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rocksdb/util/thread_local.h

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <atomic>
#include <memory>
#include <unordered_map>
#include <vector>
#include "util/autovector.h"
#include "port/port.h"
#ifndef ROCKSDB_SUPPORT_THREAD_LOCAL
#define ROCKSDB_SUPPORT_THREAD_LOCAL \
!defined(OS_WIN) && !defined(OS_MACOSX) && !defined(IOS_CROSS_COMPILE)
#endif
namespace rocksdb {
// Cleanup function that will be called for a stored thread local
// pointer (if not NULL) when one of the following happens:
// (1) a thread terminates
// (2) a ThreadLocalPtr is destroyed
typedef void (*UnrefHandler)(void* ptr);
// ThreadLocalPtr stores only values of pointer type. Different from
// the usual thread-local-storage, ThreadLocalPtr has the ability to
// distinguish data coming from different threads and different
// ThreadLocalPtr instances. For example, if a regular thread_local
// variable A is declared in DBImpl, two DBImpl objects would share
// the same A. However, a ThreadLocalPtr that is defined under the
// scope of DBImpl can avoid such confliction. As a result, its memory
// usage would be O(# of threads * # of ThreadLocalPtr instances).
class ThreadLocalPtr {
public:
explicit ThreadLocalPtr(UnrefHandler handler = nullptr);
~ThreadLocalPtr();
// Return the current pointer stored in thread local
void* Get() const;
// Set a new pointer value to the thread local storage.
void Reset(void* ptr);
// Atomically swap the supplied ptr and return the previous value
void* Swap(void* ptr);
// Atomically compare the stored value with expected. Set the new
// pointer value to thread local only if the comparison is true.
// Otherwise, expected returns the stored value.
// Return true on success, false on failure
bool CompareAndSwap(void* ptr, void*& expected);
// Reset all thread local data to replacement, and return non-nullptr
// data for all existing threads
void Scrape(autovector<void*>* ptrs, void* const replacement);
// Initialize the static singletons of the ThreadLocalPtr.
//
// If this function is not called, then the singletons will be
// automatically initialized when they are used.
//
// Calling this function twice or after the singletons have been
// initialized will be no-op.
static void InitSingletons();
protected:
struct Entry {
Entry() : ptr(nullptr) {}
Entry(const Entry& e) : ptr(e.ptr.load(std::memory_order_relaxed)) {}
std::atomic<void*> ptr;
};
// This is the structure that is declared as "thread_local" storage.
// The vector keep list of atomic pointer for all instances for "current"
// thread. The vector is indexed by an Id that is unique in process and
// associated with one ThreadLocalPtr instance. The Id is assigned by a
// global StaticMeta singleton. So if we instantiated 3 ThreadLocalPtr
// instances, each thread will have a ThreadData with a vector of size 3:
// ---------------------------------------------------
// | | instance 1 | instance 2 | instnace 3 |
// ---------------------------------------------------
// | thread 1 | void* | void* | void* | <- ThreadData
// ---------------------------------------------------
// | thread 2 | void* | void* | void* | <- ThreadData
// ---------------------------------------------------
// | thread 3 | void* | void* | void* | <- ThreadData
// ---------------------------------------------------
struct ThreadData {
ThreadData() : entries() {}
std::vector<Entry> entries;
ThreadData* next;
ThreadData* prev;
};
class StaticMeta {
public:
StaticMeta();
// Return the next available Id
uint32_t GetId();
// Return the next available Id without claiming it
uint32_t PeekId() const;
// Return the given Id back to the free pool. This also triggers
// UnrefHandler for associated pointer value (if not NULL) for all threads.
void ReclaimId(uint32_t id);
// Return the pointer value for the given id for the current thread.
void* Get(uint32_t id) const;
// Reset the pointer value for the given id for the current thread.
// It triggers UnrefHanlder if the id has existing pointer value.
void Reset(uint32_t id, void* ptr);
// Atomically swap the supplied ptr and return the previous value
void* Swap(uint32_t id, void* ptr);
// Atomically compare and swap the provided value only if it equals
// to expected value.
bool CompareAndSwap(uint32_t id, void* ptr, void*& expected);
// Reset all thread local data to replacement, and return non-nullptr
// data for all existing threads
void Scrape(uint32_t id, autovector<void*>* ptrs, void* const replacement);
// Register the UnrefHandler for id
void SetHandler(uint32_t id, UnrefHandler handler);
// Initialize all the singletons associated with StaticMeta.
//
// If this function is not called, then the singletons will be
// automatically initialized when they are used.
//
// Calling this function twice or after the singletons have been
// initialized will be no-op.
static void InitSingletons();
private:
// Get UnrefHandler for id with acquiring mutex
// REQUIRES: mutex locked
UnrefHandler GetHandler(uint32_t id);
// Triggered before a thread terminates
static void OnThreadExit(void* ptr);
// Add current thread's ThreadData to the global chain
// REQUIRES: mutex locked
void AddThreadData(ThreadData* d);
// Remove current thread's ThreadData from the global chain
// REQUIRES: mutex locked
void RemoveThreadData(ThreadData* d);
static ThreadData* GetThreadLocal();
uint32_t next_instance_id_;
// Used to recycle Ids in case ThreadLocalPtr is instantiated and destroyed
// frequently. This also prevents it from blowing up the vector space.
autovector<uint32_t> free_instance_ids_;
// Chain all thread local structure together. This is necessary since
// when one ThreadLocalPtr gets destroyed, we need to loop over each
// thread's version of pointer corresponding to that instance and
// call UnrefHandler for it.
ThreadData head_;
std::unordered_map<uint32_t, UnrefHandler> handler_map_;
// protect inst, next_instance_id_, free_instance_ids_, head_,
// ThreadData.entries
//
// Note that here we prefer function static variable instead of the usual
// global static variable. The reason is that c++ destruction order of
// static variables in the reverse order of their construction order.
// However, C++ does not guarantee any construction order when global
// static variables are defined in different files, while the function
// static variables are initialized when their function are first called.
// As a result, the construction order of the function static variables
// can be controlled by properly invoke their first function calls in
// the right order.
//
// For instance, the following function contains a function static
// variable. We place a dummy function call of this inside
// Env::Default() to ensure the construction order of the construction
// order.
static port::Mutex* Mutex();
#if ROCKSDB_SUPPORT_THREAD_LOCAL
// Thread local storage
static __thread ThreadData* tls_;
#endif
// Used to make thread exit trigger possible if !defined(OS_MACOSX).
// Otherwise, used to retrieve thread data.
pthread_key_t pthread_key_;
};
static StaticMeta* Instance();
const uint32_t id_;
};
} // namespace rocksdb