// <shared_mutex> -*- C++ -*- // Copyright (C) 2013-2020 Free Software Foundation, Inc. // // This file is part of the GNU ISO C++ Library. This library is free // software; you can redistribute it and/or modify it under the // terms of the GNU General Public License as published by the // Free Software Foundation; either version 3, or (at your option) // any later version. // This library is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU General Public License for more details. // Under Section 7 of GPL version 3, you are granted additional // permissions described in the GCC Runtime Library Exception, version // 3.1, as published by the Free Software Foundation. // You should have received a copy of the GNU General Public License and // a copy of the GCC Runtime Library Exception along with this program; // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see // <http://www.gnu.org/licenses/>. /** @file include/shared_mutex * This is a Standard C++ Library header. */ #ifndef _GLIBCXX_SHARED_MUTEX #define _GLIBCXX_SHARED_MUTEX 1 #pragma GCC system_header #if __cplusplus >= 201402L #include <bits/c++config.h> #include <condition_variable> #include <bits/functexcept.h> namespace std _GLIBCXX_VISIBILITY(default) { _GLIBCXX_BEGIN_NAMESPACE_VERSION /** * @addtogroup mutexes * @{ */ #ifdef _GLIBCXX_HAS_GTHREADS #if __cplusplus >= 201703L #define __cpp_lib_shared_mutex 201505 class shared_mutex; #endif #define __cpp_lib_shared_timed_mutex 201402 class shared_timed_mutex; /// @cond undocumented #if _GLIBCXX_USE_PTHREAD_RWLOCK_T #ifdef __gthrw #define _GLIBCXX_GTHRW(name) \ __gthrw(pthread_ ## name); \ static inline int \ __glibcxx_ ## name (pthread_rwlock_t *__rwlock) \ { \ if (__gthread_active_p ()) \ return __gthrw_(pthread_ ## name) (__rwlock); \ else \ return 0; \ } _GLIBCXX_GTHRW(rwlock_rdlock) _GLIBCXX_GTHRW(rwlock_tryrdlock) _GLIBCXX_GTHRW(rwlock_wrlock) _GLIBCXX_GTHRW(rwlock_trywrlock) _GLIBCXX_GTHRW(rwlock_unlock) # ifndef PTHREAD_RWLOCK_INITIALIZER _GLIBCXX_GTHRW(rwlock_destroy) __gthrw(pthread_rwlock_init); static inline int __glibcxx_rwlock_init (pthread_rwlock_t *__rwlock) { if (__gthread_active_p ()) return __gthrw_(pthread_rwlock_init) (__rwlock, NULL); else return 0; } # endif # if _GTHREAD_USE_MUTEX_TIMEDLOCK __gthrw(pthread_rwlock_timedrdlock); static inline int __glibcxx_rwlock_timedrdlock (pthread_rwlock_t *__rwlock, const timespec *__ts) { if (__gthread_active_p ()) return __gthrw_(pthread_rwlock_timedrdlock) (__rwlock, __ts); else return 0; } __gthrw(pthread_rwlock_timedwrlock); static inline int __glibcxx_rwlock_timedwrlock (pthread_rwlock_t *__rwlock, const timespec *__ts) { if (__gthread_active_p ()) return __gthrw_(pthread_rwlock_timedwrlock) (__rwlock, __ts); else return 0; } # endif #else static inline int __glibcxx_rwlock_rdlock (pthread_rwlock_t *__rwlock) { return pthread_rwlock_rdlock (__rwlock); } static inline int __glibcxx_rwlock_tryrdlock (pthread_rwlock_t *__rwlock) { return pthread_rwlock_tryrdlock (__rwlock); } static inline int __glibcxx_rwlock_wrlock (pthread_rwlock_t *__rwlock) { return pthread_rwlock_wrlock (__rwlock); } static inline int __glibcxx_rwlock_trywrlock (pthread_rwlock_t *__rwlock) { return pthread_rwlock_trywrlock (__rwlock); } static inline int __glibcxx_rwlock_unlock (pthread_rwlock_t *__rwlock) { return pthread_rwlock_unlock (__rwlock); } static inline int __glibcxx_rwlock_destroy(pthread_rwlock_t *__rwlock) { return pthread_rwlock_destroy (__rwlock); } static inline int __glibcxx_rwlock_init(pthread_rwlock_t *__rwlock) { return pthread_rwlock_init (__rwlock, NULL); } # if _GTHREAD_USE_MUTEX_TIMEDLOCK static inline int __glibcxx_rwlock_timedrdlock (pthread_rwlock_t *__rwlock, const timespec *__ts) { return pthread_rwlock_timedrdlock (__rwlock, __ts); } static inline int __glibcxx_rwlock_timedwrlock (pthread_rwlock_t *__rwlock, const timespec *__ts) { return pthread_rwlock_timedwrlock (__rwlock, __ts); } # endif #endif /// A shared mutex type implemented using pthread_rwlock_t. class __shared_mutex_pthread { friend class shared_timed_mutex; #ifdef PTHREAD_RWLOCK_INITIALIZER pthread_rwlock_t _M_rwlock = PTHREAD_RWLOCK_INITIALIZER; public: __shared_mutex_pthread() = default; ~__shared_mutex_pthread() = default; #else pthread_rwlock_t _M_rwlock; public: __shared_mutex_pthread() { int __ret = __glibcxx_rwlock_init(&_M_rwlock); if (__ret == ENOMEM) __throw_bad_alloc(); else if (__ret == EAGAIN) __throw_system_error(int(errc::resource_unavailable_try_again)); else if (__ret == EPERM) __throw_system_error(int(errc::operation_not_permitted)); // Errors not handled: EBUSY, EINVAL __glibcxx_assert(__ret == 0); } ~__shared_mutex_pthread() { int __ret __attribute((__unused__)) = __glibcxx_rwlock_destroy(&_M_rwlock); // Errors not handled: EBUSY, EINVAL __glibcxx_assert(__ret == 0); } #endif __shared_mutex_pthread(const __shared_mutex_pthread&) = delete; __shared_mutex_pthread& operator=(const __shared_mutex_pthread&) = delete; void lock() { int __ret = __glibcxx_rwlock_wrlock(&_M_rwlock); if (__ret == EDEADLK) __throw_system_error(int(errc::resource_deadlock_would_occur)); // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); } bool try_lock() { int __ret = __glibcxx_rwlock_trywrlock(&_M_rwlock); if (__ret == EBUSY) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } void unlock() { int __ret __attribute((__unused__)) = __glibcxx_rwlock_unlock(&_M_rwlock); // Errors not handled: EPERM, EBUSY, EINVAL __glibcxx_assert(__ret == 0); } // Shared ownership void lock_shared() { int __ret; // We retry if we exceeded the maximum number of read locks supported by // the POSIX implementation; this can result in busy-waiting, but this // is okay based on the current specification of forward progress // guarantees by the standard. do __ret = __glibcxx_rwlock_rdlock(&_M_rwlock); while (__ret == EAGAIN); if (__ret == EDEADLK) __throw_system_error(int(errc::resource_deadlock_would_occur)); // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); } bool try_lock_shared() { int __ret = __glibcxx_rwlock_tryrdlock(&_M_rwlock); // If the maximum number of read locks has been exceeded, we just fail // to acquire the lock. Unlike for lock(), we are not allowed to throw // an exception. if (__ret == EBUSY || __ret == EAGAIN) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } void unlock_shared() { unlock(); } void* native_handle() { return &_M_rwlock; } }; #endif #if ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK) /// A shared mutex type implemented using std::condition_variable. class __shared_mutex_cv { friend class shared_timed_mutex; // Based on Howard Hinnant's reference implementation from N2406. // The high bit of _M_state is the write-entered flag which is set to // indicate a writer has taken the lock or is queuing to take the lock. // The remaining bits are the count of reader locks. // // To take a reader lock, block on gate1 while the write-entered flag is // set or the maximum number of reader locks is held, then increment the // reader lock count. // To release, decrement the count, then if the write-entered flag is set // and the count is zero then signal gate2 to wake a queued writer, // otherwise if the maximum number of reader locks was held signal gate1 // to wake a reader. // // To take a writer lock, block on gate1 while the write-entered flag is // set, then set the write-entered flag to start queueing, then block on // gate2 while the number of reader locks is non-zero. // To release, unset the write-entered flag and signal gate1 to wake all // blocked readers and writers. // // This means that when no reader locks are held readers and writers get // equal priority. When one or more reader locks is held a writer gets // priority and no more reader locks can be taken while the writer is // queued. // Only locked when accessing _M_state or waiting on condition variables. mutex _M_mut; // Used to block while write-entered is set or reader count at maximum. condition_variable _M_gate1; // Used to block queued writers while reader count is non-zero. condition_variable _M_gate2; // The write-entered flag and reader count. unsigned _M_state; static constexpr unsigned _S_write_entered = 1U << (sizeof(unsigned)*__CHAR_BIT__ - 1); static constexpr unsigned _S_max_readers = ~_S_write_entered; // Test whether the write-entered flag is set. _M_mut must be locked. bool _M_write_entered() const { return _M_state & _S_write_entered; } // The number of reader locks currently held. _M_mut must be locked. unsigned _M_readers() const { return _M_state & _S_max_readers; } public: __shared_mutex_cv() : _M_state(0) {} ~__shared_mutex_cv() { __glibcxx_assert( _M_state == 0 ); } __shared_mutex_cv(const __shared_mutex_cv&) = delete; __shared_mutex_cv& operator=(const __shared_mutex_cv&) = delete; // Exclusive ownership void lock() { unique_lock<mutex> __lk(_M_mut); // Wait until we can set the write-entered flag. _M_gate1.wait(__lk, [=]{ return !