tools/include/linux/refcount.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _TOOLS_LINUX_REFCOUNT_H
   3 #define _TOOLS_LINUX_REFCOUNT_H
   4
   5 /*
   6  * Variant of atomic_t specialized for reference counts.
   7  *
   8  * The interface matches the atomic_t interface (to aid in porting) but only
   9  * provides the few functions one should use for reference counting.
  10  *
  11  * It differs in that the counter saturates at UINT_MAX and will not move once
  12  * there. This avoids wrapping the counter and causing 'spurious'
  13  * use-after-free issues.
  14  *
  15  * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
  16  * and provide only what is strictly required for refcounts.
  17  *
  18  * The increments are fully relaxed; these will not provide ordering. The
  19  * rationale is that whatever is used to obtain the object we're increasing the
  20  * reference count on will provide the ordering. For locked data structures,
  21  * its the lock acquire, for RCU/lockless data structures its the dependent
  22  * load.
  23  *
  24  * Do note that inc_not_zero() provides a control dependency which will order
  25  * future stores against the inc, this ensures we'll never modify the object
  26  * if we did not in fact acquire a reference.
  27  *
  28  * The decrements will provide release order, such that all the prior loads and
  29  * stores will be issued before, it also provides a control dependency, which
  30  * will order us against the subsequent free().
  31  *
  32  * The control dependency is against the load of the cmpxchg (ll/sc) that
  33  * succeeded. This means the stores aren't fully ordered, but this is fine
  34  * because the 1->0 transition indicates no concurrency.
  35  *
  36  * Note that the allocator is responsible for ordering things between free()
  37  * and alloc().
  38  *
  39  */
  40
  41 #include <linux/atomic.h>
  42 #include <linux/kernel.h>
  43
  44 #ifdef NDEBUG
  45 #define REFCOUNT_WARN(cond, str) (void)(cond)
  46 #define __refcount_check
  47 #else
  48 #define REFCOUNT_WARN(cond, str) BUG_ON(cond)
  49 #define __refcount_check        __must_check
  50 #endif
  51
  52 typedef struct refcount_struct {
  53         atomic_t refs;
  54 } refcount_t;
  55
  56 #define REFCOUNT_INIT(n)        { .refs = ATOMIC_INIT(n), }
  57
  58 static inline void refcount_set(refcount_t *r, unsigned int n)
  59 {
  60         atomic_set(&r->refs, n);
  61 }
  62
  63 static inline unsigned int refcount_read(const refcount_t *r)
  64 {
  65         return atomic_read(&r->refs);
  66 }
  67
  68 /*
  69  * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
  70  *
  71  * Provides no memory ordering, it is assumed the caller has guaranteed the
  72  * object memory to be stable (RCU, etc.). It does provide a control dependency
  73  * and thereby orders future stores. See the comment on top.
  74  */
  75 static inline __refcount_check
  76 bool refcount_inc_not_zero(refcount_t *r)
  77 {
  78         unsigned int old, new, val = atomic_read(&r->refs);
  79
  80         for (;;) {
  81                 new = val + 1;
  82
  83                 if (!val)
  84                         return false;
  85
  86                 if (unlikely(!new))
  87                         return true;
  88
  89                 old = atomic_cmpxchg_relaxed(&r->refs, val, new);
  90                 if (old == val)
  91                         break;
  92
  93                 val = old;
  94         }
  95
  96         REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
  97
  98         return true;
  99 }
 100
 101 /*
 102  * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
 103  *
 104  * Provides no memory ordering, it is assumed the caller already has a
 105  * reference on the object, will WARN when this is not so.
 106  */
 107 static inline void refcount_inc(refcount_t *r)
 108 {
 109         REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
 110 }
 111
 112 /*
 113  * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
 114  * decrement when saturated at UINT_MAX.
 115  *
 116  * Provides release memory ordering, such that prior loads and stores are done
 117  * before, and provides a control dependency such that free() must come after.
 118  * See the comment on top.
 119  */
 120 static inline __refcount_check
 121 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
 122 {
 123         unsigned int old, new, val = atomic_read(&r->refs);
 124
 125         for (;;) {
 126                 if (unlikely(val == UINT_MAX))
 127                         return false;
 128
 129                 new = val - i;
 130                 if (new > val) {
 131                         REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");
 132                         return false;
 133                 }
 134
 135                 old = atomic_cmpxchg_release(&r->refs, val, new);
 136                 if (old == val)
 137                         break;
 138
 139                 val = old;
 140         }
 141
 142         return !new;
 143 }
 144
 145 static inline __refcount_check
 146 bool refcount_dec_and_test(refcount_t *r)
 147 {
 148         return refcount_sub_and_test(1, r);
 149 }
 150
 151
 152 #endif /* _ATOMIC_LINUX_REFCOUNT_H */