RCU, etc.). It does provide a control dependency
117 * and thereby orders future stores. See the comment on top.
118 *
119 * Return: true if the increment was successful, false otherwise
120 */
121 bool refcount_inc_not_zero_checked(refcount_t *r)
122 {
123 unsigned int new, val = atomic_read(&r->refs);
124
125 do {
126 new = val + 1;
127
128 if (!val)
129 return false;
130
131 if (unlikely(!new))
132 return true;
133
134 } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
135
136 WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
137
138 return true;
139 }
140 EXPORT_SYMBOL(refcount_inc_not_zero_checked);
141
142 /**
143 * refcount_inc_checked - increment a refcount
144 * @r: the refcount to increment
145 *
146 * Similar to atomic_inc(), but will saturate at UINT_MAX and WARN.
147 *
148 * Provides no memory ordering, it is assumed the caller already has a
149 * reference on the object.
150 *
151 * Will WARN if the refcount is 0, as this represents a possible use-after-free
152 * condition.
153 */
154 void refcount_inc_checked(refcount_t *r)
155 {
156 WARN_ONCE(!refcount_inc_not_zero_checked(r), "refcount_t: increment on 0; use-after-free.\n");
157 }
158 EXPORT_SYMBOL(refcount_inc_checked);
159
160 /**
161 * refcount_sub_and_test_checked - subtract from a refcount and test if it is 0
162 * @i: amount to subtract from the refcount
163 * @r: the refcount
164 *
165 * Similar to atomic_dec_and_test(), but it will WARN, return false and
166 * ultimately leak on underflow and will fail to decrement when saturated
167 * at UINT_MAX.
168 *
169 * Provides release memory ordering, such that prior loads and stores are done
170 * before, and provides an acquire ordering on success such that free()
171 * must come after.
172 *
173 * Use of this function is not recommended for the normal reference counting
174 * use case in which references are taken and released one at a time. In these
175 * cases, refcount_dec(), or one of its variants, should instead be used to
176 * decrement a reference count.
177 *
178 * Return: true if the resulting refcount is 0, false otherwise
179 */
180 bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r)
181 {
182 unsigned int new, val = atomic_read(&r->refs);
183
184 do {
185 if (unlikely(val == UINT_MAX))
186 return false;
187
188 new = val - i;
189 if (new > val) {
190 WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
191 return false;
192 }
193
194 } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
195
196 if (!new) {
197 smp_acquire__after_ctrl_dep();
198 return true;
199 }
200 return false;
201
202 }
203 EXPORT_SYMBOL(refcount_sub_and_test_checked);
204
205 /**
206 * refcount_dec_and_test_checked - decrement a refcount and test if it is 0
207 * @r: the refcount
208 *
209 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
210 * decrement when saturated at UINT_MAX.
211 *
212 * Provides release memory ordering, such that prior loads and stores are done
213 * before, and provides an acquire ordering on success such that free()
214 * must come after.
215 *
216 * Return: true if the resulting refcount is 0, false otherwise
217 */
218 bool refcount_dec_and_test_checked(refcount_t *r)
219 {
220 return refcount_sub_and_test_checked(1, r);
221 }
222 EXPORT_SYMBOL(refcount_dec_and_test_checked);
223
224 /**
225 * refcount_dec_checked - decrement a refcount
226 * @r: the refcount
227 *
228 * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
229 * when saturated at UINT_MAX.
230 *
231 * Provides release memory ordering, such that prior loads and stores are done
232 * before.
233 */
234 void refcount_dec_checked(refcount_t *r)
235 {
236 WARN_ONCE(refcount_dec_and_test_checked(r), "refcount_t: decrement hit 0; leaking memory.\n");
237 }
238 EXPORT_SYMBOL(refcount_dec_checked);
239
240 /**
241 * refcount_dec_if_one - decrement a refcount if it is 1
242 * @r: the refcount
243 *
244 * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
245 * success thereof.
