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 <climits> 13 #include <atomic> 14 #include <functional> 15 16 #ifdef __linux__ 17 18 #include <unistd.h> 19 #include <linux/futex.h> 20 #include <sys/syscall.h> 21 22 // libc++ uses SYS_futex as a universal syscall name. However, on 32 bit architectures 23 // with a 64 bit time_t, we need to specify SYS_futex_time64. 24 #if !defined(SYS_futex) && defined(SYS_futex_time64) 25 # define SYS_futex SYS_futex_time64 26 #endif 27 28 #else // <- Add other operating systems here 29 30 // Baseline needs no new headers 31 32 #endif 33 34 _LIBCPP_BEGIN_NAMESPACE_STD 35 36 #ifdef __linux__ 37 38 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 39 __cxx_contention_t __val) 40 { 41 static constexpr timespec __timeout = { 2, 0 }; 42 syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0); 43 } 44 45 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, 46 bool __notify_one) 47 { 48 syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0); 49 } 50 51 #elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK) 52 53 extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value, 54 uint32_t timeout); /* timeout is specified in microseconds */ 55 extern "C" int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value); 56 57 #define UL_COMPARE_AND_WAIT 1 58 #define ULF_WAKE_ALL 0x00000100 59 60 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 61 __cxx_contention_t __val) 62 { 63 __ulock_wait(UL_COMPARE_AND_WAIT, 64 const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0); 65 } 66 67 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, 68 bool __notify_one) 69 { 70 __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL), 71 const_cast<__cxx_atomic_contention_t*>(__ptr), 0); 72 } 73 74 #else // <- Add other operating systems here 75 76 // Baseline is just a timed backoff 77 78 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 79 __cxx_contention_t __val) 80 { 81 __libcpp_thread_poll_with_backoff([=]() -> bool { 82 return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val); 83 }, __libcpp_timed_backoff_policy()); 84 } 85 86 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { } 87 88 #endif // __linux__ 89 90 static constexpr size_t __libcpp_contention_table_size = (1 << 8); /* < there's no magic in this number */ 91 92 struct alignas(64) /* aim to avoid false sharing */ __libcpp_contention_table_entry 93 { 94 __cxx_atomic_contention_t __contention_state; 95 __cxx_atomic_contention_t __platform_state; 96 inline constexpr __libcpp_contention_table_entry() : 97 __contention_state(0), __platform_state(0) { } 98 }; 99 100 static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ]; 101 102 static hash<void const volatile*> __libcpp_contention_hasher; 103 104 static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p) 105 { 106 return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)]; 107 } 108 109 /* Given an atomic to track contention and an atomic to actually wait on, which may be 110 the same atomic, we try to detect contention to avoid spuriously calling the platform. */ 111 112 static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state, 113 __cxx_atomic_contention_t const volatile* __platform_state, 114 bool __notify_one) 115 { 116 if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst)) 117 // We only call 'wake' if we consumed a contention bit here. 118 __libcpp_platform_wake_by_address(__platform_state, __notify_one); 119 } 120 static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state, 121 __cxx_atomic_contention_t const volatile* __platform_state) 122 { 123 // We will monitor this value. 124 return __cxx_atomic_load(__platform_state, memory_order_acquire); 125 } 126 static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state, 127 __cxx_atomic_contention_t const volatile* __platform_state, 128 __cxx_contention_t __old_value) 129 { 130 __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst); 131 // We sleep as long as the monitored value hasn't changed. 132 __libcpp_platform_wait_on_address(__platform_state, __old_value); 133 __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release); 134 } 135 136 /* When the incoming atomic is the wrong size for the platform wait size, need to 137 launder the value sequence through an atomic from our table. */ 138 139 static void __libcpp_atomic_notify(void const volatile* __location) 140 { 141 auto const __entry = __libcpp_contention_state(__location); 142 // The value sequence laundering happens on the next line below. 143 __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release); 144 __libcpp_contention_notify(&__entry->__contention_state, 145 &__entry->__platform_state, 146 false /* when laundering, we can't handle notify_one */); 147 } 148 _LIBCPP_EXPORTED_FROM_ABI 149 void __cxx_atomic_notify_one(void const volatile* __location) 150 { __libcpp_atomic_notify(__location); } 151 _LIBCPP_EXPORTED_FROM_ABI 152 void __cxx_atomic_notify_all(void const volatile* __location) 153 { __libcpp_atomic_notify(__location); } 154 _LIBCPP_EXPORTED_FROM_ABI 155 __cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location) 156 { 157 auto const __entry = __libcpp_contention_state(__location); 158 return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state); 159 } 160 _LIBCPP_EXPORTED_FROM_ABI 161 void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value) 162 { 163 auto const __entry = __libcpp_contention_state(__location); 164 __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value); 165 } 166 167 /* When the incoming atomic happens to be the platform wait size, we still need to use the 168 table for the contention detection, but we can use the atomic directly for the wait. */ 169 170 _LIBCPP_EXPORTED_FROM_ABI 171 void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location) 172 { 173 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true); 174 } 175 _LIBCPP_EXPORTED_FROM_ABI 176 void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location) 177 { 178 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false); 179 } 180 _LIBCPP_EXPORTED_FROM_ABI 181 __cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location) 182 { 183 return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location); 184 } 185 _LIBCPP_EXPORTED_FROM_ABI 186 void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value) 187 { 188 __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value); 189 } 190 191 _LIBCPP_END_NAMESPACE_STD 192 193 #endif //_LIBCPP_HAS_NO_THREADS 194