1// Copyright (c) 2011 The Chromium Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style license that can be 3// found in the LICENSE file. 4 5#ifndef BASE_MEMORY_SINGLETON_H_ 6#define BASE_MEMORY_SINGLETON_H_ 7#pragma once 8 9#include "base/at_exit.h" 10#include "base/atomicops.h" 11#include "base/third_party/dynamic_annotations/dynamic_annotations.h" 12#include "base/threading/platform_thread.h" 13#include "base/threading/thread_restrictions.h" 14 15// Default traits for Singleton<Type>. Calls operator new and operator delete on 16// the object. Registers automatic deletion at process exit. 17// Overload if you need arguments or another memory allocation function. 18template<typename Type> 19struct DefaultSingletonTraits { 20 // Allocates the object. 21 static Type* New() { 22 // The parenthesis is very important here; it forces POD type 23 // initialization. 24 return new Type(); 25 } 26 27 // Destroys the object. 28 static void Delete(Type* x) { 29 delete x; 30 } 31 32 // Set to true to automatically register deletion of the object on process 33 // exit. See below for the required call that makes this happen. 34 static const bool kRegisterAtExit = true; 35 36 // Set to false to disallow access on a non-joinable thread. This is 37 // different from kRegisterAtExit because StaticMemorySingletonTraits allows 38 // access on non-joinable threads, and gracefully handles this. 39 static const bool kAllowedToAccessOnNonjoinableThread = false; 40}; 41 42 43// Alternate traits for use with the Singleton<Type>. Identical to 44// DefaultSingletonTraits except that the Singleton will not be cleaned up 45// at exit. 46template<typename Type> 47struct LeakySingletonTraits : public DefaultSingletonTraits<Type> { 48 static const bool kRegisterAtExit = false; 49 static const bool kAllowedToAccessOnNonjoinableThread = true; 50}; 51 52 53// Alternate traits for use with the Singleton<Type>. Allocates memory 54// for the singleton instance from a static buffer. The singleton will 55// be cleaned up at exit, but can't be revived after destruction unless 56// the Resurrect() method is called. 57// 58// This is useful for a certain category of things, notably logging and 59// tracing, where the singleton instance is of a type carefully constructed to 60// be safe to access post-destruction. 61// In logging and tracing you'll typically get stray calls at odd times, like 62// during static destruction, thread teardown and the like, and there's a 63// termination race on the heap-based singleton - e.g. if one thread calls 64// get(), but then another thread initiates AtExit processing, the first thread 65// may call into an object residing in unallocated memory. If the instance is 66// allocated from the data segment, then this is survivable. 67// 68// The destructor is to deallocate system resources, in this case to unregister 69// a callback the system will invoke when logging levels change. Note that 70// this is also used in e.g. Chrome Frame, where you have to allow for the 71// possibility of loading briefly into someone else's process space, and 72// so leaking is not an option, as that would sabotage the state of your host 73// process once you've unloaded. 74template <typename Type> 75struct StaticMemorySingletonTraits { 76 // WARNING: User has to deal with get() in the singleton class 77 // this is traits for returning NULL. 78 static Type* New() { 79 if (base::subtle::NoBarrier_AtomicExchange(&dead_, 1)) 80 return NULL; 81 Type* ptr = reinterpret_cast<Type*>(buffer_); 82 83 // We are protected by a memory barrier. 84 new(ptr) Type(); 85 return ptr; 86 } 87 88 static void Delete(Type* p) { 89 base::subtle::NoBarrier_Store(&dead_, 1); 90 base::subtle::MemoryBarrier(); 91 if (p != NULL) 92 p->Type::~Type(); 93 } 94 95 static const bool kRegisterAtExit = true; 96 static const bool kAllowedToAccessOnNonjoinableThread = true; 97 98 // Exposed for unittesting. 99 static void Resurrect() { 100 base::subtle::NoBarrier_Store(&dead_, 0); 101 } 102 103 private: 104 static const size_t kBufferSize = (sizeof(Type) + 105 sizeof(intptr_t) - 1) / sizeof(intptr_t); 106 static intptr_t buffer_[kBufferSize]; 107 108 // Signal the object was already deleted, so it is not revived. 109 static base::subtle::Atomic32 dead_; 110}; 111 112template <typename Type> intptr_t 113 StaticMemorySingletonTraits<Type>::buffer_[kBufferSize]; 114template <typename Type> base::subtle::Atomic32 115 StaticMemorySingletonTraits<Type>::dead_ = 0; 116 117// The Singleton<Type, Traits, DifferentiatingType> class manages a single 118// instance of Type which will be created on first use and will be destroyed at 119// normal process exit). The Trait::Delete function will not be called on 120// abnormal process exit. 121// 122// DifferentiatingType is used as a key to differentiate two different 123// singletons having the same memory allocation functions but serving a 124// different purpose. This is mainly used for Locks serving different purposes. 