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TOMOYO Linux Cross Reference
Linux/lib/refcount.c

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  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  */
 37 
 38 #include <linux/refcount.h>
 39 #include <linux/bug.h>
 40 
 41 #ifdef CONFIG_REFCOUNT_FULL
 42 
 43 /**
 44  * refcount_add_not_zero - add a value to a refcount unless it is 0
 45  * @i: the value to add to the refcount
 46  * @r: the refcount
 47  *
 48  * Will saturate at UINT_MAX and WARN.
 49  *
 50  * Provides no memory ordering, it is assumed the caller has guaranteed the
 51  * object memory to be stable (RCU, etc.). It does provide a control dependency
 52  * and thereby orders future stores. See the comment on top.
 53  *
 54  * Use of this function is not recommended for the normal reference counting
 55  * use case in which references are taken and released one at a time.  In these
 56  * cases, refcount_inc(), or one of its variants, should instead be used to
 57  * increment a reference count.
 58  *
 59  * Return: false if the passed refcount is 0, true otherwise
 60  */
 61 bool refcount_add_not_zero(unsigned int i, refcount_t *r)
 62 {
 63         unsigned int new, val = atomic_read(&r->refs);
 64 
 65         do {
 66                 if (!val)
 67                         return false;
 68 
 69                 if (unlikely(val == UINT_MAX))
 70                         return true;
 71 
 72                 new = val + i;
 73                 if (new < val)
 74                         new = UINT_MAX;
 75 
 76         } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
 77 
 78         WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
 79 
 80         return true;
 81 }
 82 EXPORT_SYMBOL(refcount_add_not_zero);
 83 
 84 /**
 85  * refcount_add - add a value to a refcount
 86  * @i: the value to add to the refcount
 87  * @r: the refcount
 88  *
 89  * Similar to atomic_add(), but will saturate at UINT_MAX and WARN.
 90  *
 91  * Provides no memory ordering, it is assumed the caller has guaranteed the
 92  * object memory to be stable (RCU, etc.). It does provide a control dependency
 93  * and thereby orders future stores. See the comment on top.
 94  *
 95  * Use of this function is not recommended for the normal reference counting
 96  * use case in which references are taken and released one at a time.  In these
 97  * cases, refcount_inc(), or one of its variants, should instead be used to
 98  * increment a reference count.
 99  */
100 void refcount_add(unsigned int i, refcount_t *r)
101 {
102         WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
103 }
104 EXPORT_SYMBOL(refcount_add);
105 
106 /**
107  * refcount_inc_not_zero - increment a refcount unless it is 0
108  * @r: the refcount to increment
109  *
110  * Similar to atomic_inc_not_zero(), but will saturate at UINT_MAX and WARN.
111  *
112  * Provides no memory ordering, it is assumed the caller has guaranteed the
113  * object memory to be stable (RCU, etc.). It does provide a control dependency
114  * and thereby orders future stores. See the comment on top.
115  *
116  * Return: true if the increment was successful, false otherwise
117  */
118 bool refcount_inc_not_zero(refcount_t *r)
119 {
120         unsigned int new, val = atomic_read(&r->refs);
121 
122         do {
123                 new = val + 1;
124 
125                 if (!val)
126                         return false;
127 
128                 if (unlikely(!new))
129                         return true;
130 
131         } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
132 
133         WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
134 
135         return true;
136 }
137 EXPORT_SYMBOL(refcount_inc_not_zero);
138 
139 /**
140  * refcount_inc - increment a refcount
141  * @r: the refcount to increment
142  *
143  * Similar to atomic_inc(), but will saturate at UINT_MAX and WARN.
144  *
145  * Provides no memory ordering, it is assumed the caller already has a
146  * reference on the object.
147  *
148  * Will WARN if the refcount is 0, as this represents a possible use-after-free
149  * condition.
150  */
151 void refcount_inc(refcount_t *r)
152 {
153         WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
154 }
155 EXPORT_SYMBOL(refcount_inc);
156 
157 /**
158  * refcount_sub_and_test - subtract from a refcount and test if it is 0
159  * @i: amount to subtract from the refcount
160  * @r: the refcount
161  *
162  * Similar to atomic_dec_and_test(), but it will WARN, return false and
163  * ultimately leak on underflow and will fail to decrement when saturated
164  * at UINT_MAX.
165  *
166  * Provides release memory ordering, such that prior loads and stores are done
167  * before, and provides a control dependency such that free() must come after.
168  * See the comment on top.
169  *
170  * Use of this function is not recommended for the normal reference counting
171  * use case in which references are taken and released one at a time.  In these
172  * cases, refcount_dec(), or one of its variants, should instead be used to
173  * decrement a reference count.
