sequence_lock.h | Explore in Territory

//
// Copyright 2020 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.

#ifndef ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_
#define ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_

#include <stddef.h>
#include <stdint.h>

#include <atomic>
#include <cassert>
#include <cstring>

#include "absl/base/optimization.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace flags_internal {

// Align 'x' up to the nearest 'align' bytes.
inline constexpr size_t AlignUp(size_t x, size_t align) { … }

// A SequenceLock implements lock-free reads. A sequence counter is incremented
// before and after each write, and readers access the counter before and after
// accessing the protected data. If the counter is verified to not change during
// the access, and the sequence counter value was even, then the reader knows
// that the read was race-free and valid. Otherwise, the reader must fall back
// to a Mutex-based code path.
//
// This particular SequenceLock starts in an "uninitialized" state in which
// TryRead() returns false. It must be enabled by calling MarkInitialized().
// This serves as a marker that the associated flag value has not yet been
// initialized and a slow path needs to be taken.
//
// The memory reads and writes protected by this lock must use the provided
// `TryRead()` and `Write()` functions. These functions behave similarly to
// `memcpy()`, with one oddity: the protected data must be an array of
// `std::atomic<uint64>`. This is to comply with the C++ standard, which
// considers data races on non-atomic objects to be undefined behavior. See "Can
// Seqlocks Get Along With Programming Language Memory Models?"[1] by Hans J.
// Boehm for more details.
//
// [1] https://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf
class SequenceLock { … };

}  // namespace flags_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_
chromium/third_party/abseil-cpp/absl/flags/internal/sequence_lock.h