125// 126// Example usage: 127// 128// In your header: 129// #include "base/memory/singleton.h" 130// class FooClass { 131// public: 132// static FooClass* GetInstance(); <-- See comment below on this. 133// void Bar() { ... } 134// private: 135// FooClass() { ... } 136// friend struct DefaultSingletonTraits<FooClass>; 137// 138// DISALLOW_COPY_AND_ASSIGN(FooClass); 139// }; 140// 141// In your source file: 142// FooClass* FooClass::GetInstance() { 143// return Singleton<FooClass>::get(); 144// } 145// 146// And to call methods on FooClass: 147// FooClass::GetInstance()->Bar(); 148// 149// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance 150// and it is important that FooClass::GetInstance() is not inlined in the 151// header. This makes sure that when source files from multiple targets include 152// this header they don't end up with different copies of the inlined code 153// creating multiple copies of the singleton. 154// 155// Singleton<> has no non-static members and doesn't need to actually be 156// instantiated. 157// 158// This class is itself thread-safe. The underlying Type must of course be 159// thread-safe if you want to use it concurrently. Two parameters may be tuned 160// depending on the user's requirements. 161// 162// Glossary: 163// RAE = kRegisterAtExit 164// 165// On every platform, if Traits::RAE is true, the singleton will be destroyed at 166// process exit. More precisely it uses base::AtExitManager which requires an 167// object of this type to be instantiated. AtExitManager mimics the semantics 168// of atexit() such as LIFO order but under Windows is safer to call. For more 169// information see at_exit.h. 170// 171// If Traits::RAE is false, the singleton will not be freed at process exit, 172// thus the singleton will be leaked if it is ever accessed. Traits::RAE 173// shouldn't be false unless absolutely necessary. Remember that the heap where 174// the object is allocated may be destroyed by the CRT anyway. 175// 176// Caveats: 177// (a) Every call to get(), operator->() and operator*() incurs some overhead 178// (16ns on my P4/2.8GHz) to check whether the object has already been 179// initialized. You may wish to cache the result of get(); it will not 180// change. 181// 182// (b) Your factory function must never throw an exception. This class is not 183// exception-safe. 184// 185template <typename Type, 186 typename Traits = DefaultSingletonTraits<Type>, 187 typename DifferentiatingType = Type> 188class Singleton { 189 private: 190 // Classes using the Singleton<T> pattern should declare a GetInstance() 191 // method and call Singleton::get() from within that. 192 friend Type* Type::GetInstance(); 193 194 // This class is safe to be constructed and copy-constructed since it has no 195 // member. 196 197 // Return a pointer to the one true instance of the class. 198 static Type* get() { 199 if (!Traits::kAllowedToAccessOnNonjoinableThread) 200 base::ThreadRestrictions::AssertSingletonAllowed(); 201 202 // Our AtomicWord doubles as a spinlock, where a value of 203 // kBeingCreatedMarker means the spinlock is being held for creation. 204 static const base::subtle::AtomicWord kBeingCreatedMarker = 1; 205 206 base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_); 207 if (value != 0 && value != kBeingCreatedMarker) { 208 // See the corresponding HAPPENS_BEFORE below. 209 ANNOTATE_HAPPENS_AFTER(&instance_); 210 return reinterpret_cast<Type*>(value); 211 } 212 213 // Object isn't created yet, maybe we will get to create it, let's try... 214 if (base::subtle::Acquire_CompareAndSwap(&instance_, 215 0, 216 kBeingCreatedMarker) == 0) { 217 // instance_ was NULL and is now kBeingCreatedMarker. Only one thread 218 // will ever get here. Threads might be spinning on us, and they will 219 // stop right after we do this store. 220 Type* newval = Traits::New(); 221 222 // This annotation helps race detectors recognize correct lock-less 223 // synchronization between different threads calling get(). 224 // See the corresponding HAPPENS_AFTER below and above. 225 ANNOTATE_HAPPENS_BEFORE(&instance_); 226 base::subtle::Release_Store( 227 &instance_, reinterpret_cast<base::subtle::AtomicWord>(newval)); 228 229 if (newval != NULL && Traits::kRegisterAtExit) 230 base::AtExitManager::RegisterCallback(OnExit, NULL); 231 232 return newval; 233 } 234 235 // We hit a race. Another thread beat us and either: 236 // - Has the object in BeingCreated state 237 // - Already has the object created... 238 // We know value != NULL. It could be kBeingCreatedMarker, or a valid ptr. 239 // Unless your constructor can be very time consuming, it is very unlikely 240 // to hit this race. When it does, we just spin and yield the thread until 241 // the object has been created. 242 while (true) { 243 value = base::subtle::NoBarrier_Load(&instance_); 244 if (value != kBeingCreatedMarker) 245 break; 246 base::PlatformThread::YieldCurrentThread(); 247 } 248 249 // See the corresponding HAPPENS_BEFORE above. 250 ANNOTATE_HAPPENS_AFTER(&instance_); 251 return reinterpret_cast<Type*>(value); 252 } 253 254 // Adapter function for use with AtExit(). This should be called single 255 // threaded, so don't use atomic operations. 256 // Calling OnExit while singleton is in use by other threads is a mistake. 257 static void OnExit(void* /*unused*/) { 258 // AtExit should only ever be register after the singleton instance was 259 // created. We should only ever get here with a valid instance_ pointer. 260 Traits::Delete( 261 reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_))); 262 instance_ = 0; 263 } 264 static base::subtle::AtomicWord instance_; 265}; 266 267template <typename Type, typename Traits, typename DifferentiatingType> 268base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>:: 269 instance_ = 0; 270 271#endif // BASE_MEMORY_SINGLETON_H_ 272