lib/refcount.c - kernel/common - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * 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>

 #ifdef CONFIG_REFCOUNT_FULL

 /**
  * 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 new, val = atomic_read(&r->refs);

 	do {
 		if (!val)
 			return false;

 		if (unlikely(val == UINT_MAX))
 			return true;

 		new = val + i;
 		if (new < val)
 			new = UINT_MAX;

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

 	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

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

 	do {
 		new = val + 1;

 		if (!val)
 			return false;

 		if (unlikely(!new))
 			return true;

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

 	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

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

 	do {
 		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;
 		}

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

 	return !new;
 }
 EXPORT_SYMBOL(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(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(refcount_dec);
 #endif /* CONFIG_REFCOUNT_FULL */

 /**
  * 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 == 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;
 		}

 	} 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 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(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(refcount_dec_and_lock);
	// SPDX-License-Identifier: GPL-2.0
	/*
	* 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>

	#ifdef CONFIG_REFCOUNT_FULL

	/**
	* 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 new, val = atomic_read(&r->refs);

	do {
	if (!val)
	return false;

	if (unlikely(val == UINT_MAX))
	return true;

	new = val + i;
	if (new < val)
	new = UINT_MAX;

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

	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

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

	do {
	new = val + 1;

	if (!val)
	return false;

	if (unlikely(!new))
	return true;

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

	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

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

	do {
	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;
	}

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

	return !new;
	}
	EXPORT_SYMBOL(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(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(refcount_dec);
	#endif /* CONFIG_REFCOUNT_FULL */

	/**
	* 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 == 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;
	}

	} 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 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(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(refcount_dec_and_lock);