shared_mutex
Go to the documentation of this file.
// <shared_mutex> -*- C++ -*-
// Copyright (C) 2013-2018 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
/**
* @ingroup mutexes
* @{
*/
#ifdef _GLIBCXX_USE_C99_STDINT_TR1
#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;
#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
/// 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 = pthread_rwlock_init(&_M_rwlock, NULL);
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__)) = pthread_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 = pthread_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 = pthread_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__)) = pthread_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 = pthread_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 = pthread_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
#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
#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
/// 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.
typedef chrono::system_clock __clock_t;
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>& __rel_time)
{
return try_lock_until(__clock_t::now() + __rel_time);
}
// 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>& __rel_time)
{
return try_lock_shared_until(__clock_t::now() + __rel_time);
}
#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
// Exclusive ownership
template<typename _Duration>
bool
try_lock_until(const chrono::time_point<__clock_t, _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_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;
}
template<typename _Clock, typename _Duration>
bool
try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
{
// DR 887 - Sync unknown clock to known clock.
const typename _Clock::time_point __c_entry = _Clock::now();
const __clock_t::time_point __s_entry = __clock_t::now();
const auto __delta = __abs_time - __c_entry;
const auto __s_atime = __s_entry + __delta;
return try_lock_until(__s_atime);
}
// Shared ownership
template<typename _Duration>
bool
try_lock_shared_until(const chrono::time_point<__clock_t,
_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 = pthread_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;
}
template<typename _Clock, typename _Duration>
bool
try_lock_shared_until(const chrono::time_point<_Clock,
_Duration>& __abs_time)
{
// DR 887 - Sync unknown clock to known clock.
const typename _Clock::time_point __c_entry = _Clock::now();
const __clock_t::time_point __s_entry = __clock_t::now();
const auto __delta = __abs_time - __c_entry;
const auto __s_atime = __s_entry + __delta;
return try_lock_shared_until(__s_atime);
}
#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
template<typename _Mutex>
void
swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) noexcept
{ __x.swap(__y); }
#endif // _GLIBCXX_USE_C99_STDINT_TR1
// @} group mutexes
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // C++14
#endif // _GLIBCXX_SHARED_MUTEX