246 *
247 * Like all decrement operations, it provides release memory order and provides
248 * a control dependency.
249 *
250 * It can be used like a try-delete operator; this explicit case is provided
251 * and not cmpxchg in generic, because that would allow implementing unsafe
252 * operations.
253 *
254 * Return: true if the resulting refcount is 0, false otherwise
255 */
256 bool refcount_dec_if_one(refcount_t *r)
257 {
258 int val = 1;
259
260 return atomic_try_cmpxchg_release(&r->refs, &val, 0);
261 }
262 EXPORT_SYMBOL(refcount_dec_if_one);
263
264 /**
265 * refcount_dec_not_one - decrement a refcount if it is not 1
266 * @r: the refcount
267 *
268 * No atomic_t counterpart, it decrements unless the value is 1, in which case
269 * it will return false.
270 *
271 * Was often done like: atomic_add_unless(&var, -1, 1)
272 *
273 * Return: true if the decrement operation was successful, false otherwise
274 */
275 bool refcount_dec_not_one(refcount_t *r)
276 {
277 unsigned int new, val = atomic_read(&r->refs);
278
279 do {
280 if (unlikely(val == UINT_MAX))
281 return true;
282
283 if (val == 1)
284 return false;
285
286 new = val - 1;
287 if (new > val) {
288 WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
289 return true;
290 }
291
292 } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
293
294 return true;
295 }
296 EXPORT_SYMBOL(refcount_dec_not_one);
297
298 /**
299 * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
300 * refcount to 0
301 * @r: the refcount
302 * @lock: the mutex to be locked
303 *
304 * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
305 * to decrement when saturated at UINT_MAX.
306 *
307 * Provides release memory ordering, such that prior loads and stores are done
308 * before, and provides a control dependency such that free() must come after.
309 * See the comment on top.
310 *
311 * Return: true and hold mutex if able to decrement refcount to 0, false
312 * otherwise
313 */
314 bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
315 {
316 if (refcount_dec_not_one(r))
317 return false;
318
319 mutex_lock(lock);
320 if (!refcount_dec_and_test(r)) {
321 mutex_unlock(lock);
322 return false;
323 }
324
325 return true;
326 }
327 EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
328
329 /**
330 * refcount_dec_and_lock - return holding spinlock if able to decrement
331 * refcount to 0
332 * @r: the refcount
333 * @lock: the spinlock to be locked
334 *
335 * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
336 * decrement when saturated at UINT_MAX.
337 *
338 * Provides release memory ordering, such that prior loads and stores are done
339 * before, and provides a control dependency such that free() must come after.
340 * See the comment on top.
341 *
342 * Return: true and hold spinlock if able to decrement refcount to 0, false
343 * otherwise
344 */
345 bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
346 {
347 if (refcount_dec_not_one(r))
348 return false;
349
350 spin_lock(lock);
351 if (!refcount_dec_and_test(r)) {
352 spin_unlock(lock);
353 return false;
354 }
355
356 return true;
357 }
358 EXPORT_SYMBOL(refcount_dec_and_lock);
359
360 /**
361 * refcount_dec_and_lock_irqsave - return holding spinlock with disabled
362 * interrupts if able to decrement refcount to 0
363 * @r: the refcount
364 * @lock: the spinlock to be locked
365 * @flags: saved IRQ-flags if the is acquired
366 *
367 * Same as refcount_dec_and_lock() above except that the spinlock is acquired
368 * with disabled interupts.
369 *
370 * Return: true and hold spinlock if able to decrement refcount to 0, false
371 * otherwise
372 */
373 bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock,
374 unsigned long *flags)
375 {
376 if (refcount_dec_not_one(r))
377 return false;
378
379 spin_lock_irqsave(lock, *flags);
380 if (!refcount_dec_and_test(r)) {
381 spin_unlock_irqrestore(lock, *flags);
382 return false;
383 }
384
385 return true;
386 }
387 EXPORT_SYMBOL(refcount_dec_and_lock_irqsave);