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
__libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile * __ptr,__cxx_contention_t __val)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
__libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile * __ptr,bool __notify_one)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__) && __SIZEOF_LONG__ == 8
81 /*
82 * Since __cxx_contention_t is int64_t even on 32bit FreeBSD
83 * platforms, we have to use umtx ops that work on the long type, and
84 * limit its use to architectures where long and int64_t are synonyms.
85 */
86
87 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
88 __cxx_contention_t __val)
89 {
90 _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr),
91 UMTX_OP_WAIT, __val, NULL, NULL);
92 }
93
94 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
95 bool __notify_one)
96 {
97 _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr),
98 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 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
106 __cxx_contention_t __val)
107 {
108 __libcpp_thread_poll_with_backoff([=]() -> bool {
109 return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val);
110 }, __libcpp_timed_backoff_policy());
111 }
112
113 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { }
114
115 #endif // __linux__
116
117 static constexpr size_t __libcpp_contention_table_size = (1 << 8); /* < there's no magic in this number */
118
119 struct alignas(64) /* aim to avoid false sharing */ __libcpp_contention_table_entry
120 {
121 __cxx_atomic_contention_t __contention_state;
122 __cxx_atomic_contention_t __platform_state;
__libcpp_contention_table_entry__libcpp_contention_table_entry123 inline constexpr __libcpp_contention_table_entry() :
124 __contention_state(0), __platform_state(0) { }
125 };
126
127 static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ];
128
129 static hash<void const volatile*> __libcpp_contention_hasher;
130
__libcpp_contention_state(void const volatile * p)131 static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p)
132 {
133 return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)];
134 }
135
136 /* Given an atomic to track contention and an atomic to actually wait on, which may be
137 the same atomic, we try to detect contention to avoid spuriously calling the platform. */
138
__libcpp_contention_notify(__cxx_atomic_contention_t volatile * __contention_state,__cxx_atomic_contention_t const volatile * __platform_state,bool __notify_one)139 static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state,
140 __cxx_atomic_contention_t const volatile* __platform_state,
141 bool __notify_one)
142 {
143 if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst))
144 // We only call 'wake' if we consumed a contention bit here.
145 __libcpp_platform_wake_by_address(__platform_state, __notify_one);
146 }
__libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile * __contention_state,__cxx_atomic_contention_t const volatile * __platform_state)147 static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state,
148 __cxx_atomic_contention_t const volatile* __platform_state)
149 {
150 // We will monitor this value.
151 return __cxx_atomic_load(__platform_state, memory_order_acquire);
152 }
__libcpp_contention_wait(__cxx_atomic_contention_t volatile * __contention_state,__cxx_atomic_contention_t const volatile * __platform_state,__cxx_contention_t __old_value)153 static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state,
154 __cxx_atomic_contention_t const volatile* __platform_state,
155 __cxx_contention_t __old_value)
156 {
157 __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
158 // We sleep as long as the monitored value hasn't changed.
159 __libcpp_platform_wait_on_address(__platform_state, __old_value);
160 __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
161 }
162
163 /* When the incoming atomic is the wrong size for the platform wait size, need to
164 launder the value sequence through an atomic from our table. */
165
__libcpp_atomic_notify(void const volatile * __location)166 static void __libcpp_atomic_notify(void const volatile* __location)
167 {
168 auto const __entry = __libcpp_contention_state(__location);
169 // The value sequence laundering happens on the next line below.
170 __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
171 __libcpp_contention_notify(&__entry->__contention_state,
172 &__entry->__platform_state,
173 false /* when laundering, we can't handle notify_one */);
174 }
175 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_one(void const volatile * __location)176 void __cxx_atomic_notify_one(void const volatile* __location)
177 { __libcpp_atomic_notify(__location); }
178 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_all(void const volatile * __location)179 void __cxx_atomic_notify_all(void const volatile* __location)
180 { __libcpp_atomic_notify(__location); }
181 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_monitor(void const volatile * __location)182 __cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location)
183 {
184 auto const __entry = __libcpp_contention_state(__location);
185 return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state);
186 }
187 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_wait(void const volatile * __location,__cxx_contention_t __old_value)188 void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value)
189 {
190 auto const __entry = __libcpp_contention_state(__location);
191 __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value);
192 }
193
194 /* When the incoming atomic happens to be the platform wait size, we still need to use the
195 table for the contention detection, but we can use the atomic directly for the wait. */
196
197 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile * __location)198 void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location)
199 {
200 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true);
201 }
202 _LIBCPP_EXPORTED_FROM_ABI
__cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile * __location)203 void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location)
204 {
205 __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false);
206 }
207 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile * __location)208 __cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location)
209 {
210 return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location);
211 }
212 _LIBCPP_EXPORTED_FROM_ABI
__libcpp_atomic_wait(__cxx_atomic_contention_t const volatile * __location,__cxx_contention_t __old_value)213 void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value)
214 {
215 __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value);
216 }
217
218 _LIBCPP_END_NAMESPACE_STD
219
220 #endif //_LIBCPP_HAS_NO_THREADS
221