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Implement LWG-2309 mutex::lock() should not throw device_or_resource_busy #3469

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merged 4 commits into from
Feb 23, 2023
Merged

Implement LWG-2309 mutex::lock() should not throw device_or_resource_busy #3469

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dd2fef5
Implement LWG-2309
frederick-vs-ja Feb 15, 2023
b2d678e
Code review feedback.
StephanTLavavej Feb 15, 2023
c1b114a
Failures of `try_lock` for recursive mutexes are well-defined
frederick-vs-ja Feb 15, 2023
fbfc328
Address @barcharcraz
frederick-vs-ja Feb 16, 2023
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31 changes: 28 additions & 3 deletions stl/inc/mutex
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Original file line number Diff line number Diff line change
Expand Up @@ -45,11 +45,30 @@ public:
_Mutex_base& operator=(const _Mutex_base&) = delete;

void lock() {
_Check_C_return(_Mtx_lock(_Mymtx()));
if (_Mtx_lock(_Mymtx()) != _Thrd_success) {
// undefined behavior, only occurs for plain mutexes (N4928 [thread.mutex.requirements.mutex.general]/6)
_STD _Throw_Cpp_error(_RESOURCE_DEADLOCK_WOULD_OCCUR);
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}
if (_Ownership_levels() == INT_MAX) {
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// only occurs for recursive mutexes (N4928 [thread.mutex.recursive]/3)
--_Ownership_levels();
// POSIX specifies EAGAIN in the corresponding situation:
// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html
_STD _Throw_Cpp_error(_RESOURCE_UNAVAILABLE_TRY_AGAIN);
}
}

_NODISCARD_TRY_CHANGE_STATE bool try_lock() noexcept /* strengthened */ {
return _Mtx_trylock(_Mymtx()) == _Thrd_success;
if (_Mtx_trylock(_Mymtx()) != _Thrd_success) {
// undefined behavior, only occurs for plain mutexes (N4928 [thread.mutex.requirements.mutex.general]/6)
return false;
}
if (_Ownership_levels() == INT_MAX) {
// only occurs for recursive mutexes (N4928 [thread.mutex.recursive]/3)
--_Ownership_levels();
return false;
}
return true;
}

void unlock() noexcept /* strengthened */ {
Expand All @@ -71,6 +90,10 @@ private:
_Mtx_t _Mymtx() noexcept { // get pointer to _Mtx_internal_imp_t inside _Mtx_storage
return reinterpret_cast<_Mtx_t>(&_Mtx_storage);
}

int& _Ownership_levels() noexcept { // the count is denoted by an int subobject at the end of _Mtx_storage
return *reinterpret_cast<int*>(reinterpret_cast<unsigned char*>(&_Mtx_storage + 1) - sizeof(int));
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}
};

_EXPORT_STD class mutex : public _Mutex_base { // class for mutual exclusion
Expand Down Expand Up @@ -852,7 +875,9 @@ public:
if (_My_locked < UINT_MAX) {
++_My_locked;
} else {
_Throw_system_error(errc::device_or_resource_busy);
// POSIX specifies EAGAIN in the corresponding situation:
// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html
_STD _Throw_system_error(errc::resource_unavailable_try_again);
}
} else {
while (_My_locked != 0) {
Expand Down