lib/refcount.c

/*
 * 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>

/**
 * refcount_add_not_zero - add a value to a refcount unless it is 0
 * @i: the value to add to the refcount
 * @r: the refcount
 *
 * 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.
 *
 * Use of this function is not recommended for the normal reference counting
 * use case in which references are taken and released one at a time.  In these
 * cases, refcount_inc(), or one of its variants, should instead be used to
 * increment a reference count.
 *
 * Return: false if the passed refcount is 0, true otherwise
 */
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);

/**
 * refcount_add - add a value to a refcount
 * @i: the value to add to the refcount
 * @r: the refcount
 *
 * Similar to atomic_add(), but 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.
 *
 * Use of this function is not recommended for the normal reference counting
 * use case in which references are taken and released one at a time.  In these
 * cases, refcount_inc(), or one of its variants, should instead be used to
 * increment a reference count.
 */
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);

/**
 * refcount_inc_not_zero - increment a refcount unless it is 0
 * @r: the refcount to increment
 *
 * Similar to atomic_inc_not_zero(), but 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.
 *
 * Return: true if the increment was successful, false otherwise
 */
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);

/**
 * refcount_inc - increment a refcount
 * @r: the refcount to increment
 *
 * Similar to atomic_inc(), but 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 if the refcount is 0, as this represents a possible use-after-free
 * condition.
 */
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);

/**
 * refcount_sub_and_test - subtract from a refcount and test if it is 0
 * @i: amount to subtract from the refcount
 * @r: the refcount
 *
 * Similar to atomic_dec_and_test(), but it will WARN, return false and
 * ultimately leak on underflow and will 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.
 *
 * Use of this function is not recommended for the normal reference counting
 * use case in which references are taken and released one at a time.  In these
 * cases, refcount_dec(), or one of its variants, should instead be used to
 * decrement a reference count.
 *
 * Return: true if the resulting refcount is 0, false otherwise
 */
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);

/**
 * refcount_dec_and_test - decrement a refcount and test if it is 0
 * @r: the refcount
 *
 * 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.
 *
 * Return: true if the resulting refcount is 0, false otherwise
 */
bool refcount_dec_and_test(refcount_t *r)
{
	return refcount_sub_and_test(1, r);
}
EXPORT_SYMBOL_GPL(refcount_dec_and_test);

/**
 * refcount_dec - decrement a refcount
 * @r: the refcount
 *
 * 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);

/**
 * refcount_dec_if_one - decrement a refcount if it is 1
 * @r: the refcount
 *
 * 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.
 *
 * Return: true if the resulting refcount is 0, false otherwise
 */
bool refcount_dec_if_one(refcount_t *r)
{
	return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
}
EXPORT_SYMBOL_GPL(refcount_dec_if_one);

/**
 * refcount_dec_not_one - decrement a refcount if it is not 1
 * @r: the refcount
 *
 * 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)
 *
 * Return: true if the decrement operation was successful, false otherwise
 */
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);

/**
 * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
 *                               refcount to 0
 * @r: the refcount
 * @lock: the mutex to be locked
 *
 * 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.
 *
 * Return: true and hold mutex if able to decrement refcount to 0, false
 *         otherwise
 */
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);

/**
 * refcount_dec_and_lock - return holding spinlock if able to decrement
 *                         refcount to 0
 * @r: the refcount
 * @lock: the spinlock to be locked
 *
 * 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.
 *
 * Return: true and hold spinlock if able to decrement refcount to 0, false
 *         otherwise
 */
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);