mfbt/RefCounted.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* CRTP refcounting templates.  Do not use unless you are an Expert. */
   8
   9 #ifndef mozilla_RefCounted_h
  10 #define mozilla_RefCounted_h
  11
  12 #include <utility>
  13
  14 #include "mozilla/AlreadyAddRefed.h"
  15 #include "mozilla/Assertions.h"
  16 #include "mozilla/Atomics.h"
  17 #include "mozilla/Attributes.h"
  18 #include "mozilla/RefCountType.h"
  19
  20 #ifdef __wasi__
  21 #  include "mozilla/WasiAtomic.h"
  22 #else
  23 #  include <atomic>
  24 #endif  // __wasi__
  25
  26 #if defined(MOZILLA_INTERNAL_API)
  27 #  include "nsXPCOM.h"
  28 #endif
  29
  30 #if defined(MOZILLA_INTERNAL_API) && defined(NS_BUILD_REFCNT_LOGGING)
  31 #  define MOZ_REFCOUNTED_LEAK_CHECKING
  32 #endif
  33
  34 namespace mozilla {
  35
  36 /**
  37  * RefCounted<T> is a sort of a "mixin" for a class T.  RefCounted
  38  * manages, well, refcounting for T, and because RefCounted is
  39  * parameterized on T, RefCounted<T> can call T's destructor directly.
  40  * This means T doesn't need to have a virtual dtor and so doesn't
  41  * need a vtable.
  42  *
  43  * RefCounted<T> is created with refcount == 0.  Newly-allocated
  44  * RefCounted<T> must immediately be assigned to a RefPtr to make the
  45  * refcount > 0.  It's an error to allocate and free a bare
  46  * RefCounted<T>, i.e. outside of the RefPtr machinery.  Attempts to
  47  * do so will abort DEBUG builds.
  48  *
  49  * Live RefCounted<T> have refcount > 0.  The lifetime (refcounts) of
  50  * live RefCounted<T> are controlled by RefPtr<T> and
  51  * RefPtr<super/subclass of T>.  Upon a transition from refcounted==1
  52  * to 0, the RefCounted<T> "dies" and is destroyed.  The "destroyed"
  53  * state is represented in DEBUG builds by refcount==0xffffdead.  This
  54  * state distinguishes use-before-ref (refcount==0) from
  55  * use-after-destroy (refcount==0xffffdead).
  56  *
  57  * Note that when deriving from RefCounted or AtomicRefCounted, you
  58  * should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public
  59  * section of your class, where ClassName is the name of your class.
  60  *
  61  * Note: SpiderMonkey should use js::RefCounted instead since that type
  62  * will use appropriate js_delete and also not break ref-count logging.
  63  */
  64 namespace detail {
  65 const MozRefCountType DEAD = 0xffffdead;
  66
  67 // When building code that gets compiled into Gecko, try to use the
  68 // trace-refcount leak logging facilities.
  69 class RefCountLogger {
  70  public:
  71   // Called by `RefCounted`-like classes to log a successful AddRef call in the
  72   // Gecko leak-logging system. This call is a no-op outside of Gecko. Should be
  73   // called afer incrementing the reference count.
  74   template <class T>
  75   static void logAddRef(const T* aPointer, MozRefCountType aRefCount) {
  76 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
  77     const void* pointer = aPointer;
  78     const char* typeName = aPointer->typeName();
  79     uint32_t typeSize = aPointer->typeSize();
  80     NS_LogAddRef(const_cast<void*>(pointer), aRefCount, typeName, typeSize);
  81 #endif
  82   }
  83
  84   // Created by `RefCounted`-like classes to log a successful Release call in
  85   // the Gecko leak-logging system. The constructor should be invoked before the
  86   // refcount is decremented to avoid invoking `typeName()` with a zero
  87   // reference count. This call is a no-op outside of Gecko.
