// 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.
  ");

  
  	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.
  ");

  }

  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.
  ");

  
  	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.
  ");

  }

  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.
  ");

  			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.
  ");

  }

  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.
  ");

  			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);