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Improve robustness of atomics

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And fix increment in `CowData` not being conditional anymore after the recent changes.

Co-authored-by: Hein-Pieter van Braam-Stewart <hp@tmm.cx>
This commit is contained in:
Pedro J. Estébanez 2021-02-18 23:31:30 +01:00
parent 247b7e2448
commit e3ec31f999
2 changed files with 30 additions and 12 deletions
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32
core/templates/cowdata.h
View file

@ -45,8 +45,9 @@ class CharString;
template <class T, class V>
class VMap;
// CowData is relying on this to be true
static_assert(sizeof(SafeNumeric<uint32_t>) == sizeof(uint32_t));
#if !defined(NO_THREADS)
SAFE_NUMERIC_TYPE_PUN_GUARANTEES(uint32_t)
#endif
template <class T>
class CowData {
@ -114,7 +115,7 @@ private:
void _unref(void *p_data);
void _ref(const CowData *p_from);
void _ref(const CowData &p_from);
void _copy_on_write();
uint32_t _copy_on_write();
public:
void operator=(const CowData<T> &p_from) { _ref(p_from); }
@ -217,20 +218,21 @@ void CowData<T>::_unref(void *p_data) {
}
template <class T>
void CowData<T>::_copy_on_write() {
uint32_t CowData<T>::_copy_on_write() {
if (!_ptr) {
return;
return 0;
}
SafeNumeric<uint32_t> *refc = _get_refcount();
if (unlikely(refc->get() > 1)) {
uint32_t rc = refc->get();
if (unlikely(rc > 1)) {
/* in use by more than me */
uint32_t current_size = *_get_size();
uint32_t *mem_new = (uint32_t *)Memory::alloc_static(_get_alloc_size(current_size), true);
reinterpret_cast<SafeNumeric<uint32_t> *>(mem_new - 2)->set(1); //refcount
new (mem_new - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(1); //refcount
*(mem_new - 1) = current_size; //size
T *_data = (T *)(mem_new);
@ -247,7 +249,10 @@ void CowData<T>::_copy_on_write() {
_unref(_ptr);
_ptr = _data;
rc = 1;
}
return rc;
}
template <class T>
@ -268,7 +273,7 @@ Error CowData<T>::resize(int p_size) {
}
// possibly changing size, copy on write
_copy_on_write();
uint32_t rc = _copy_on_write();
size_t current_alloc_size = _get_alloc_size(current_size);
size_t alloc_size;
@ -281,13 +286,15 @@ Error CowData<T>::resize(int p_size) {
uint32_t *ptr = (uint32_t *)Memory::alloc_static(alloc_size, true);
ERR_FAIL_COND_V(!ptr, ERR_OUT_OF_MEMORY);
*(ptr - 1) = 0; //size, currently none
reinterpret_cast<SafeNumeric<uint32_t> *>(ptr - 2)->set(1); //refcount
new (ptr - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(1); //refcount
_ptr = (T *)ptr;
} else {
void *_ptrnew = (T *)Memory::realloc_static(_ptr, alloc_size, true);
uint32_t *_ptrnew = (uint32_t *)Memory::realloc_static(_ptr, alloc_size, true);
ERR_FAIL_COND_V(!_ptrnew, ERR_OUT_OF_MEMORY);
new (_ptrnew - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(rc); //refcount
_ptr = (T *)(_ptrnew);
}
}
@ -314,8 +321,9 @@ Error CowData<T>::resize(int p_size) {
}
if (alloc_size != current_alloc_size) {
void *_ptrnew = (T *)Memory::realloc_static(_ptr, alloc_size, true);
uint32_t *_ptrnew = (uint32_t *)Memory::realloc_static(_ptr, alloc_size, true);
ERR_FAIL_COND_V(!_ptrnew, ERR_OUT_OF_MEMORY);
new (_ptrnew - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(rc); //refcount
_ptr = (T *)(_ptrnew);
}
@ -362,7 +370,7 @@ void CowData<T>::_ref(const CowData &p_from) {
return; //nothing to do
}
if (p_from._get_refcount()->increment() > 0) { // could reference
if (p_from._get_refcount()->conditional_increment() > 0) { // could reference
_ptr = p_from._ptr;
}
}

10
core/templates/safe_refcount.h
View file

@ -47,10 +47,18 @@
// value and, as an important benefit, you can be sure the value is properly synchronized
// even with threads that are already running.
// This is used in very specific areas of the engine where it's critical that these guarantees are held
#define SAFE_NUMERIC_TYPE_PUN_GUARANTEES(m_type) \
static_assert(sizeof(SafeNumeric<m_type>) == sizeof(m_type)); \
static_assert(alignof(SafeNumeric<m_type>) == alignof(m_type)); \
static_assert(std::is_trivially_destructible<std::atomic<m_type>>::value);
template <class T>
class SafeNumeric {
std::atomic<T> value;
static_assert(std::atomic<T>::is_always_lock_free);
public:
_ALWAYS_INLINE_ void set(T p_value) {
value.store(p_value, std::memory_order_release);
@ -128,6 +136,8 @@ public:
class SafeFlag {
std::atomic_bool flag;
static_assert(std::atomic_bool::is_always_lock_free);
public:
_ALWAYS_INLINE_ bool is_set() const {
return flag.load(std::memory_order_acquire);