atomic_wait.h

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// -*- C++ -*- header.
 
// Copyright (C) 2020-2025 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_wait.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{atomic}
 */
 
#ifndef _GLIBCXX_ATOMIC_WAIT_H
#define _GLIBCXX_ATOMIC_WAIT_H 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/version.h>
 
#if __glibcxx_atomic_wait
#include <cstdint>
#include <bits/functional_hash.h>
#include <bits/gthr.h>
#include <ext/numeric_traits.h>
 
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
# include <cerrno>
# include <climits>
# include <unistd.h>
# include <syscall.h>
# include <bits/functexcept.h>
#endif
 
# include <bits/std_mutex.h>  // std::mutex, std::__condvar
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
  namespace __detail
  {
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
#define _GLIBCXX_HAVE_PLATFORM_WAIT 1
    using __platform_wait_t = int;
    inline constexpr size_t __platform_wait_alignment = 4;
#else
// define _GLIBCX_HAVE_PLATFORM_WAIT and implement __platform_wait()
// and __platform_notify() if there is a more efficient primitive supported
// by the platform (e.g. __ulock_wait()/__ulock_wake()) which is better than
// a mutex/condvar based wait.
# if ATOMIC_LONG_LOCK_FREE == 2
    using __platform_wait_t = unsigned long;
# else
    using __platform_wait_t = unsigned int;
# endif
    inline constexpr size_t __platform_wait_alignment
      = __alignof__(__platform_wait_t);
#endif
  } // namespace __detail
 
  template<typename _Tp>
    inline constexpr bool __platform_wait_uses_type
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
      = is_scalar_v<_Tp>
        && ((sizeof(_Tp) == sizeof(__detail::__platform_wait_t))
        && (alignof(_Tp*) >= __detail::__platform_wait_alignment));
#else
      = false;
#endif
 
  namespace __detail
  {
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
    enum class __futex_wait_flags : int
    {
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE
      __private_flag = 128,
#else
      __private_flag = 0,
#endif
      __wait = 0,
      __wake = 1,
      __wait_bitset = 9,
      __wake_bitset = 10,
      __wait_private = __wait | __private_flag,
      __wake_private = __wake | __private_flag,
      __wait_bitset_private = __wait_bitset | __private_flag,
      __wake_bitset_private = __wake_bitset | __private_flag,
      __bitset_match_any = -1
    };
 
    template<typename _Tp>
      void
      __platform_wait(const _Tp* __addr, __platform_wait_t __val) noexcept
      {
        auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
                            static_cast<int>(__futex_wait_flags::__wait_private),
                            __val, nullptr);
        if (!__e || errno == EAGAIN)
          return;
        if (errno != EINTR)
          __throw_system_error(errno);
      }
 
    template<typename _Tp>
      void
      __platform_notify(const _Tp* __addr, bool __all) noexcept
      {
        syscall (SYS_futex, static_cast<const void*>(__addr),
                 static_cast<int>(__futex_wait_flags::__wake_private),
                 __all ? INT_MAX : 1);
      }
#endif
 
    inline void
    __thread_yield() noexcept
    {
#if defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
     __gthread_yield();
#endif
    }
 
    inline void
    __thread_relax() noexcept
    {
#if defined __i386__ || defined __x86_64__
      __builtin_ia32_pause();
#else
      __thread_yield();
#endif
    }
 
    inline constexpr auto __atomic_spin_count_relax = 12;
    inline constexpr auto __atomic_spin_count = 16;
 
    struct __default_spin_policy
    {
      bool
      operator()() const noexcept
      { return false; }
    };
 
    template<typename _Pred,
             typename _Spin = __default_spin_policy>
      bool
      __atomic_spin(_Pred& __pred, _Spin __spin = _Spin{ }) noexcept
      {
        for (auto __i = 0; __i < __atomic_spin_count; ++__i)
          {
            if (__pred())
              return true;
 
            if (__i < __atomic_spin_count_relax)
              __detail::__thread_relax();
            else
              __detail::__thread_yield();
          }
 
        while (__spin())
          {
            if (__pred())
              return true;
          }
 
        return false;
      }
 
    // return true if equal
    template<typename _Tp>
      bool __atomic_compare(const _Tp& __a, const _Tp& __b)
      {
        // TODO make this do the correct padding bit ignoring comparison
        return __builtin_memcmp(std::addressof(__a), std::addressof(__b),
                                sizeof(_Tp)) == 0;
      }
 
    struct __waiter_pool_base
    {
      // Don't use std::hardware_destructive_interference_size here because we
      // don't want the layout of library types to depend on compiler options.
      static constexpr auto _S_align = 64;
 
      alignas(_S_align) __platform_wait_t _M_wait = 0;
 
