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 #elif defined(__FreeBSD__) 30 31 #include <sys/types.h> 32 #include <sys/umtx.h> 33 34 #else // <- Add other operating systems here 35 36 // Baseline needs no new headers 37 38 #endif 39 40 _LIBCPP_BEGIN_NAMESPACE_STD 41 42 #ifdef __linux__ 43 44 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 45 __cxx_contention_t __val) 46 { 47 static constexpr timespec __timeout = { 2, 0 }; 48 syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0); 49 } 50 51 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, 52 bool __notify_one) 53 { 54 syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0); 55 } 56 57 #elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK) 58 59 extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value, 60 uint32_t timeout); /* timeout is specified in microseconds */ 61 extern "C" int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value); 62 63 #define UL_COMPARE_AND_WAIT 1 64 #define ULF_WAKE_ALL 0x00000100 65 66 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 67 __cxx_contention_t __val) 68 { 69 __ulock_wait(UL_COMPARE_AND_WAIT, 70 const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0); 71 } 72 73 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, 74 bool __notify_one) 75 { 76 __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL), 77 const_cast<__cxx_atomic_contention_t*>(__ptr), 0); 78 } 79 80 #elif defined(__FreeBSD__) && defined(__LP64__) 81 82 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 83 __cxx_contention_t __val) 84 { 85 _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), 86 UMTX_OP_WAIT, __val, NULL, NULL); 87 } 88 89 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, 90 bool __notify_one) 91 { 92 _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), 93 UMTX_OP_WAKE, __notify_one ? 1 : INT_MAX, NULL, NULL); 94 } 95 96 #else // <- Add other operating systems here 97 98 // Baseline is just a timed backoff 99 100 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, 101 __cxx_contention_t __val) 102 { 103 __libcpp_thread_poll_with_backoff([=]() -> bool { 104 return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val); 105 }, __libcpp_timed_backoff_policy()); 106 } 107 108 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { } 109 110 #endif // __linux__ 111 112 static constexpr size_t __libcpp_contention_table_size = (1 << 8); /* < there's no magic in this number */ 113 114 struct alignas(64) /* aim to avoid false sharing */ __libcpp_contention_table_entry 115 { 116 __cxx_atomic_contention_t __contention_state; 117 __cxx_atomic_contention_t __platform_state; 118 inline constexpr __libcpp_contention_table_entry() : 119 __contention_state(0), __platform_state(0) { } 120 }; 121 122 static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ]; 123 124 static hash<void const volatile*> __libcpp_contention_hasher; 125 126 static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p) 127 { 128 return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)]; 129 } 130 131 /* Given an atomic to track contention and an atomic to actually wait on, which may be 132 the same atomic, we try to detect contention to avoid spuriously calling the platform. */ 133 134 static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state, 135 __cxx_atomic_contention_t const volatile* __platform_state, 136 bool __notify_one) 137 { 138 if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst)) 139 // We only call 'wake' if we consumed a contention bit here. 140 __libcpp_platform_wake_by_address(__platform_state, __notify_one); 141 } 142 static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state, 143 __cxx_atomic_contention_t const volatile* __platform_state) 144 { 145 // We will monitor this value. 146 return __cxx_atomic_load(__platform_state, memory_order_acquire); 147 } 148 static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state, 149 __cxx_atomic_contention_t const volatile* __platform_state, 150 __cxx_contention_t __old_value) 151 { 152 __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst); 153 // We sleep as long as the monitored value hasn't changed. 154 __libcpp_platform_wait_on_address(__platform_state, __old_value); 155 __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release); 156 } 157 158 /* When the incoming atomic is the wrong size for the platform wait size, need to 159 launder the value sequence through an atomic from our table. */ 160 161 static void __libcpp_atomic_notify(void const volatile* __location) 162 { 163 auto const __entry = __libcpp_contention_state(__location); 164 // The value sequence laundering happens on the next line below. 165 __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release); 166 __libcpp_contention_notify(&__entry->__contention_state, 167 &__entry->__platform_state, 168 false /* when laundering, we can't handle notify_one */); 169 } 170 _LIBCPP_EXPORTED_FROM_ABI 171 void __cxx_atomic_notify_one(void const volatile* __location) 172 { __libcpp_atomic_notify(__location); } 173 _LIBCPP_EXPORTED_FROM_ABI 174 void __cxx_atomic_notify_all(void const volatile* __location) 175 { __libcpp_atomic_notify(__location); } 176 _LIBCPP_EXPORTED_FROM_ABI 177 __cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location) 178 { 179 auto const __entry = __libcpp_contention_state(__location); 180 return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state); 181 } 182 _LIBCPP_EXPORTED_FROM_ABI 183 void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value) 184 { 185 auto const __entry = __libcpp_contention_state(__location); 186 __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value); 187 } 188 189 /* When the incoming atomic happens to be the platform wait size, we still need to use the 190 table for the contention detection, but we can use the atomic directly for the wait. */ 191 192 _LIBCPP_EXPORTED_FROM_ABI 193 void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location) 194 { 195 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true); 196 } 197 _LIBCPP_EXPORTED_FROM_ABI 198 void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location) 199 { 200 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false); 201 } 202 _LIBCPP_EXPORTED_FROM_ABI 203 __cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location) 204 { 205 return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location); 206 } 207 _LIBCPP_EXPORTED_FROM_ABI 208 void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value) 209 { 210 __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value); 211 } 212 213 _LIBCPP_END_NAMESPACE_STD 214 215 #endif //_LIBCPP_HAS_NO_THREADS 216