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 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 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; 97 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; 104 105 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. */ 112 113 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 } 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 } 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. */ 139 140 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 150 void __cxx_atomic_notify_one(void const volatile* __location) 151 { __libcpp_atomic_notify(__location); } 152 _LIBCPP_EXPORTED_FROM_ABI 153 void __cxx_atomic_notify_all(void const volatile* __location) 154 { __libcpp_atomic_notify(__location); } 155 _LIBCPP_EXPORTED_FROM_ABI 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 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 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 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 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 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