atomic_futex.h

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// -*- C++ -*- header.
 
// Copyright (C) 2015-2025 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 bits/atomic_futex.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly.
 */
 
#ifndef _GLIBCXX_ATOMIC_FUTEX_H
#define _GLIBCXX_ATOMIC_FUTEX_H 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <atomic>
#if ! (defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1)
#include <mutex>
#include <condition_variable>
#endif
#include <bits/chrono.h>
 
#ifndef _GLIBCXX_ALWAYS_INLINE
#define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
#ifdef _GLIBCXX_HAS_GTHREADS
#if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
  struct __atomic_futex_unsigned_base
  {
    // __s and __ns are measured against CLOCK_REALTIME. Returns false
    // iff a timeout occurred.
    bool
    _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
        chrono::seconds __s, chrono::nanoseconds __ns);
 
    // __s and __ns are measured against CLOCK_MONOTONIC. Returns
    // false iff a timeout occurred.
    bool
    _M_futex_wait_until_steady(unsigned *__addr, unsigned __val,
        bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns);
 
    // This can be executed after the object has been destroyed.
    static void _M_futex_notify_all(unsigned* __addr);
  };
 
  template <unsigned _Waiter_bit = 0x80000000>
  class __atomic_futex_unsigned : __atomic_futex_unsigned_base
  {
    typedef chrono::steady_clock __clock_t;
 
    // This must be lock-free and at offset 0.
    atomic<unsigned> _M_data;
 
  public:
    explicit
    __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
    { }
 
    _GLIBCXX_ALWAYS_INLINE unsigned
    _M_load(memory_order __mo)
    {
      return _M_data.load(__mo) & ~_Waiter_bit;
    }
 
  private:
    // If a timeout occurs, returns a current value after the timeout;
    // otherwise, returns the operand's value if equal is true or a different
    // value if equal is false.
    // The assumed value is the caller's assumption about the current value
    // when making the call.
    // __s and __ns are measured against CLOCK_REALTIME.
    unsigned
    _M_load_and_test_until(unsigned __assumed, unsigned __operand,
        bool __equal, memory_order __mo, bool __has_timeout,
        chrono::seconds __s, chrono::nanoseconds __ns)
    {
      for (;;)
        {
          // Don't bother checking the value again because we expect the caller
          // to have done it recently.
          // memory_order_relaxed is sufficient because we can rely on just the
          // modification order (store_notify uses an atomic RMW operation too),
          // and the futex syscalls synchronize between themselves.
          _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
          bool __ret = _M_futex_wait_until((unsigned*)(void*)&_M_data,
                                           __assumed | _Waiter_bit,
                                           __has_timeout, __s, __ns);
          // Fetch the current value after waiting (clears _Waiter_bit).
          __assumed = _M_load(__mo);
          if (!__ret || ((__operand == __assumed) == __equal))
            return __assumed;
          // TODO adapt wait time
        }
    }
 
    // If a timeout occurs, returns a current value after the timeout;
    // otherwise, returns the operand's value if equal is true or a different
    // value if equal is false.
    // The assumed value is the caller's assumption about the current value
    // when making the call.
    // __s and __ns are measured against CLOCK_MONOTONIC.
    unsigned
    _M_load_and_test_until_steady(unsigned __assumed, unsigned __operand,
        bool __equal, memory_order __mo, bool __has_timeout,
        chrono::seconds __s, chrono::nanoseconds __ns)
    {
      for (;;)
        {
          // Don't bother checking the value again because we expect the caller
          // to have done it recently.
          // memory_order_relaxed is sufficient because we can rely on just the
          // modification order (store_notify uses an atomic RMW operation too),
          // and the futex syscalls synchronize between themselves.
          _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
          bool __ret = _M_futex_wait_until_steady((unsigned*)(void*)&_M_data,
                                           __assumed | _Waiter_bit,
                                           __has_timeout, __s, __ns);
          // Fetch the current value after waiting (clears _Waiter_bit).
          __assumed = _M_load(__mo);
          if (!__ret || ((__operand == __assumed) == __equal))
            return __assumed;
          // TODO adapt wait time
        }
    }
 
    // Returns the operand's value if equal is true or a different value if
    // equal is false.
    // The assumed value is the caller's assumption about the current value
    // when making the call.
    unsigned
    _M_load_and_test(unsigned __assumed, unsigned __operand,
        bool __equal, memory_order __mo)
    {
      return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
                                    false, {}, {});
    }
 
    // If a timeout occurs, returns a current value after the timeout;
    // otherwise, returns the operand's value if equal is true or a different
    // value if equal is false.
    // The assumed value is the caller's assumption about the current value
    // when making the call.
    template<typename _Dur>
    unsigned
    _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
        bool __equal, memory_order __mo,
        const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
    {
      auto __d = __atime.time_since_epoch();
      if (__d < __d.zero()) [[__unlikely__]]
        return false;
      auto __s = chrono::duration_cast<chrono::seconds>(__d);
      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__d - __s);
      return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
                                    true, __s, __ns);
    }
 
    template<typename _Dur>
    unsigned
    _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
        bool __equal, memory_order __mo,
        const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
    {
      auto __d = __atime.time_since_epoch();
      if (__d < __d.zero()) [[__unlikely__]]
        return false;
      auto __s = chrono::duration_cast<chrono::seconds>(__d);
      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__d - __s);
      return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
          true, __s, __ns);
    }
 
  public:
 
    _GLIBCXX_ALWAYS_INLINE unsigned
    _M_load_when_not_equal(unsigned __val, memory_order __mo)
    {
      unsigned __i = _M_load(__mo);
      if ((__i & ~_Waiter_bit) != __val)
        return (__i & ~_Waiter_bit);
      // TODO Spin-wait first.
      return _M_load_and_test(__i, __val, false, __mo);
    }
 
