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