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 <__thread/timed_backoff_policy.h> 10 #include <atomic> 11 #include <climits> 12 #include <functional> 13 #include <thread> 14 15 #include "include/apple_availability.h" 16 17 #ifdef __linux__ 18 19 # include <linux/futex.h> 20 # include <sys/syscall.h> 21 # include <unistd.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 # define _LIBCPP_FUTEX(...) syscall(SYS_futex, __VA_ARGS__) 29 30 #elif defined(__FreeBSD__) 31 32 # include <sys/types.h> 33 # include <sys/umtx.h> 34 35 # define _LIBCPP_FUTEX(...) syscall(SYS_futex, __VA_ARGS__) 36 37 #elif defined(__OpenBSD__) 38 39 # include <sys/futex.h> 40 41 // OpenBSD has no indirect syscalls 42 # define _LIBCPP_FUTEX(...) futex(__VA_ARGS__) 43 44 #else // <- Add other operating systems here 45 46 // Baseline needs no new headers 47 48 # define _LIBCPP_FUTEX(...) syscall(SYS_futex, __VA_ARGS__) 49 50 #endif 51 52 _LIBCPP_BEGIN_NAMESPACE_STD 53 54 #ifdef __linux__ 55 56 static void 57 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) { 58 static constexpr timespec __timeout = {2, 0}; 59 _LIBCPP_FUTEX(__ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0); 60 } 61 62 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) { 63 _LIBCPP_FUTEX(__ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0); 64 } 65 66 #elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK) 67 68 extern "C" int __ulock_wait( 69 uint32_t operation, void* addr, uint64_t value, uint32_t timeout); /* timeout is specified in microseconds */ 70 extern "C" int __ulock_wake(uint32_t operation, void* addr, uint64_t wake_value); 71 72 # define UL_COMPARE_AND_WAIT 1 73 # define ULF_WAKE_ALL 0x00000100 74 75 static void 76 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) { 77 __ulock_wait(UL_COMPARE_AND_WAIT, const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0); 78 } 79 80 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) { 81 __ulock_wake( 82 UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL), const_cast<__cxx_atomic_contention_t*>(__ptr), 0); 83 } 84 85 #elif defined(__FreeBSD__) && __SIZEOF_LONG__ == 8 86 /* 87 * Since __cxx_contention_t is int64_t even on 32bit FreeBSD 88 * platforms, we have to use umtx ops that work on the long type, and 89 * limit its use to architectures where long and int64_t are synonyms. 90 */ 91 92 static void 93 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) { 94 _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), UMTX_OP_WAIT, __val, NULL, NULL); 95 } 96 97 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) { 98 _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), UMTX_OP_WAKE, __notify_one ? 1 : INT_MAX, NULL, NULL); 99 } 100 101 #else // <- Add other operating systems here 102 103 // Baseline is just a timed backoff 104 105 static void 106 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) { 107 __libcpp_thread_poll_with_backoff( 108 [=]() -> bool { return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val); }, 109 __libcpp_timed_backoff_policy()); 110 } 111 112 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) {} 113 114 #endif // __linux__ 115 116 static constexpr size_t __libcpp_contention_table_size = (1 << 8); /* < there's no magic in this number */ 117 118 struct alignas(64) /* aim to avoid false sharing */ __libcpp_contention_table_entry { 119 __cxx_atomic_contention_t __contention_state; 120 __cxx_atomic_contention_t __platform_state; 121 inline constexpr __libcpp_contention_table_entry() : __contention_state(0), __platform_state(0) {} 122 }; 123 124 static __libcpp_contention_table_entry __libcpp_contention_table[__libcpp_contention_table_size]; 125 126 static hash<void const volatile*> __libcpp_contention_hasher; 127 128 static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile* p) { 129 return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)]; 130 } 131 132 /* Given an atomic to track contention and an atomic to actually wait on, which may be 133 the same atomic, we try to detect contention to avoid spuriously calling the platform. */ 134 135 static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state, 136 __cxx_atomic_contention_t const volatile* __platform_state, 137 bool __notify_one) { 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 143 __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* /*__contention_state*/, 144 __cxx_atomic_contention_t const volatile* __platform_state) { 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 __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst); 152 // We sleep as long as the monitored value hasn't changed. 153 __libcpp_platform_wait_on_address(__platform_state, __old_value); 154 __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release); 155 } 156 157 /* When the incoming atomic is the wrong size for the platform wait size, need to 158 launder the value sequence through an atomic from our table. */ 159 160 static void __libcpp_atomic_notify(void const volatile* __location) { 161 auto const __entry = __libcpp_contention_state(__location); 162 // The value sequence laundering happens on the next line below. 163 __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release); 164 __libcpp_contention_notify( 165 &__entry->__contention_state, 166 &__entry->__platform_state, 167 false /* when laundering, we can't handle notify_one */); 168 } 169 _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(void const volatile* __location) { 170 __libcpp_atomic_notify(__location); 171 } 172 _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(void const volatile* __location) { 173 __libcpp_atomic_notify(__location); 174 } 175 _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location) { 176 auto const __entry = __libcpp_contention_state(__location); 177 return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state); 178 } 179 _LIBCPP_EXPORTED_FROM_ABI void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value) { 180 auto const __entry = __libcpp_contention_state(__location); 181 __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value); 182 } 183 184 /* When the incoming atomic happens to be the platform wait size, we still need to use the 185 table for the contention detection, but we can use the atomic directly for the wait. */ 186 187 _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location) { 188 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true); 189 } 190 _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location) { 191 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false); 192 } 193 _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t 194 __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location) { 195 return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location); 196 } 197 _LIBCPP_EXPORTED_FROM_ABI void 198 __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value) { 199 __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value); 200 } 201 202 _LIBCPP_END_NAMESPACE_STD 203