lib/refcount.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Out-of-line refcount functions common to all refcount implementations.
 */

#include <linux/mutex.h>
#include <linux/refcount.h>
#include <linux/spinlock.h>
#include <linux/bug.h>

/**
 * 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)
{
	int val = 1;

	return atomic_try_cmpxchg_release(&r->refs, &val, 0);
}
EXPORT_SYMBOL(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 new, val = atomic_read(&r->refs);

	do {
		if (unlikely(val == REFCOUNT_SATURATED))
			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;
		}

	} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));

	return true;
}
EXPORT_SYMBOL(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 REFCOUNT_SATURATED.
 *
 * 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(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 REFCOUNT_SATURATED.
 *
 * 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(refcount_dec_and_lock);

/**
 * refcount_dec_and_lock_irqsave - return holding spinlock with disabled
 *                                 interrupts if able to decrement refcount to 0
 * @r: the refcount
 * @lock: the spinlock to be locked
 * @flags: saved IRQ-flags if the is acquired
 *
 * Same as refcount_dec_and_lock() above except that the spinlock is acquired
 * with disabled interupts.
 *
 * Return: true and hold spinlock if able to decrement refcount to 0, false
 *         otherwise
 */
bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock,
				   unsigned long *flags)
{
	if (refcount_dec_not_one(r))
		return false;

	spin_lock_irqsave(lock, *flags);
	if (!refcount_dec_and_test(r)) {
		spin_unlock_irqrestore(lock, *flags);
		return false;
	}

	return true;
}
EXPORT_SYMBOL(refcount_dec_and_lock_irqsave);