_M_write_entered(); }); _M_state |= _S_write_entered; // Then wait until there are no more readers. _M_gate2.wait(__lk, [=]{ return _M_readers() == 0; }); } bool try_lock() { unique_lock<mutex> __lk(_M_mut, try_to_lock); if (__lk.owns_lock() && _M_state == 0) { _M_state = _S_write_entered; return true; } return false; } void unlock() { lock_guard<mutex> __lk(_M_mut); __glibcxx_assert( _M_write_entered() ); _M_state = 0; // call notify_all() while mutex is held so that another thread can't // lock and unlock the mutex then destroy *this before we make the call. _M_gate1.notify_all(); } // Shared ownership void lock_shared() { unique_lock<mutex> __lk(_M_mut); _M_gate1.wait(__lk, [=]{ return _M_state < _S_max_readers; }); ++_M_state; } bool try_lock_shared() { unique_lock<mutex> __lk(_M_mut, try_to_lock); if (!__lk.owns_lock()) return false; if (_M_state < _S_max_readers) { ++_M_state; return true; } return false; } void unlock_shared() { lock_guard<mutex> __lk(_M_mut); __glibcxx_assert( _M_readers() > 0 ); auto __prev = _M_state--; if (_M_write_entered()) { // Wake the queued writer if there are no more readers. if (_M_readers() == 0) _M_gate2.notify_one(); // No need to notify gate1 because we give priority to the queued // writer, and that writer will eventually notify gate1 after it // clears the write-entered flag. } else { // Wake any thread that was blocked on reader overflow. if (__prev == _S_max_readers) _M_gate1.notify_one(); } } }; #endif /// @endcond #if __cplusplus > 201402L /// The standard shared mutex type. class shared_mutex { public: shared_mutex() = default; ~shared_mutex() = default; shared_mutex(const shared_mutex&) = delete; shared_mutex& operator=(const shared_mutex&) = delete; // Exclusive ownership void lock() { _M_impl.lock(); } bool try_lock() { return _M_impl.try_lock(); } void unlock() { _M_impl.unlock(); } // Shared ownership void lock_shared() { _M_impl.lock_shared(); } bool try_lock_shared() { return _M_impl.try_lock_shared(); } void unlock_shared() { _M_impl.unlock_shared(); } #if _GLIBCXX_USE_PTHREAD_RWLOCK_T typedef void* native_handle_type; native_handle_type native_handle() { return _M_impl.native_handle(); } private: __shared_mutex_pthread _M_impl; #else private: __shared_mutex_cv _M_impl; #endif }; #endif // C++17 /// @cond undocumented #if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK using __shared_timed_mutex_base = __shared_mutex_pthread; #else using __shared_timed_mutex_base = __shared_mutex_cv; #endif /// @endcond /// The standard shared timed mutex type. class shared_timed_mutex : private __shared_timed_mutex_base { using _Base = __shared_timed_mutex_base; // Must use the same clock as condition_variable for __shared_mutex_cv. #ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK using __clock_t = chrono::steady_clock; #else using __clock_t = chrono::system_clock; #endif public: shared_timed_mutex() = default; ~shared_timed_mutex() = default; shared_timed_mutex(const shared_timed_mutex&) = delete; shared_timed_mutex& operator=(const shared_timed_mutex&) = delete; // Exclusive ownership void lock() { _Base::lock(); } bool try_lock() { return _Base::try_lock(); } void unlock() { _Base::unlock(); } template<typename _Rep, typename _Period> bool try_lock_for(const chrono::duration<_Rep, _Period>& __rtime) { auto __rt = chrono::duration_cast<__clock_t::duration>(__rtime); if (ratio_greater<__clock_t::period, _Period>()) ++__rt; return try_lock_until(__clock_t::now() + __rt); } // Shared ownership void lock_shared() { _Base::lock_shared(); } bool try_lock_shared() { return _Base::try_lock_shared(); } void unlock_shared() { _Base::unlock_shared(); } template<typename _Rep, typename _Period> bool try_lock_shared_for(const chrono::duration<_Rep, _Period>& __rtime) { auto __rt = chrono::duration_cast<__clock_t::duration>(__rtime); if (ratio_greater<__clock_t::period, _Period>()) ++__rt; return try_lock_shared_until(__clock_t::now() + __rt); } #if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK // Exclusive ownership template<typename _Duration> bool try_lock_until(const chrono::time_point<chrono::system_clock, _Duration>& __atime) { auto __s = chrono::time_point_cast<chrono::seconds>(__atime); auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s); __gthread_time_t __ts = { static_cast<std::time_t>(__s.time_since_epoch().count()), static_cast<long>(__ns.count()) }; int __ret = __glibcxx_rwlock_timedwrlock(&_M_rwlock, &__ts); // On self-deadlock, we just fail to acquire the lock. Technically, // the program violated the precondition. if (__ret == ETIMEDOUT || __ret == EDEADLK) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } #ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK template<typename _Duration> bool try_lock_until(const chrono::time_point<chrono::steady_clock, _Duration>& __atime) { auto __s = chrono::time_point_cast<chrono::seconds>(__atime); auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s); __gthread_time_t __ts = { static_cast<std::time_t>(__s.time_since_epoch().count()), static_cast<long>(__ns.count()) }; int __ret = pthread_rwlock_clockwrlock(&_M_rwlock, CLOCK_MONOTONIC, &__ts); // On self-deadlock, we just fail to acquire the lock. Technically, // the program violated the precondition. if (__ret == ETIMEDOUT || __ret == EDEADLK) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } #endif template<typename _Clock, typename _Duration> bool try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime) { #if __cplusplus > 201703L static_assert(chrono::is_clock_v<_Clock>); #endif // The user-supplied clock may not tick at the same rate as // steady_clock, so we must loop in order to guarantee that // the timeout has expired before returning false. typename _Clock::time_point __now = _Clock::now(); do { auto __rtime = __atime - __now; if (try_lock_for(__rtime)) return true; __now = _Clock::now(); } while (__atime > __now); return false; } // Shared ownership template<typename _Duration> bool try_lock_shared_until(const chrono::time_point<chrono::system_clock, _Duration>& __atime) { auto __s = chrono::time_point_cast<chrono::seconds>(__atime); auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s); __gthread_time_t __ts = { static_cast<std::time_t>(__s.time_since_epoch().count()), static_cast<long>(__ns.count()) }; int __ret; // Unlike for lock(), we are not allowed to throw an exception so if // the maximum number of read locks has been exceeded, or we would // deadlock, we just try to acquire the lock again (and will time out // eventually). // In cases where we would exceed the maximum number of read locks // throughout the whole time until the timeout, we will fail to // acquire the lock even if it would be logically free; however, this // is allowed by the standard, and we made a "strong effort" // (see C++14 30.4.1.4p26). // For cases where the implementation detects a deadlock we // intentionally block and timeout so that an early return isn't // mistaken for a spurious failure, which might help users realise // there is a deadlock. do __ret = __glibcxx_rwlock_timedrdlock(&_M_rwlock, &__ts); while (__ret == EAGAIN || __ret == EDEADLK); if (__ret == ETIMEDOUT) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } #ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK template<typename _Duration> bool try_lock_shared_until(const chrono::time_point<chrono::steady_clock, _Duration>& __atime) { auto __s = chrono::time_point_cast<chrono::seconds>(__atime); auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s); __gthread_time_t __ts = { static_cast<std::time_t>(__s.time_since_epoch().count()), static_cast<long>(__ns.count()) }; int __ret = pthread_rwlock_clockrdlock(&_M_rwlock, CLOCK_MONOTONIC, &__ts); // On self-deadlock, we just fail to acquire the lock. Technically, // the program violated the precondition. if (__ret == ETIMEDOUT || __ret == EDEADLK) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } #endif template<typename _Clock, typename _Duration> bool try_lock_shared_until(const chrono::time_point<_Clock, _Duration>& __atime) { #if __cplusplus > 201703L static_assert(chrono::is_clock_v<_Clock>); #endif // The user-supplied clock may not tick at the same rate as // steady_clock, so we must loop in order to guarantee that // the timeout has expired before returning false. typename _Clock::time_point __now = _Clock::now(); do { auto __rtime = __atime - __now; if (try_lock_shared_for(__rtime)) return true; __now = _Clock::now(); } while (__atime > __now); return false; } #else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK) // Exclusive ownership template<typename _Clock, typename _Duration> bool try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { unique_lock<mutex> __lk(_M_mut); if (!