/* * 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/refcount.h>#include <linux/bug.h> bool refcount_add_not_zero(unsigned int i, refcount_t *r){	unsigned int old, new, val = atomic_read(&r->refs); 	for (;;) {		if (!val)			return false; 		if (unlikely(val == UINT_MAX))			return true; 		new = val + i;		if (new < val)			new = UINT_MAX;		old = atomic_cmpxchg_relaxed(&r->refs, val, new);		if (old == val)			break; 		val = old;	} 	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); 	return true;}EXPORT_SYMBOL_GPL(refcount_add_not_zero); void refcount_add(unsigned int i, refcount_t *r){	WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");}EXPORT_SYMBOL_GPL(refcount_add); /* * 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. */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;	} 	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); 	return true;}EXPORT_SYMBOL_GPL(refcount_inc_not_zero); /* * 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. */void refcount_inc(refcount_t *r){	WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");}EXPORT_SYMBOL_GPL(refcount_inc); 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) {			WARN_ONCE(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;}EXPORT_SYMBOL_GPL(refcount_sub_and_test); /* * 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. */bool refcount_dec_and_test(refcount_t *r){	return refcount_sub_and_test(1, r);}EXPORT_SYMBOL_GPL(refcount_dec_and_test); /* * Similar to atomic_dec(), 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. */ void refcount_dec(refcount_t *r){	WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");}EXPORT_SYMBOL_GPL(refcount_dec); /* * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the * success thereof. * * Like all decrement operations, it provides release memory order and provides * a control dependency. * * It can be used like a try-delete operator; this explicit case is provided * and not cmpxchg in generic, because that would allow implementing unsafe * operations. */bool refcount_dec_if_one(refcount_t *r){	return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;}EXPORT_SYMBOL_GPL(refcount_dec_if_one); /* * No atomic_t counterpart, it decrements unless the value is 1, in which case * it will return false. * * Was often done like: atomic_add_unless(&var, -1, 1) */bool refcount_dec_not_one(refcount_t *r){	unsigned int old, new, val = atomic_read(&r->refs); 	for (;;) {		if (unlikely(val == UINT_MAX))			return true; 		if (val == 1)			return false; 		new = val - 1;		if (new > val) {			WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");			return true;		} 		old = atomic_cmpxchg_release(&r->refs, val, new);		if (old == val)			break; 		val = old;	} 	return true;}EXPORT_SYMBOL_GPL(refcount_dec_not_one); /* * Similar to atomic_dec_and_mutex_lock(), 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. */bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock){	if (refcount_dec_not_one(r))		return false; 	mutex_lock(lock);	if (!refcount_dec_and_test(r)) {		mutex_unlock(lock);		return false;	} 	return true;}EXPORT_SYMBOL_GPL(refcount_dec_and_mutex_lock); /* * Similar to atomic_dec_and_lock(), 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. */bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock){	if (refcount_dec_not_one(r))		return false; 	spin_lock(lock);	if (!refcount_dec_and_test(r)) {		spin_unlock(lock);		return false;	} 	return true;}EXPORT_SYMBOL_GPL(refcount_dec_and_lock);