projects/linux·blob v4.11 · refs

267 lines · 6.8 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
40bool refcount_add_not_zero(unsigned int i, refcount_t *r)
41{
42 unsigned int old, new, val = atomic_read(&r->refs);
43
44 for (;;) {
45 if (!val)
46 return false;
47
48 if (unlikely(val == UINT_MAX))
49 return true;
50
51 new = val + i;
52 if (new < val)
53 new = UINT_MAX;
54 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
55 if (old == val)
56 break;
57
58 val = old;
59 }
60
61 WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
62
63 return true;
64}
65EXPORT_SYMBOL_GPL(refcount_add_not_zero);
66
67void refcount_add(unsigned int i, refcount_t *r)
68{
69 WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
70}
71EXPORT_SYMBOL_GPL(refcount_add);
72
73/*
74 * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
75 *
76 * Provides no memory ordering, it is assumed the caller has guaranteed the
77 * object memory to be stable (RCU, etc.). It does provide a control dependency
78 * and thereby orders future stores. See the comment on top.
79 */
80bool refcount_inc_not_zero(refcount_t *r)
81{
82 unsigned int old, new, val = atomic_read(&r->refs);
83
84 for (;;) {
85 new = val + 1;
86
87 if (!val)
88 return false;
89
90 if (unlikely(!new))
91 return true;
92
93 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
94 if (old == val)
95 break;
96
97 val = old;
98 }
99
100 WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
101
102 return true;
103}
104EXPORT_SYMBOL_GPL(refcount_inc_not_zero);
105
106/*
107 * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
108 *
109 * Provides no memory ordering, it is assumed the caller already has a
110 * reference on the object, will WARN when this is not so.
111 */
112void refcount_inc(refcount_t *r)
113{
114 WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
115}
116EXPORT_SYMBOL_GPL(refcount_inc);
117
118bool refcount_sub_and_test(unsigned int i, refcount_t *r)
119{
120 unsigned int old, new, val = atomic_read(&r->refs);
121
122 for (;;) {
123 if (unlikely(val == UINT_MAX))
124 return false;
125
126 new = val - i;
127 if (new > val) {
128 WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
129 return false;
130 }
131
132 old = atomic_cmpxchg_release(&r->refs, val, new);
133 if (old == val)
134 break;
135
136 val = old;
137 }
138
139 return !new;
140}
141EXPORT_SYMBOL_GPL(refcount_sub_and_test);
142
143/*
144 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
145 * decrement when saturated at UINT_MAX.
146 *
147 * Provides release memory ordering, such that prior loads and stores are done
148 * before, and provides a control dependency such that free() must come after.
149 * See the comment on top.
150 */
151bool refcount_dec_and_test(refcount_t *r)
152{
153 return refcount_sub_and_test(1, r);
154}
155EXPORT_SYMBOL_GPL(refcount_dec_and_test);
156
157/*
158 * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
159 * when saturated at UINT_MAX.
160 *
161 * Provides release memory ordering, such that prior loads and stores are done
162 * before.
163 */
164
165void refcount_dec(refcount_t *r)
166{
167 WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
168}
169EXPORT_SYMBOL_GPL(refcount_dec);
170
171/*
172 * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
173 * success thereof.
174 *
175 * Like all decrement operations, it provides release memory order and provides
176 * a control dependency.
177 *
178 * It can be used like a try-delete operator; this explicit case is provided
179 * and not cmpxchg in generic, because that would allow implementing unsafe
180 * operations.
181 */
182bool refcount_dec_if_one(refcount_t *r)
183{
184 return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
185}
186EXPORT_SYMBOL_GPL(refcount_dec_if_one);
187
188/*
189 * No atomic_t counterpart, it decrements unless the value is 1, in which case
190 * it will return false.
191 *
192 * Was often done like: atomic_add_unless(&var, -1, 1)
193 */
194bool refcount_dec_not_one(refcount_t *r)
195{
196 unsigned int old, new, val = atomic_read(&r->refs);
197
198 for (;;) {
199 if (unlikely(val == UINT_MAX))
200 return true;
201
202 if (val == 1)
203 return false;
204
205 new = val - 1;
206 if (new > val) {
207 WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
208 return true;
209 }
210
211 old = atomic_cmpxchg_release(&r->refs, val, new);
212 if (old == val)
213 break;
214
215 val = old;
216 }
217
218 return true;
219}
220EXPORT_SYMBOL_GPL(refcount_dec_not_one);
221
222/*
223 * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
224 * to decrement when saturated at UINT_MAX.
225 *
226 * Provides release memory ordering, such that prior loads and stores are done
227 * before, and provides a control dependency such that free() must come after.
228 * See the comment on top.
229 */
230bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
231{
232 if (refcount_dec_not_one(r))
233 return false;
234
235 mutex_lock(lock);
236 if (!refcount_dec_and_test(r)) {
237 mutex_unlock(lock);
238 return false;
239 }
240
241 return true;
242}
243EXPORT_SYMBOL_GPL(refcount_dec_and_mutex_lock);
244
245/*
246 * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
247 * decrement when saturated at UINT_MAX.
248 *
249 * Provides release memory ordering, such that prior loads and stores are done
250 * before, and provides a control dependency such that free() must come after.
251 * See the comment on top.
252 */
253bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
254{
255 if (refcount_dec_not_one(r))
256 return false;
257
258 spin_lock(lock);
259 if (!refcount_dec_and_test(r)) {
260 spin_unlock(lock);
261 return false;
262 }
263
264 return true;
265}
266EXPORT_SYMBOL_GPL(refcount_dec_and_lock);
267