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 <__config>
10 #include <__mutex/once_flag.h>
11 #include <__utility/exception_guard.h>
12
13 #if _LIBCPP_HAS_THREADS
14 # include <__thread/support.h>
15 #endif
16
17 #include "include/atomic_support.h"
18
19 _LIBCPP_BEGIN_NAMESPACE_STD
20
21 // If dispatch_once_f ever handles C++ exceptions, and if one can get to it
22 // without illegal macros (unexpected macros not beginning with _UpperCase or
23 // __lowercase), and if it stops spinning waiting threads, then call_once should
24 // call into dispatch_once_f instead of here. Relevant radar this code needs to
25 // keep in sync with: 7741191.
26
27 #if _LIBCPP_HAS_THREADS
28 static constinit __libcpp_mutex_t mut = _LIBCPP_MUTEX_INITIALIZER;
29 static constinit __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
30 #endif
31
__call_once(volatile once_flag::_State_type & flag,void * arg,void (* func)(void *))32 void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
33 #if !_LIBCPP_HAS_THREADS
34
35 if (flag == once_flag::_Unset) {
36 auto guard = std::__make_exception_guard([&flag] { flag = once_flag::_Unset; });
37 flag = once_flag::_Pending;
38 func(arg);
39 flag = once_flag::_Complete;
40 guard.__complete();
41 }
42
43 #else // !_LIBCPP_HAS_THREADS
44
45 __libcpp_mutex_lock(&mut);
46 while (flag == once_flag::_Pending)
47 __libcpp_condvar_wait(&cv, &mut);
48 if (flag == once_flag::_Unset) {
49 auto guard = std::__make_exception_guard([&flag] {
50 __libcpp_mutex_lock(&mut);
51 __libcpp_relaxed_store(&flag, once_flag::_Unset);
52 __libcpp_mutex_unlock(&mut);
53 __libcpp_condvar_broadcast(&cv);
54 });
55
56 __libcpp_relaxed_store(&flag, once_flag::_Pending);
57 __libcpp_mutex_unlock(&mut);
58 func(arg);
59 __libcpp_mutex_lock(&mut);
60 __libcpp_atomic_store(&flag, once_flag::_Complete, _AO_Release);
61 __libcpp_mutex_unlock(&mut);
62 __libcpp_condvar_broadcast(&cv);
63 guard.__complete();
64 } else {
65 __libcpp_mutex_unlock(&mut);
66 }
67
68 #endif // !_LIBCPP_HAS_THREADS
69 }
70
71 _LIBCPP_END_NAMESPACE_STD
72