tools/include/linux/refcount.h

   1 #ifndef _TOOLS_LINUX_REFCOUNT_H
   2 #define _TOOLS_LINUX_REFCOUNT_H
   3
   4 /*
   5  * Variant of atomic_t specialized for reference counts.
   6  *
   7  * The interface matches the atomic_t interface (to aid in porting) but only
   8  * provides the few functions one should use for reference counting.
   9  *
  10  * It differs in that the counter saturates at UINT_MAX and will not move once
  11  * there. This avoids wrapping the counter and causing 'spurious'
  12  * use-after-free issues.
  13  *
  14  * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
  15  * and provide only what is strictly required for refcounts.
  16  *
  17  * The increments are fully relaxed; these will not provide ordering. The
  18  * rationale is that whatever is used to obtain the object we're increasing the
  19  * reference count on will provide the ordering. For locked data structures,
  20  * its the lock acquire, for RCU/lockless data structures its the dependent
  21  * load.
  22  *
  23  * Do note that inc_not_zero() provides a control dependency which will order
  24  * future stores against the inc, this ensures we'll never modify the object
  25  * if we did not in fact acquire a reference.
  26  *
  27  * The decrements will provide release order, such that all the prior loads and
  28  * stores will be issued before, it also provides a control dependency, which
  29  * will order us against the subsequent free().
  30  *
  31  * The control dependency is against the load of the cmpxchg (ll/sc) that
  32  * succeeded. This means the stores aren't fully ordered, but this is fine
  33  * because the 1->0 transition indicates no concurrency.
  34  *
  35  * Note that the allocator is responsible for ordering things between free()
  36  * and alloc().
  37  *
  38  */
  39
  40 #include <linux/atomic.h>
  41 #include <linux/kernel.h>
  42
  43 #ifdef NDEBUG
  44 #define REFCOUNT_WARN(cond, str) (void)(cond)
  45 #define __refcount_check
  46 #else
  47 #define REFCOUNT_WARN(cond, str) BUG_ON(cond)
  48 #define __refcount_check        __must_check
  49 #endif
  50
  51 typedef struct refcount_struct {
  52         atomic_t refs;
  53 } refcount_t;
  54
  55 #define REFCOUNT_INIT(n)        { .refs = ATOMIC_INIT(n), }
  56
  57 static inline void refcount_set(refcount_t *r, unsigned int n)
  58 {
  59         atomic_set(&r->refs, n);
  60 }
  61
  62 static inline unsigned int refcount_read(const refcount_t *r)
  63 {
  64         return atomic_read(&r->refs);
  65 }
  66
  67 /*
  68  * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
  69  *
  70  * Provides no memory ordering, it is assumed the caller has guaranteed the
  71  * object memory to be stable (RCU, etc.). It does provide a control dependency
  72  * and thereby orders future stores. See the comment on top.
  73  */
  74 static inline __refcount_check
  75 bool refcount_inc_not_zero(refcount_t *r)
  76 {
  77         unsigned int old, new, val = atomic_read(&r->refs);
  78
  79         for (;;) {
  80                 new = val + 1;
  81
  82                 if (!val)
  83                         return false;
  84
  85                 if (unlikely(!new))
  86                         return true;
  87
  88                 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
  89                 if (old == val)
  90                         break;
  91
  92                 val = old;
  93         }
  94
  95         REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
  96
  97         return true;
  98 }
  99
 100 /*
 101  * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
 102  *
 103  * Provides no memory ordering, it is assumed the caller already has a
 104  * reference on the object, will WARN when this is not so.
 105  */
 106 static inline void refcount_inc(refcount_t *r)
 107 {
 108         REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
 109 }
 110
 111 /*
 112  * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
 113  * decrement when saturated at UINT_MAX.
 114  *
 115  * Provides release memory ordering, such that prior loads and stores are done
 116  * before, and provides a control dependency such that free() must come after.
 117  * See the comment on top.
 118  */
 119 static inline __refcount_check
 120 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
 121 {
 122         unsigned int old, new, val = atomic_read(&r->refs);
 123
 124         for (;;) {
 125                 if (unlikely(val == UINT_MAX))
 126                         return false;
 127
 128                 new = val - i;
 129                 if (new > val) {
 130                         REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");
 131                         return false;
 132                 }
 133
 134                 old = atomic_cmpxchg_release(&r->refs, val, new);
 135                 if (old == val)
 136                         break;
 137
 138                 val = old;
 139         }
 140
 141         return !new;
 142 }
 143
 144 static inline __refcount_check
 145 bool refcount_dec_and_test(refcount_t *r)
 146 {
 147         return refcount_sub_and_test(1, r);
 148 }
 149
 150
 151 #endif /* _ATOMIC_LINUX_REFCOUNT_H */