174  *
175  * Return: true if the resulting refcount is 0, false otherwise
176  */
177 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
178 {
179         unsigned int new, val = atomic_read(&r->refs);
180 
181         do {
182                 if (unlikely(val == UINT_MAX))
183                         return false;
184 
185                 new = val - i;
186                 if (new > val) {
187                         WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
188                         return false;
189                 }
190 
191         } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
192 
193         return !new;
194 }
195 EXPORT_SYMBOL(refcount_sub_and_test);
196 
197 /**
198  * refcount_dec_and_test - decrement a refcount and test if it is 0
199  * @r: the refcount
200  *
201  * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
202  * decrement when saturated at UINT_MAX.
203  *
204  * Provides release memory ordering, such that prior loads and stores are done
205  * before, and provides a control dependency such that free() must come after.
206  * See the comment on top.
207  *
208  * Return: true if the resulting refcount is 0, false otherwise
209  */
210 bool refcount_dec_and_test(refcount_t *r)
211 {
212         return refcount_sub_and_test(1, r);
213 }
214 EXPORT_SYMBOL(refcount_dec_and_test);
215 
216 /**
217  * refcount_dec - decrement a refcount
218  * @r: the refcount
219  *
220  * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
221  * when saturated at UINT_MAX.
222  *
223  * Provides release memory ordering, such that prior loads and stores are done
224  * before.
225  */
226 void refcount_dec(refcount_t *r)
227 {
228         WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
229 }
230 EXPORT_SYMBOL(refcount_dec);
231 #endif /* CONFIG_REFCOUNT_FULL */
232 
233 /**
234  * refcount_dec_if_one - decrement a refcount if it is 1
235  * @r: the refcount
236  *
237  * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
238  * success thereof.
239  *
240  * Like all decrement operations, it provides release memory order and provides
241  * a control dependency.
242  *
243  * It can be used like a try-delete operator; this explicit case is provided
244  * and not cmpxchg in generic, because that would allow implementing unsafe
245  * operations.
246  *
247  * Return: true if the resulting refcount is 0, false otherwise
248  */
249 bool refcount_dec_if_one(refcount_t *r)
250 {
251         int val = 1;
252 
253         return atomic_try_cmpxchg_release(&r->refs, &val, 0);
254 }
255 EXPORT_SYMBOL(refcount_dec_if_one);
256 
257 /**
258  * refcount_dec_not_one - decrement a refcount if it is not 1
259  * @r: the refcount
260  *
261  * No atomic_t counterpart, it decrements unless the value is 1, in which case
262  * it will return false.
263  *
264  * Was often done like: atomic_add_unless(&var, -1, 1)
265  *
266  * Return: true if the decrement operation was successful, false otherwise
267  */
268 bool refcount_dec_not_one(refcount_t *r)
269 {
270         unsigned int new, val = atomic_read(&r->refs);
271 
272         do {
273                 if (unlikely(val == UINT_MAX))
274                         return true;
275 
276                 if (val == 1)
277                         return false;
278 
279                 new = val - 1;
280                 if (new > val) {
281                         WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
282                         return true;
283                 }
284 
285         } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
286 
287         return true;
288 }
289 EXPORT_SYMBOL(refcount_dec_not_one);
290 
291 /**
292  * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
293  *                               refcount to 0
294  * @r: the refcount
295  * @lock: the mutex to be locked
296  *
297  * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
298  * to decrement when saturated at UINT_MAX.
299  *
300  * Provides release memory ordering, such that prior loads and stores are done
301  * before, and provides a control dependency such that free() must come after.
302  * See the comment on top.
303  *
304  * Return: true and hold mutex if able to decrement refcount to 0, false
305  *         otherwise
306  */
307 bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
308 {
309         if (refcount_dec_not_one(r))
310                 return false;
311 
312         mutex_lock(lock);
313         if (!refcount_dec_and_test(r)) {
314                 mutex_unlock(lock);
315                 return false;
316         }
317 
318         return true;
319 }
320 EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
321 
322 /**
323  * refcount_dec_and_lock - return holding spinlock if able to decrement
324  *                         refcount to 0
325  * @r: the refcount
326  * @lock: the spinlock to be locked
327  *
328  * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
329  * decrement when saturated at UINT_MAX.
330  *
331  * Provides release memory ordering, such that prior loads and stores are done
332  * before, and provides a control dependency such that free() must come after.
333  * See the comment on top.
334  *
335  * Return: true and hold spinlock if able to decrement refcount to 0, false
336  *         otherwise
337  */
338 bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
339 {
340         if (refcount_dec_not_one(r))
341                 return false;
342 
343         spin_lock(lock);
344         if (!refcount_dec_and_test(r)) {
345                 spin_unlock(lock);
346                 return false;
347         }
348 
349         return true;
350 }
351 EXPORT_SYMBOL(refcount_dec_and_lock);
352 
353 

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