_M_gate1.wait_until(__lk, __abs_time, [=]{ return !_M_write_entered(); })) { return false; } _M_state |= _S_write_entered; if (!_M_gate2.wait_until(__lk, __abs_time, [=]{ return _M_readers() == 0; })) { _M_state ^= _S_write_entered; // Wake all threads blocked while the write-entered flag was set. _M_gate1.notify_all(); return false; } return true; } // Shared ownership template <typename _Clock, typename _Duration> bool try_lock_shared_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { unique_lock<mutex> __lk(_M_mut); if (!_M_gate1.wait_until(__lk, __abs_time, [=]{ return _M_state < _S_max_readers; })) { return false; } ++_M_state; return true; } #endif // _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK }; #endif // _GLIBCXX_HAS_GTHREADS /// shared_lock template<typename _Mutex> class shared_lock { public: typedef _Mutex mutex_type; // Shared locking shared_lock() noexcept : _M_pm(nullptr), _M_owns(false) { } explicit shared_lock(mutex_type& __m) : _M_pm(std::__addressof(__m)), _M_owns(true) { __m.lock_shared(); } shared_lock(mutex_type& __m, defer_lock_t) noexcept : _M_pm(std::__addressof(__m)), _M_owns(false) { } shared_lock(mutex_type& __m, try_to_lock_t) : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared()) { } shared_lock(mutex_type& __m, adopt_lock_t) : _M_pm(std::__addressof(__m)), _M_owns(true) { } template<typename _Clock, typename _Duration> shared_lock(mutex_type& __m, const chrono::time_point<_Clock, _Duration>& __abs_time) : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared_until(__abs_time)) { } template<typename _Rep, typename _Period> shared_lock(mutex_type& __m, const chrono::duration<_Rep, _Period>& __rel_time) : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared_for(__rel_time)) { } ~shared_lock() { if (_M_owns) _M_pm->unlock_shared(); } shared_lock(shared_lock const&) = delete; shared_lock& operator=(shared_lock const&) = delete; shared_lock(shared_lock&& __sl) noexcept : shared_lock() { swap(__sl); } shared_lock& operator=(shared_lock&& __sl) noexcept { shared_lock(std::move(__sl)).swap(*this); return *this; } void lock() { _M_lockable(); _M_pm->lock_shared(); _M_owns = true; } bool try_lock() { _M_lockable(); return _M_owns = _M_pm->try_lock_shared(); } template<typename _Rep, typename _Period> bool try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time) { _M_lockable(); return _M_owns = _M_pm->try_lock_shared_for(__rel_time); } template<typename _Clock, typename _Duration> bool try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { _M_lockable(); return _M_owns = _M_pm->try_lock_shared_until(__abs_time); } void unlock() { if (!_M_owns) __throw_system_error(int(errc::resource_deadlock_would_occur)); _M_pm->unlock_shared(); _M_owns = false; } // Setters void swap(shared_lock& __u) noexcept { std::swap(_M_pm, __u._M_pm); std::swap(_M_owns, __u._M_owns); } mutex_type* release() noexcept { _M_owns = false; return std::exchange(_M_pm, nullptr); } // Getters bool owns_lock() const noexcept { return _M_owns; } explicit operator bool() const noexcept { return _M_owns; } mutex_type* mutex() const noexcept { return _M_pm; } private: void _M_lockable() const { if (_M_pm == nullptr) __throw_system_error(int(errc::operation_not_permitted)); if (_M_owns) __throw_system_error(int(errc::resource_deadlock_would_occur)); } mutex_type* _M_pm; bool _M_owns; }; /// Swap specialization for shared_lock /// @relates shared_mutex template<typename _Mutex> void swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) noexcept { __x.swap(__y); } // @} group mutexes _GLIBCXX_END_NAMESPACE_VERSION } // namespace #endif // C++14 #endif // _GLIBCXX_SHARED_MUTEX