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 #ifndef _LIBCPP_HAS_NO_THREADS
11
12 #include <atomic>
13 #include <climits>
14 #include <functional>
15 #include <thread>
16
17 #ifdef __linux__
18
19 #include <unistd.h>
20 #include <linux/futex.h>
21 #include <sys/syscall.h>
22
23 // libc++ uses SYS_futex as a universal syscall name. However, on 32 bit architectures
24 // with a 64 bit time_t, we need to specify SYS_futex_time64.
25 #if !defined(SYS_futex) && defined(SYS_futex_time64)
26 # define SYS_futex SYS_futex_time64
27 #endif
28
29 #else // <- Add other operating systems here
30
31 // Baseline needs no new headers
32
33 #endif
34
35 _LIBCPP_BEGIN_NAMESPACE_STD
36
37 #ifdef __linux__
38
__libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile * __ptr,__cxx_contention_t __val)39 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
40 __cxx_contention_t __val)
41 {
42 static constexpr timespec __timeout = { 2, 0 };
43 syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
44 }
45
__libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile * __ptr,bool __notify_one)46 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
47 bool __notify_one)
48 {
49 syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
50 }
51
52 #elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK)
53
54 extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value,
55 uint32_t timeout); /* timeout is specified in microseconds */
56 extern "C" int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value);
57
58 #define UL_COMPARE_AND_WAIT 1
59 #define ULF_WAKE_ALL 0x00000100
60
61 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
62 __cxx_contention_t __val)
63 {
64 __ulock_wait(UL_COMPARE_AND_WAIT,
65 const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0);
66 }
67
68 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
69 bool __notify_one)
70 {
71 __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL),
72 const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
73 }
74
75 #else // <- Add other operating systems here
76
77 // Baseline is just a timed backoff
78
79 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
80 __cxx_contention_t __val)
81 {
82 __libcpp_thread_poll_with_backoff([=]() -> bool {
83 return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val);
84 }, __libcpp_timed_backoff_policy());
85 }
86
87 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { }
88
89 #endif // __linux__
90
91 static constexpr size_t __libcpp_contention_table_size = (1 << 8); /* < there's no magic in this number */
92
93 struct alignas(64) /* aim to avoid false sharing */ __libcpp_contention_table_entry
94 {
95 __cxx_atomic_contention_t __contention_state;
96 __cxx_atomic_contention_t __platform_state;
__libcpp_contention_table_entry__libcpp_contention_table_entry97 inline constexpr __libcpp_contention_table_entry() :
98 __contention_state(0), __platform_state(0) { }
99 };
100
101 static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ];
102
103 static hash<void const volatile*> __libcpp_contention_hasher;
104
__libcpp_contention_state(void const volatile * p)105 static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p)
106 {
107 return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)];
108 }
109
110 /* Given an atomic to track contention and an atomic to actually wait on, which may be
111 the same atomic, we try to detect contention to avoid spuriously calling the platform. */
112
__libcpp_contention_notify(__cxx_atomic_contention_t volatile * __contention_state,__cxx_atomic_contention_t const volatile * __platform_state,bool __notify_one)113 static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state,
114 __cxx_atomic_contention_t const volatile* __platform_state,
115 bool __notify_one)
116 {
117 if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst))
118 // We only call 'wake' if we consumed a contention bit here.
119 __libcpp_platform_wake_by_address(__platform_state, __notify_one);
120 }
__libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile * __contention_state,__cxx_atomic_contention_t const volatile * __platform_state)121 static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state,
122 __cxx_atomic_contention_t const volatile* __platform_state)
123 {
124 // We will monitor this value.
125 return __cxx_atomic_load(__platform_state, memory_order_acquire);
126 }
__libcpp_contention_wait(__cxx_atomic_contention_t volatile * __contention_state,__cxx_atomic_contention_t const volatile * __platform_state,__cxx_contention_t __old_value)127 static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state,
128 __cxx_atomic_contention_t const volatile* __platform_state,
129 __cxx_contention_t __old_value)
130 {
131 __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
132 // We sleep as long as the monitored value hasn't changed.
133 __libcpp_platform_wait_on_address(__platform_state, __old_value);
134 __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
135 }
136
137 /* When the incoming atomic is the wrong size for the platform wait size, need to
138 launder the value sequence through an atomic from our table. */
139
__libcpp_atomic_notify(void const volatile * __location)140 static void __libcpp_atomic_notify(void const volatile* __location)
141 {
142 auto const __entry = __libcpp_contention_state(__location);
143 // The value sequence laundering happens on the next line below.
144 __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
145 __libcpp_contention_notify(&__entry->__contention_state,
146 &__entry->__platform_state,
147 false /* when laundering, we can't handle notify_one */);
148 }
149 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_one(void const volatile * __location)150 void __cxx_atomic_notify_one(void const volatile* __location)
151 { __libcpp_atomic_notify(__location); }
152 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_all(void const volatile * __location)153 void __cxx_atomic_notify_all(void const volatile* __location)
154 { __libcpp_atomic_notify(__location); }
155 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_monitor(void const volatile * __location)156 __cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location)
157 {
158 auto const __entry = __libcpp_contention_state(__location);
159 return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state);
160 }
161 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_wait(void const volatile * __location,__cxx_contention_t __old_value)162 void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value)
163 {
164 auto const __entry = __libcpp_contention_state(__location);
165 __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value);
166 }
167
168 /* When the incoming atomic happens to be the platform wait size, we still need to use the
169 table for the contention detection, but we can use the atomic directly for the wait. */
170
171 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile * __location)172 void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location)
173 {
174 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true);
175 }
176 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile * __location)177 void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location)
178 {
179 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false);
180 }
181 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile * __location)182 __cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location)
183 {
184 return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location);
185 }
186 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_wait(__cxx_atomic_contention_t const volatile * __location,__cxx_contention_t __old_value)187 void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value)
188 {
189 __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value);
190 }
191
192 _LIBCPP_END_NAMESPACE_STD
193
194 #endif //_LIBCPP_HAS_NO_THREADS
195