// 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 "absl/base/internal/invoke.h"
#include "absl/base/internal/low_level_scheduling.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/scheduling_mode.h"
#include "absl/base/internal/spinlock_wait.h"
#include "absl/base/macros.h"
#include "absl/base/optimization.h"
#include "absl/base/port.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
class once_flag;
namespace base_internal {
std::atomic<uint32_t>* ControlWord(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 absl::once_flag once_;
//
// MyInitClass* init() const {
// absl::call_once(once_, &MyInitClass::Init, this);
// return ptr_;
// }
//
template <typename Callable, typename... Args>
void call_once(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(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)...);
// The call to SpinLockWake below is an optimization, because the waiter
// in SpinLockWait is waiting with a short timeout. The atomic load/store
// sequence is slightly faster than an atomic exchange:
// old_control = control->exchange(base_internal::kOnceDone,
// std::memory_order_release);
// We opt for a slightly faster case when there are no waiters, in spite
// of longer tail latency when there are waiters.
old_control = control->load(std::memory_order_relaxed);
control->store(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(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(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 absl
#endif // ABSL_BASE_CALL_ONCE_H_
