folly/folly/synchronization/detail/ThreadCachedReaders.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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
 *
 *     http://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.
 */

#pragma once

#include <atomic>
#include <cstdint>
#include <limits>
#include <folly/Function.h>
#include <folly/ThreadLocal.h>
#include <folly/synchronization/AsymmetricThreadFence.h>
#include <folly/synchronization/RelaxedAtomic.h>
#include <folly/synchronization/detail/ThreadCachedTag.h>

namespace folly {

namespace detail {

// Use memory_order_seq_cst for accesses to increments/decrements if we're
// running under TSAN, because TSAN ignores barriers completely.
template <bool>
struct thread_cached_readers_atomic_;
template <>
struct thread_cached_readers_atomic_<false> { … };
template <>
struct thread_cached_readers_atomic_<true> { … };
thread_cached_readers_atomic;

// A data structure that keeps a per-thread cache of a bitfield that contains
// the current active epoch for readers in the thread, and the number of active
// readers:
//
//                _______________________________________
//                |   Current Epoch    |    # Readers   |
// epoch_readers: | 63 62 ... 34 33 32 | 31 30 ... 2 1 0|
//                o--------------------|----------------o
//
// There are several important implications with this data structure:
//
// 1. Read regions must be entered and exited on the same thread.
// 2. Read regions are fully nested. That is, two read regions in a single
//    thread may not overlap across two epochs.
//
// These implications afford us debugging opportunities, such
// as being able to detect long-running readers (T113951078).
class ThreadCachedReaders { … };

} // namespace detail
} // namespace folly