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1/* SPDX-License-Identifier: GPL-2.0 */2#ifndef _TOOLS_LINUX_REFCOUNT_H3#define _TOOLS_LINUX_REFCOUNT_H4 5/*6 * Variant of atomic_t specialized for reference counts.7 *8 * The interface matches the atomic_t interface (to aid in porting) but only9 * 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 once12 * 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 functions16 * and provide only what is strictly required for refcounts.17 *18 * The increments are fully relaxed; these will not provide ordering. The19 * rationale is that whatever is used to obtain the object we're increasing the20 * reference count on will provide the ordering. For locked data structures,21 * its the lock acquire, for RCU/lockless data structures its the dependent22 * load.23 *24 * Do note that inc_not_zero() provides a control dependency which will order25 * future stores against the inc, this ensures we'll never modify the object26 * if we did not in fact acquire a reference.27 *28 * The decrements will provide release order, such that all the prior loads and29 * stores will be issued before, it also provides a control dependency, which30 * will order us against the subsequent free().31 *32 * The control dependency is against the load of the cmpxchg (ll/sc) that33 * succeeded. This means the stores aren't fully ordered, but this is fine34 * 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 NDEBUG45#define REFCOUNT_WARN(cond, str) (void)(cond)46#define __refcount_check47#else48#define REFCOUNT_WARN(cond, str) BUG_ON(cond)49#define __refcount_check __must_check50#endif51 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 the72 * object memory to be stable (RCU, etc.). It does provide a control dependency73 * and thereby orders future stores. See the comment on top.74 */75static inline __refcount_check76bool 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 a105 * 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 to114 * decrement when saturated at UINT_MAX.115 *116 * Provides release memory ordering, such that prior loads and stores are done117 * before, and provides a control dependency such that free() must come after.118 * See the comment on top.119 */120static inline __refcount_check121bool 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_check146bool 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