1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
|
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: call_once.h
// -----------------------------------------------------------------------------
//
// This header file provides an Abseil version of `std::call_once` for invoking
// a given function at most once, across all threads. This Abseil version is
// faster than the C++11 version and incorporates the C++17 argument-passing
// fix, so that (for example) non-const references may be passed to the invoked
// function.
#ifndef ABSL_BASE_CALL_ONCE_H_
#define ABSL_BASE_CALL_ONCE_H_
#include <algorithm>
#include <atomic>
#include <cstdint>
#include <type_traits>
#include <utility>
#include "y_absl/base/internal/invoke.h"
#include "y_absl/base/internal/low_level_scheduling.h"
#include "y_absl/base/internal/raw_logging.h"
#include "y_absl/base/internal/scheduling_mode.h"
#include "y_absl/base/internal/spinlock_wait.h"
#include "y_absl/base/macros.h"
#include "y_absl/base/optimization.h"
#include "y_absl/base/port.h"
namespace y_absl {
ABSL_NAMESPACE_BEGIN
class once_flag;
namespace base_internal {
std::atomic<uint32_t>* ControlWord(y_absl::once_flag* flag);
} // namespace base_internal
// call_once()
//
// For all invocations using a given `once_flag`, invokes a given `fn` exactly
// once across all threads. The first call to `call_once()` with a particular
// `once_flag` argument (that does not throw an exception) will run the
// specified function with the provided `args`; other calls with the same
// `once_flag` argument will not run the function, but will wait
// for the provided function to finish running (if it is still running).
//
// This mechanism provides a safe, simple, and fast mechanism for one-time
// initialization in a multi-threaded process.
//
// Example:
//
// class MyInitClass {
// public:
// ...
// mutable y_absl::once_flag once_;
//
// MyInitClass* init() const {
// y_absl::call_once(once_, &MyInitClass::Init, this);
// return ptr_;
// }
//
template <typename Callable, typename... Args>
void call_once(y_absl::once_flag& flag, Callable&& fn, Args&&... args);
// once_flag
//
// Objects of this type are used to distinguish calls to `call_once()` and
// ensure the provided function is only invoked once across all threads. This
// type is not copyable or movable. However, it has a `constexpr`
// constructor, and is safe to use as a namespace-scoped global variable.
class once_flag {
public:
constexpr once_flag() : control_(0) {}
once_flag(const once_flag&) = delete;
once_flag& operator=(const once_flag&) = delete;
private:
friend std::atomic<uint32_t>* base_internal::ControlWord(once_flag* flag);
std::atomic<uint32_t> control_;
};
//------------------------------------------------------------------------------
// End of public interfaces.
// Implementation details follow.
//------------------------------------------------------------------------------
namespace base_internal {
// Like call_once, but uses KERNEL_ONLY scheduling. Intended to be used to
// initialize entities used by the scheduler implementation.
template <typename Callable, typename... Args>
void LowLevelCallOnce(y_absl::once_flag* flag, Callable&& fn, Args&&... args);
// Disables scheduling while on stack when scheduling mode is non-cooperative.
// No effect for cooperative scheduling modes.
class SchedulingHelper {
public:
explicit SchedulingHelper(base_internal::SchedulingMode mode) : mode_(mode) {
if (mode_ == base_internal::SCHEDULE_KERNEL_ONLY) {
guard_result_ = base_internal::SchedulingGuard::DisableRescheduling();
}
}
~SchedulingHelper() {
if (mode_ == base_internal::SCHEDULE_KERNEL_ONLY) {
base_internal::SchedulingGuard::EnableRescheduling(guard_result_);
}
}
private:
base_internal::SchedulingMode mode_;
bool guard_result_;
};
// Bit patterns for call_once state machine values. Internal implementation
// detail, not for use by clients.
//
// The bit patterns are arbitrarily chosen from unlikely values, to aid in
// debugging. However, kOnceInit must be 0, so that a zero-initialized
// once_flag will be valid for immediate use.
enum {
kOnceInit = 0,
kOnceRunning = 0x65C2937B,
kOnceWaiter = 0x05A308D2,
// A very small constant is chosen for kOnceDone so that it fit in a single
// compare with immediate instruction for most common ISAs. This is verified
// for x86, POWER and ARM.
kOnceDone = 221, // Random Number
};
template <typename Callable, typename... Args>
ABSL_ATTRIBUTE_NOINLINE
void CallOnceImpl(std::atomic<uint32_t>* control,
base_internal::SchedulingMode scheduling_mode, Callable&& fn,
Args&&... args) {
#ifndef NDEBUG
{
uint32_t old_control = control->load(std::memory_order_relaxed);
if (old_control != kOnceInit &&
old_control != kOnceRunning &&
old_control != kOnceWaiter &&
old_control != kOnceDone) {
ABSL_RAW_LOG(FATAL, "Unexpected value for control word: 0x%lx",
static_cast<unsigned long>(old_control)); // NOLINT
}
}
#endif // NDEBUG
static const base_internal::SpinLockWaitTransition trans[] = {
{kOnceInit, kOnceRunning, true},
{kOnceRunning, kOnceWaiter, false},
{kOnceDone, kOnceDone, true}};
// Must do this before potentially modifying control word's state.
base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode);
// Short circuit the simplest case to avoid procedure call overhead.
// The base_internal::SpinLockWait() call returns either kOnceInit or
// kOnceDone. If it returns kOnceDone, it must have loaded the control word
// with std::memory_order_acquire and seen a value of kOnceDone.
uint32_t old_control = kOnceInit;
if (control->compare_exchange_strong(old_control, kOnceRunning,
std::memory_order_relaxed) ||
base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans,
scheduling_mode) == kOnceInit) {
base_internal::invoke(std::forward<Callable>(fn),
std::forward<Args>(args)...);
old_control =
control->exchange(base_internal::kOnceDone, std::memory_order_release);
if (old_control == base_internal::kOnceWaiter) {
base_internal::SpinLockWake(control, true);
}
} // else *control is already kOnceDone
}
inline std::atomic<uint32_t>* ControlWord(once_flag* flag) {
return &flag->control_;
}
template <typename Callable, typename... Args>
void LowLevelCallOnce(y_absl::once_flag* flag, Callable&& fn, Args&&... args) {
std::atomic<uint32_t>* once = base_internal::ControlWord(flag);
uint32_t s = once->load(std::memory_order_acquire);
if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) {
base_internal::CallOnceImpl(once, base_internal::SCHEDULE_KERNEL_ONLY,
std::forward<Callable>(fn),
std::forward<Args>(args)...);
}
}
} // namespace base_internal
template <typename Callable, typename... Args>
void call_once(y_absl::once_flag& flag, Callable&& fn, Args&&... args) {
std::atomic<uint32_t>* once = base_internal::ControlWord(&flag);
uint32_t s = once->load(std::memory_order_acquire);
if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) {
base_internal::CallOnceImpl(
once, base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL,
std::forward<Callable>(fn), std::forward<Args>(args)...);
}
}
ABSL_NAMESPACE_END
} // namespace y_absl
#endif // ABSL_BASE_CALL_ONCE_H_
|
