lib/refcount.c

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