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
      mutex _M_mtx;
#endif
 
      alignas(_S_align) __platform_wait_t _M_ver = 0;
 
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
      __condvar _M_cv;
#endif
      __waiter_pool_base() = default;
 
      void
      _M_enter_wait() noexcept
      { __atomic_fetch_add(&_M_wait, 1, __ATOMIC_SEQ_CST); }
 
      void
      _M_leave_wait() noexcept
      { __atomic_fetch_sub(&_M_wait, 1, __ATOMIC_RELEASE); }
 
      bool
      _M_waiting() const noexcept
      {
        __platform_wait_t __res;
        __atomic_load(&_M_wait, &__res, __ATOMIC_SEQ_CST);
        return __res != 0;
      }
 
      void
      _M_notify(__platform_wait_t* __addr, [[maybe_unused]] bool __all,
                bool __bare) noexcept
      {
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
        if (__addr == &_M_ver)
          {
            __atomic_fetch_add(__addr, 1, __ATOMIC_SEQ_CST);
            __all = true;
          }
 
        if (__bare || _M_waiting())
          __platform_notify(__addr, __all);
#else
        {
          lock_guard<mutex> __l(_M_mtx);
          __atomic_fetch_add(__addr, 1, __ATOMIC_RELAXED);
        }
        if (__bare || _M_waiting())
          _M_cv.notify_all();
#endif
      }
 
      static __waiter_pool_base&
      _S_for(const void* __addr) noexcept
      {
        constexpr __UINTPTR_TYPE__ __ct = 16;
        static __waiter_pool_base __w[__ct];
        auto __key = ((__UINTPTR_TYPE__)__addr >> 2) % __ct;
        return __w[__key];
      }
    };
 
    struct __waiter_pool : __waiter_pool_base
    {
      void
      _M_do_wait(const __platform_wait_t* __addr, __platform_wait_t __old) noexcept
      {
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
        __platform_wait(__addr, __old);
#else
        __platform_wait_t __val;
        __atomic_load(__addr, &__val, __ATOMIC_SEQ_CST);
        if (__val == __old)
          {
            lock_guard<mutex> __l(_M_mtx);
            __atomic_load(__addr, &__val, __ATOMIC_RELAXED);
            if (__val == __old)
              _M_cv.wait(_M_mtx);
          }
#endif // __GLIBCXX_HAVE_PLATFORM_WAIT
      }
    };
 
    template<typename _Tp>
      struct __waiter_base
      {
        using __waiter_type = _Tp;
 
        __waiter_type& _M_w;
        __platform_wait_t* _M_addr;
 
        template<typename _Up>
          static __platform_wait_t*
          _S_wait_addr(const _Up* __a, __platform_wait_t* __b)
          {
            if constexpr (__platform_wait_uses_type<_Up>)
              return reinterpret_cast<__platform_wait_t*>(const_cast<_Up*>(__a));
            else
              return __b;
          }
 
        static __waiter_type&
        _S_for(const void* __addr) noexcept
        {
          static_assert(sizeof(__waiter_type) == sizeof(__waiter_pool_base));
          auto& res = __waiter_pool_base::_S_for(__addr);
          return reinterpret_cast<__waiter_type&>(res);
        }
 
        template<typename _Up>
          explicit __waiter_base(const _Up* __addr) noexcept
            : _M_w(_S_for(__addr))
            , _M_addr(_S_wait_addr(__addr, &_M_w._M_ver))
          { }
 
        void
        _M_notify(bool __all, bool __bare = false) noexcept
        { _M_w._M_notify(_M_addr, __all, __bare); }
 
        template<typename _Up, typename _ValFn,
                 typename _Spin = __default_spin_policy>
          static bool
          _S_do_spin_v(__platform_wait_t* __addr,
                       const _Up& __old, _ValFn __vfn,
                       __platform_wait_t& __val,
                       _Spin __spin = _Spin{ })
          {
            auto const __pred = [=]
              { return !__detail::__atomic_compare(__old, __vfn()); };
 
            if constexpr (__platform_wait_uses_type<_Up>)
              {
                __builtin_memcpy(&__val, &__old, sizeof(__val));
              }
            else
              {
                __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
              }
            return __atomic_spin(__pred, __spin);
          }
 
        template<typename _Up, typename _ValFn,
                 typename _Spin = __default_spin_policy>
          bool
          _M_do_spin_v(const _Up& __old, _ValFn __vfn,
                       __platform_wait_t& __val,
                       _Spin __spin = _Spin{ })
          { return _S_do_spin_v(_M_addr, __old, __vfn, __val, __spin); }
 
        template<typename _Pred,
                 typename _Spin = __default_spin_policy>
          static bool
          _S_do_spin(const __platform_wait_t* __addr,
                     _Pred __pred,
                     __platform_wait_t& __val,
                     _Spin __spin = _Spin{ })
          {
            __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
            return __atomic_spin(__pred, __spin);
          }
 
        template<typename _Pred,
                 typename _Spin = __default_spin_policy>
          bool
          _M_do_spin(_Pred __pred, __platform_wait_t& __val,
                     _Spin __spin = _Spin{ })
          { return _S_do_spin(_M_addr, __pred, __val, __spin); }
      };
 
    template<typename _EntersWait>
      struct __waiter : __waiter_base<__waiter_pool>
      {
        using __base_type = __waiter_base<__waiter_pool>;
 
        template<typename _Tp>
          explicit __waiter(const _Tp* __addr) noexcept
            : __base_type(__addr)
          {
            if constexpr (_EntersWait::value)
              _M_w._M_enter_wait();
          }
 
        ~__waiter()
        {
          if constexpr (_EntersWait::value)
            _M_w._M_leave_wait();
        }
 
        template<typename _Tp, typename _ValFn>
          void
          _M_do_wait_v(_Tp __old, _ValFn __vfn)
          {
            do
              {
                __platform_wait_t __val;
                if (__base_type::_M_do_spin_v(__old, __vfn, __val))
                  return;
                __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
              }
            while (__detail::__atomic_compare(__old, __vfn()));
          }
 
        template<typename _Pred>
          void
          _M_do_wait(_Pred __pred) noexcept
          {
            do
              {
                __platform_wait_t __val;
                if (__base_type::_M_do_spin(__pred, __val))
                  return;
                __base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
              }
            while (!__pred());
          }
      };
 
    using __enters_wait = __waiter<std::true_type>;
    using __bare_wait = __waiter<std::false_type>;
  } // namespace __detail
 
  template<typename _Tp, typename _ValFn>
    void
    __atomic_wait_address_v(const _Tp* __addr, _Tp __old,
                            _ValFn __vfn) noexcept
    {
      __detail::__enters_wait __w(__addr);
      __w._M_do_wait_v(__old, __vfn);
    }
 
  template<typename _Tp, typename _Pred>
    void
    __atomic_wait_address(const _Tp* __addr, _Pred __pred) noexcept
    {
      __detail::__enters_wait __w(__addr);
      __w._M_do_wait(__pred);
    }
 
  // This call is to be used by atomic types which track contention externally
  template<typename _Pred>
    void
    __atomic_wait_address_bare(const __detail::__platform_wait_t* __addr,
                               _Pred __pred) noexcept
    {
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
      do
        {
          __detail::__platform_wait_t __val;
          if (__detail::__bare_wait::_S_do_spin(__addr, __pred, __val))
            return;
          __detail::__platform_wait(__addr, __val);
        }
      while (!__pred());
#else // !_GLIBCXX_HAVE_PLATFORM_WAIT
      __detail::__bare_wait __w(__addr);
      __w._M_do_wait(__pred);
#endif
    }
 
  template<typename _Tp>
    void
    __atomic_notify_address(const _Tp* __addr, bool __all) noexcept
    {
      __detail::__bare_wait __w(__addr);
      __w._M_notify(__all);
    }
 
  // This call is to be used by atomic types which track contention externally
  inline void
  __atomic_notify_address_bare(const __detail::__platform_wait_t* __addr,
                               bool __all) noexcept
  {
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
    __detail::__platform_notify(__addr, __all);
#else
    __detail::__bare_wait __w(__addr);
    __w._M_notify(__all, true);
#endif
  }
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif // __glibcxx_atomic_wait
#endif // _GLIBCXX_ATOMIC_WAIT_H