| // 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); |
| |