blob: 1d33366189d10c88bf10616e6a9b25c64c4bf570 [file] [log] [blame]
/*
* 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>
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(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
return true;
}
EXPORT_SYMBOL_GPL(refcount_add_not_zero);
void refcount_add(unsigned int i, refcount_t *r)
{
WARN(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
}
EXPORT_SYMBOL_GPL(refcount_add);
/*
* Similar to atomic_inc_not_zero(), 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.
*/
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(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
return true;
}
EXPORT_SYMBOL_GPL(refcount_inc_not_zero);
/*
* Similar to atomic_inc(), 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 when this is not so.
*/
void refcount_inc(refcount_t *r)
{
WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
}
EXPORT_SYMBOL_GPL(refcount_inc);
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(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);
/*
* 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.
*/
bool refcount_dec_and_test(refcount_t *r)
{
return refcount_sub_and_test(1, r);
}
EXPORT_SYMBOL_GPL(refcount_dec_and_test);
/*
* 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(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
}
EXPORT_SYMBOL_GPL(refcount_dec);
/*
* 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.
*/
bool refcount_dec_if_one(refcount_t *r)
{
return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
}
EXPORT_SYMBOL_GPL(refcount_dec_if_one);
/*
* 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)
*/
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(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);
/*
* 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.
*/
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);
/*
* 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.
*/
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);