LXR linux/lib/refcount.c

   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * Out-of-line refcount functions.
   4 */
   5
   6#include <linux/mutex.h>
   7#include <linux/refcount.h>
   8#include <linux/spinlock.h>
   9#include <linux/bug.h>
  10
  11#define REFCOUNT_WARN(str)      WARN_ONCE(1, "refcount_t: " str ".\n")
  12
  13void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
  14{
  15        refcount_set(r, REFCOUNT_SATURATED);
  16
  17        switch (t) {
  18        case REFCOUNT_ADD_NOT_ZERO_OVF:
  19                REFCOUNT_WARN("saturated; leaking memory");
  20                break;
  21        case REFCOUNT_ADD_OVF:
  22                REFCOUNT_WARN("saturated; leaking memory");
  23                break;
  24        case REFCOUNT_ADD_UAF:
  25                REFCOUNT_WARN("addition on 0; use-after-free");
  26                break;
  27        case REFCOUNT_SUB_UAF:
  28                REFCOUNT_WARN("underflow; use-after-free");
  29                break;
  30        case REFCOUNT_DEC_LEAK:
  31                REFCOUNT_WARN("decrement hit 0; leaking memory");
  32                break;
  33        default:
  34                REFCOUNT_WARN("unknown saturation event!?");
  35        }
  36}
  37EXPORT_SYMBOL(refcount_warn_saturate);
  38
  39/**
  40 * refcount_dec_if_one - decrement a refcount if it is 1
  41 * @r: the refcount
  42 *
  43 * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
  44 * success thereof.
  45 *
  46 * Like all decrement operations, it provides release memory order and provides
  47 * a control dependency.
  48 *
  49 * It can be used like a try-delete operator; this explicit case is provided
  50 * and not cmpxchg in generic, because that would allow implementing unsafe
  51 * operations.
  52 *
  53 * Return: true if the resulting refcount is 0, false otherwise
  54 */
  55bool refcount_dec_if_one(refcount_t *r)
  56{
  57        int val = 1;
  58
  59        return atomic_try_cmpxchg_release(&r->refs, &val, 0);
  60}
  61EXPORT_SYMBOL(refcount_dec_if_one);
  62
  63/**
  64 * refcount_dec_not_one - decrement a refcount if it is not 1
  65 * @r: the refcount
  66 *
  67 * No atomic_t counterpart, it decrements unless the value is 1, in which case
  68 * it will return false.
  69 *
  70 * Was often done like: atomic_add_unless(&var, -1, 1)
  71 *
  72 * Return: true if the decrement operation was successful, false otherwise
  73 */
  74bool refcount_dec_not_one(refcount_t *r)
  75{
  76        unsigned int new, val = atomic_read(&r->refs);
  77
  78        do {
  79                if (unlikely(val == REFCOUNT_SATURATED))
  80                        return true;
  81
  82                if (val == 1)
  83                        return false;
  84
  85                new = val - 1;
  86                if (new > val) {
  87                        WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
  88                        return true;
  89                }
  90
  91        } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
  92
  93        return true;
  94}
  95EXPORT_SYMBOL(refcount_dec_not_one);
  96
  97/**
  98 * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
  99 *                               refcount to 0
 100 * @r: the refcount
 101 * @lock: the mutex to be locked
 102 *
 103 * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
 104 * to decrement when saturated at REFCOUNT_SATURATED.
 105 *
 106 * Provides release memory ordering, such that prior loads and stores are done
 107 * before, and provides a control dependency such that free() must come after.
 108 * See the comment on top.
 109 *
 110 * Return: true and hold mutex if able to decrement refcount to 0, false
 111 *         otherwise
 112 */
 113bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
 114{
 115        if (refcount_dec_not_one(r))
 116                return false;
 117
 118        mutex_lock(lock);
 119        if (!refcount_dec_and_test(r)) {
 120                mutex_unlock(lock);
 121                return false;
 122        }
 123
 124        return true;
 125}
 126EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
 127
 128/**
 129 * refcount_dec_and_lock - return holding spinlock if able to decrement
 130 *                         refcount to 0
 131 * @r: the refcount
 132 * @lock: the spinlock to be locked
 133 *
 134 * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
 135 * decrement when saturated at REFCOUNT_SATURATED.
 136 *
 137 * Provides release memory ordering, such that prior loads and stores are done
 138 * before, and provides a control dependency such that free() must come after.
 139 * See the comment on top.
 140 *
 141 * Return: true and hold spinlock if able to decrement refcount to 0, false
 142 *         otherwise
 143 */
 144bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
 145{
 146        if (refcount_dec_not_one(r))
 147                return false;
 148
 149        spin_lock(lock);
 150        if (!refcount_dec_and_test(r)) {
 151                spin_unlock(lock);
 152                return false;
 153        }
 154
 155        return true;
 156}
 157EXPORT_SYMBOL(refcount_dec_and_lock);
 158
 159/**
 160 * refcount_dec_and_lock_irqsave - return holding spinlock with disabled
 161 *                                 interrupts if able to decrement refcount to 0
 162 * @r: the refcount
 163 * @lock: the spinlock to be locked
 164 * @flags: saved IRQ-flags if the is acquired
 165 *
 166 * Same as refcount_dec_and_lock() above except that the spinlock is acquired
 167 * with disabled interupts.
 168 *
 169 * Return: true and hold spinlock if able to decrement refcount to 0, false
 170 *         otherwise
 171 */
 172bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock,
 173                                   unsigned long *flags)
 174{
 175        if (refcount_dec_not_one(r))
 176                return false;
 177
 178        spin_lock_irqsave(lock, *flags);
 179        if (!refcount_dec_and_test(r)) {
 180                spin_unlock_irqrestore(lock, *flags);
 181                return false;
 182        }
 183
 184        return true;
 185}
 186EXPORT_SYMBOL(refcount_dec_and_lock_irqsave);
 187