/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _TOOLS_LINUX_REFCOUNT_H
#define _TOOLS_LINUX_REFCOUNT_H
/*
* Variant of atomic_t specialized for reference counts.
*
* The interface matches the atomic_t interface (to aid in porting) but only
* provides the few functions one should use for reference counting.
*
* It differs in that the counter saturates at UINT_MAX and will not move once
* there. This avoids wrapping the counter and causing 'spurious'
* use-after-free issues.
*
* Memory ordering rules are slightly relaxed wrt regular atomic_t functions
* and provide only what is strictly required for refcounts.
*
* The increments are fully relaxed; these will not provide ordering. The
* rationale is that whatever is used to obtain the object we're increasing the
* reference count on will provide the ordering. For locked data structures,
* its the lock acquire, for RCU/lockless data structures its the dependent
* load.
*
* Do note that inc_not_zero() provides a control dependency which will order
* future stores against the inc, this ensures we'll never modify the object
* if we did not in fact acquire a reference.
*
* The decrements will provide release order, such that all the prior loads and
* stores will be issued before, it also provides a control dependency, which
* will order us against the subsequent free().
*
* The control dependency is against the load of the cmpxchg (ll/sc) that
* succeeded. This means the stores aren't fully ordered, but this is fine
* because the 1->0 transition indicates no concurrency.
*
* Note that the allocator is responsible for ordering things between free()
* and alloc().
*
*/
#include <linux/atomic.h>
#include <linux/kernel.h>
#ifdef NDEBUG
#define REFCOUNT_WARN(cond, str) (void)(cond)
#define __refcount_check
#else
#define REFCOUNT_WARN(cond, str) BUG_ON(cond)
#define __refcount_check __must_check
#endif
typedef struct refcount_struct {
atomic_t refs;
} refcount_t;
#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
static inline void refcount_set(refcount_t *r, unsigned int n)
{
atomic_set(&r->refs, n);
}
static inline unsigned int refcount_read(const refcount_t *r)
{
return atomic_read(&r->refs);
}
/*
* Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
*
* Provides no memory ordering, it is assumed the caller has guaranteed the
* object memory to be stable (RCU, etc.). It does provide a control dependency
* and thereby orders future stores. See the comment on top.
*/
static inline __refcount_check
bool refcount_inc_not_zero(refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);
for (;;) {
new = val + 1;
if (!val)
return false;
if (unlikely(!new))
return true;
old = atomic_cmpxchg_relaxed(&r->refs, val, new);
if (old == val)
break;
val = old;
}
REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
return true;
}
/*
* Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
*
* Provides no memory ordering, it is assumed the caller already has a
* reference on the object, will WARN when this is not so.
*/
static inline void refcount_inc(refcount_t *r)
{
REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
}
/*
* Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
* decrement when saturated at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides a control dependency such that free() must come after.
* See the comment on top.
*/
static inline __refcount_check
bool refcount_sub_and_test(unsigned int i, refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);
for (;;) {
if (unlikely(val == UINT_MAX))
return false;
new = val - i;
if (new > val) {
REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");
return false;
}
old = atomic_cmpxchg_release(&r->refs, val, new);
if (old == val)
break;
val = old;
}
return !new;
}
static inline __refcount_check
bool refcount_dec_and_test(refcount_t *r)
{
return refcount_sub_and_test(1, r);
}
#endif /* _ATOMIC_LINUX_REFCOUNT_H */
