<atomic>, <memory>, <execution>: make sure acquire and release are safe to use and start using them

[AlexGuteniev]

Currently memory_order_acquire and memory_order_release are considered unsafe:
The problem is critical sections overlap in the following situation with mutexes or other synch object:

T1: a.acquire(); a.release(); b.acquire(); b.release(), 
T2: b.acquire(); b.release(); a.acquire(); a.release();

Release reorders past subsequent unrelated acquire, so sections overlap and deadlock occurs.
The current resolution is believed to be the following:

1. Acquire should observe the release result in a finite time, so release operations cannot be reordered past infinite amount of acquire attempts
2. In hardware, memory changes take time to propagate, but relatively a very small time, definitely not infinite time
3. In software, the compiler either does not reorder operations at all, or does not reorder them past potentially infinite amount of other operations

Unfortunately, 1 is not what Standard currently says, and 2 and 3 has to be confirmed with compiler vendors

---

Before the status of acquire / release is clarified, currently seq_cst is used in some places, specifically:

1. atomic_shared_ptr internal spinlock:
   

   STL/stl/inc/memory

   Line 3130 in 12c684b

   if (!_Repptr.compare_exchange_weak(_Rep, (_Rep & _Ptr_value_mask) | _Locked_notify_needed)) {

   
   

   STL/stl/inc/memory

   Line 3150 in 12c684b

   uintptr_t _Rep = _Repptr.exchange(reinterpret_cast<uintptr_t>(_Value));

2. Non-lock-free atomic
   

   STL/stl/inc/atomic

   Lines 394 to 407 in 12c684b

   	inline void _Atomic_lock_spinlock(long& _Spinlock) noexcept {
   	while (_InterlockedExchange(&_Spinlock, 1)) {
   	_YIELD_PROCESSOR();
   	}
   	}
   	inline void _Atomic_unlock_spinlock(long& _Spinlock) noexcept {
   	#if defined(_M_ARM) || defined(_M_ARM64)
   	_Memory_barrier();
   	__iso_volatile_store32(reinterpret_cast<int*>(&_Spinlock), 0);
   	_Memory_barrier();
   	#else // ^^^ ARM32/ARM64 hardware / x86/x64 hardware vvv
   	_InterlockedExchange(&_Spinlock, 0);
   	#endif // hardware

3. Parallel algorithms in <execution> (more than just this occurrence):
   

   STL/stl/inc/execution

   Line 3624 in 12c684b

   _State.store(_New_state);

4. memory_resource.cpp
   

   STL/stl/src/memory_resource.cpp

   Line 24 in 12c684b

   memory_resource* const _Temp = __crt_interlocked_read_pointer(&_Default_resource);

   
   

   STL/stl/src/memory_resource.cpp

   Line 33 in 12c684b

   memory_resource* const _Temp = __crt_interlocked_read_pointer(&_Default_resource);

   
   

   STL/stl/src/memory_resource.cpp

   Line 43 in 12c684b

   memory_resource* const _Temp = __crt_interlocked_exchange_pointer(&_Default_resource, _Resource);

   
   

   STL/stl/src/memory_resource.cpp

   Line 53 in 12c684b

   memory_resource* const _Temp = __crt_interlocked_exchange_pointer(&_Default_resource, _Resource);

5. filesystem.cpp
   

   STL/stl/src/filesystem.cpp

   Lines 36 to 50 in 12c684b

   	auto _Result = __crt_interlocked_read_pointer(_Cache);
   	if (_Result) {
   	return _Result;
   	}
   	const HMODULE _HMod = GetModuleHandleW(_Module);
   	if (_HMod) {
   	_Result = reinterpret_cast<_Fn_ptr>(GetProcAddress(_HMod, _Fn_name));
   	}
   	if (!_Result) {
   	_Result = _Fallback;
   	}
   	__crt_interlocked_exchange_pointer(_Cache, _Result);

---

Some places believed to be not affected by the issue still use acquire / release, specifically:

1. shared_ptr external spinlock:
   

   STL/stl/src/atomic.cpp

   Lines 16 to 34 in 12c684b

   	_CRTIMP2_PURE void __cdecl _Lock_shared_ptr_spin_lock() { // spin until _Shared_ptr_flag successfully set
   	#ifdef _M_ARM
   	while (_InterlockedExchange_acq(&_Shared_ptr_flag, 1)) {
   	__yield();
   	}
   	#else // _M_ARM
   	while (_interlockedbittestandset(&_Shared_ptr_flag, 0)) { // set bit 0
   	}
   	#endif // _M_ARM
   	}
   	_CRTIMP2_PURE void __cdecl _Unlock_shared_ptr_spin_lock() { // release previously obtained lock
   	#ifdef _M_ARM
   	__dmb(_ARM_BARRIER_ISH);
   	__iso_volatile_store32(reinterpret_cast<volatile int*>(&_Shared_ptr_flag), 0);
   	#else // _M_ARM
   	_interlockedbittestandreset(&_Shared_ptr_flag, 0); // reset bit 0
   	#endif // _M_ARM
   	}

2. <system_error>
   

   STL/stl/inc/system_error

   Lines 590 to 597 in 12c684b

   	if (_Storage[0].load(memory_order_acquire) != 0) {
   	return reinterpret_cast<_Ty&>(_Storage);
   	}
   	const _Ty _Target;
   	const auto _Target_iter = reinterpret_cast<const uintptr_t*>(_STD addressof(_Target));
   	_CSTD memcpy(_Storage + 1, _Target_iter + 1, sizeof(_Ty) - sizeof(uintptr_t));
   	_Storage[0].store(_Target_iter[0], memory_order_release);

3. atomic_wait.cpp
   

   STL/stl/src/atomic_wait.cpp

   Lines 147 to 152 in 12c684b

   	_Wait_functions._Api_level.store(_Level, _STD memory_order_release);
   	return _Level;
   	}
   	[[nodiscard]] __std_atomic_api_level _Acquire_wait_functions() noexcept {
   	auto _Level = _Wait_functions._Api_level.load(_STD memory_order_acquire);

---

The task is to confirm the situation with compiler team and decide on using memory_order_acquire / memory_order_release in mentioned and possibly unmentioned preexisting code and new code

Note also that memory model implementation on arm may change in the future, see #83 , see also ##488 , #775 , #1082

[StephanTLavavej]

I would also want to make sure that any use of acq/rel doesn't lead to Independent Reads, Independent Writes problems (which full sequential consistency solves; some algorithms are affected by IRIW).

[BillyONeal]

giroux said he is working on the memory model implications in a paper for C++23 (if pandemic lets us have a c++23)