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 bool 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 }
  65 EXPORT_SYMBOL_GPL(refcount_add_not_zero);
  66
  67 void 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 }
  71 EXPORT_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  */
  80 bool 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 }
 104 EXPORT_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  */
 112 void refcount_inc(refcount_t *r)
 113 {
 114         WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
 115 }
 116 EXPORT_SYMBOL_GPL(refcount_inc);
 117
 118 bool 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 }
 141 EXPORT_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  */
 151 bool refcount_dec_and_test(refcount_t *r)
 152 {
 153         return refcount_sub_and_test(1, r);
 154 }
 155 EXPORT_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
 165 void refcount_dec(refcount_t *r)
 166 {
 167         WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
 168 }
 169 EXPORT_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  */
 182 bool refcount_dec_if_one(refcount_t *r)
 183 {
 184         return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
 185 }
 186 EXPORT_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  */
 194 bool 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 }
 220 EXPORT_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  */
 230 bool 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 }
 243 EXPORT_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  */
 253 bool 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 }
 266 EXPORT_SYMBOL_GPL(refcount_dec_and_lock);
 267