LXR linux/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
  52typedef struct refcount_struct {
  53        atomic_t refs;
  54} refcount_t;
  55
  56#define REFCOUNT_INIT(n)        { .refs = ATOMIC_INIT(n), }
  57
  58static inline void refcount_set(refcount_t *r, unsigned int n)
  59{
  60        atomic_set(&r->refs, n);
  61}
  62
  63static 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 */
  75static inline __refcount_check
  76bool 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 */
 107static 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 */
 120static inline __refcount_check
 121bool 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
 145static inline __refcount_check
 146bool 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 */
 153