1//===------------------------- atomic.cpp ---------------------------------===//
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
38static 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
45static 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
53extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value,
54		uint32_t timeout); /* timeout is specified in microseconds */
55extern "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
60static 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
67static 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
78static 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
86static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { }
87
88#endif // __linux__
89
90static constexpr size_t __libcpp_contention_table_size = (1 << 8);  /* < there's no magic in this number */
91
92struct 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
100static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ];
101
102static hash<void const volatile*> __libcpp_contention_hasher;
103
104static __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
112static 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}
120static __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}
126static 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
139static 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
149void __cxx_atomic_notify_one(void const volatile* __location)
150    { __libcpp_atomic_notify(__location); }
151_LIBCPP_EXPORTED_FROM_ABI
152void __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
161void __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
171void __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
176void __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
186void __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