    _GLIBCXX_ALWAYS_INLINE void
    _M_load_when_equal(unsigned __val, memory_order __mo)
    {
      unsigned __i = _M_load(__mo);
      if ((__i & ~_Waiter_bit) == __val)
        return;
      // TODO Spin-wait first.
      _M_load_and_test(__i, __val, true, __mo);
    }
 
    // Returns false iff a timeout occurred.
    template<typename _Rep, typename _Period>
      _GLIBCXX_ALWAYS_INLINE bool
      _M_load_when_equal_for(unsigned __val, memory_order __mo,
          const chrono::duration<_Rep, _Period>& __rtime)
      {
        using __dur = typename __clock_t::duration;
        return _M_load_when_equal_until(__val, __mo,
                    __clock_t::now() + chrono::__detail::ceil<__dur>(__rtime));
      }
 
    // Returns false iff a timeout occurred.
    template<typename _Clock, typename _Duration>
      _GLIBCXX_ALWAYS_INLINE bool
      _M_load_when_equal_until(unsigned __val, memory_order __mo,
          const chrono::time_point<_Clock, _Duration>& __atime)
      {
        typename _Clock::time_point __c_entry = _Clock::now();
        do {
          const __clock_t::time_point __s_entry = __clock_t::now();
          const auto __delta = __atime - __c_entry;
          const auto __s_atime = __s_entry +
              chrono::__detail::ceil<__clock_t::duration>(__delta);
          if (_M_load_when_equal_until(__val, __mo, __s_atime))
            return true;
          __c_entry = _Clock::now();
        } while (__c_entry < __atime);
        return false;
      }
 
    // Returns false iff a timeout occurred.
    template<typename _Duration>
    _GLIBCXX_ALWAYS_INLINE bool
    _M_load_when_equal_until(unsigned __val, memory_order __mo,
        const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
    {
      unsigned __i = _M_load(__mo);
      if ((__i & ~_Waiter_bit) == __val)
        return true;
      // TODO Spin-wait first.  Ignore effect on timeout.
      __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
      return (__i & ~_Waiter_bit) == __val;
    }
 
    // Returns false iff a timeout occurred.
    template<typename _Duration>
    _GLIBCXX_ALWAYS_INLINE bool
    _M_load_when_equal_until(unsigned __val, memory_order __mo,
        const chrono::time_point<std::chrono::steady_clock, _Duration>& __atime)
    {
      unsigned __i = _M_load(__mo);
      if ((__i & ~_Waiter_bit) == __val)
        return true;
      // TODO Spin-wait first.  Ignore effect on timeout.
      __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
      return (__i & ~_Waiter_bit) == __val;
    }
 
    _GLIBCXX_ALWAYS_INLINE void
    _M_store_notify_all(unsigned __val, memory_order __mo)
    {
      unsigned* __futex = (unsigned *)(void *)&_M_data;
      if (_M_data.exchange(__val, __mo) & _Waiter_bit)
        _M_futex_notify_all(__futex);
    }
  };
 
#else // ! (_GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1)
 
  // If futexes are not available, use a mutex and a condvar to wait.
  // Because we access the data only within critical sections, all accesses
  // are sequentially consistent; thus, we satisfy any provided memory_order.
  template <unsigned _Waiter_bit = 0x80000000>
  class __atomic_futex_unsigned
  {
    typedef chrono::system_clock __clock_t;
 
    unsigned _M_data;
    mutex _M_mutex;
    condition_variable _M_condvar;
 
  public:
    explicit
    __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
    { }
 
    _GLIBCXX_ALWAYS_INLINE unsigned
    _M_load(memory_order)
    {
      unique_lock<mutex> __lock(_M_mutex);
      return _M_data;
    }
 
    _GLIBCXX_ALWAYS_INLINE unsigned
    _M_load_when_not_equal(unsigned __val, memory_order)
    {
      unique_lock<mutex> __lock(_M_mutex);
      while (_M_data == __val)
        _M_condvar.wait(__lock);
      return _M_data;
    }
 
    _GLIBCXX_ALWAYS_INLINE void
    _M_load_when_equal(unsigned __val, memory_order)
    {
      unique_lock<mutex> __lock(_M_mutex);
      while (_M_data != __val)
        _M_condvar.wait(__lock);
    }
 
    template<typename _Rep, typename _Period>
      _GLIBCXX_ALWAYS_INLINE bool
      _M_load_when_equal_for(unsigned __val, memory_order,
          const chrono::duration<_Rep, _Period>& __rtime)
      {
        unique_lock<mutex> __lock(_M_mutex);
        return _M_condvar.wait_for(__lock, __rtime,
                                   [&] { return _M_data == __val;});
      }
 
    template<typename _Clock, typename _Duration>
      _GLIBCXX_ALWAYS_INLINE bool
      _M_load_when_equal_until(unsigned __val, memory_order,
          const chrono::time_point<_Clock, _Duration>& __atime)
      {
        unique_lock<mutex> __lock(_M_mutex);
        return _M_condvar.wait_until(__lock, __atime,
                                     [&] { return _M_data == __val;});
      }
 
    _GLIBCXX_ALWAYS_INLINE void
    _M_store_notify_all(unsigned __val, memory_order)
    {
      unique_lock<mutex> __lock(_M_mutex);
      _M_data = __val;
      _M_condvar.notify_all();
    }
  };
 
#endif // _GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1
#endif // _GLIBCXX_HAS_GTHREADS
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif