libcxx/src/call_once.cpp

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include <__mutex/once_flag.h>
  10 #include <__utility/exception_guard.h>
  11
  12 #if _LIBCPP_HAS_THREADS
  13 #  include <__thread/support.h>
  14 #endif
  15
  16 #include "include/atomic_support.h"
  17
  18 _LIBCPP_BEGIN_NAMESPACE_STD
  19
  20 // If dispatch_once_f ever handles C++ exceptions, and if one can get to it
  21 // without illegal macros (unexpected macros not beginning with _UpperCase or
  22 // __lowercase), and if it stops spinning waiting threads, then call_once should
  23 // call into dispatch_once_f instead of here. Relevant radar this code needs to
  24 // keep in sync with:  7741191.
  25
  26 #if _LIBCPP_HAS_THREADS
  27 static constinit __libcpp_mutex_t mut  = _LIBCPP_MUTEX_INITIALIZER;
  28 static constinit __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
  29 #endif
  30
  31 void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
  32 #if !_LIBCPP_HAS_THREADS
  33
  34   if (flag == once_flag::_Unset) {
  35     auto guard = std::__make_exception_guard([&flag] { flag = once_flag::_Unset; });
  36     flag       = once_flag::_Pending;
  37     func(arg);
  38     flag = once_flag::_Complete;
  39     guard.__complete();
  40   }
  41
  42 #else // !_LIBCPP_HAS_THREADS
  43
  44   __libcpp_mutex_lock(&mut);
  45   while (flag == once_flag::_Pending)
  46     __libcpp_condvar_wait(&cv, &mut);
  47   if (flag == once_flag::_Unset) {
  48     auto guard = std::__make_exception_guard([&flag] {
  49       __libcpp_mutex_lock(&mut);
  50       __libcpp_relaxed_store(&flag, once_flag::_Unset);
  51       __libcpp_mutex_unlock(&mut);
  52       __libcpp_condvar_broadcast(&cv);
  53     });
  54
  55     __libcpp_relaxed_store(&flag, once_flag::_Pending);
  56     __libcpp_mutex_unlock(&mut);
  57     func(arg);
  58     __libcpp_mutex_lock(&mut);
  59     __libcpp_atomic_store(&flag, once_flag::_Complete, _AO_Release);
  60     __libcpp_mutex_unlock(&mut);
  61     __libcpp_condvar_broadcast(&cv);
  62     guard.__complete();
  63   } else {
  64     __libcpp_mutex_unlock(&mut);
  65   }
  66
  67 #endif // !_LIBCPP_HAS_THREADS
  68 }
  69
  70 _LIBCPP_END_NAMESPACE_STD