  88   class MOZ_STACK_CLASS ReleaseLogger final {
  89    public:
  90     template <class T>
  91     explicit ReleaseLogger(const T* aPointer)
  92 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
  93         : mPointer(aPointer),
  94           mTypeName(aPointer->typeName())
  95 #endif
  96     {
  97     }
  98
  99     void logRelease(MozRefCountType aRefCount) {
 100 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
 101       MOZ_ASSERT(aRefCount != DEAD);
 102       NS_LogRelease(const_cast<void*>(mPointer), aRefCount, mTypeName);
 103 #endif
 104     }
 105
 106 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
 107     const void* mPointer;
 108     const char* mTypeName;
 109 #endif
 110   };
 111 };
 112
 113 // This is used WeakPtr.h as well as this file.
 114 enum RefCountAtomicity { AtomicRefCount, NonAtomicRefCount };
 115
 116 template <typename T, RefCountAtomicity Atomicity>
 117 class RC {
 118  public:
 119   explicit RC(T aCount) : mValue(aCount) {}
 120
 121   RC(const RC&) = delete;
 122   RC& operator=(const RC&) = delete;
 123   RC(RC&&) = delete;
 124   RC& operator=(RC&&) = delete;
 125
 126   T operator++() { return ++mValue; }
 127   T operator--() { return --mValue; }
 128
 129 #ifdef DEBUG
 130   void operator=(const T& aValue) { mValue = aValue; }
 131 #endif
 132
 133   operator T() const { return mValue; }
 134
 135  private:
 136   T mValue;
 137 };
 138
 139 template <typename T>
 140 class RC<T, AtomicRefCount> {
 141  public:
 142   explicit RC(T aCount) : mValue(aCount) {}
 143
 144   RC(const RC&) = delete;
 145   RC& operator=(const RC&) = delete;
 146   RC(RC&&) = delete;
 147   RC& operator=(RC&&) = delete;
 148
 149   T operator++() {
 150     // Memory synchronization is not required when incrementing a
 151     // reference count.  The first increment of a reference count on a
 152     // thread is not important, since the first use of the object on a
 153     // thread can happen before it.  What is important is the transfer
 154     // of the pointer to that thread, which may happen prior to the
 155     // first increment on that thread.  The necessary memory
 156     // synchronization is done by the mechanism that transfers the
 157     // pointer between threads.
 158     return mValue.fetch_add(1, std::memory_order_relaxed) + 1;
 159   }
 160
 161   T operator--() {
 162     // Since this may be the last release on this thread, we need
 163     // release semantics so that prior writes on this thread are visible
 164     // to the thread that destroys the object when it reads mValue with
 165     // acquire semantics.
 166     T result = mValue.fetch_sub(1, std::memory_order_release) - 1;
 167     if (result == 0) {
 168       // We're going to destroy the object on this thread, so we need
 169       // acquire semantics to synchronize with the memory released by
 170       // the last release on other threads, that is, to ensure that
 171       // writes prior to that release are now visible on this thread.
 172 #if defined(MOZ_TSAN) || defined(__wasi__)
 173       // TSan doesn't understand std::atomic_thread_fence, so in order
 174       // to avoid a false positive for every time a refcounted object
 175       // is deleted, we replace the fence with an atomic operation.
 176       mValue.load(std::memory_order_acquire);
 177 #else
 178       std::atomic_thread_fence(std::memory_order_acquire);
 179 #endif
 180     }
 181     return result;
 182   }
 183
 184 #ifdef DEBUG
 185   // This method is only called in debug builds, so we're not too concerned
 186   // about its performance.
 187   void operator=(const T& aValue) {
 188     mValue.store(aValue, std::memory_order_seq_cst);
 189   }
 190 #endif
 191
 192   operator T() const {
 193     // Use acquire semantics since we're not sure what the caller is
 194     // doing.
 195     return mValue.load(std::memory_order_acquire);
 196   }
 197
 198   T IncrementIfNonzero() {
 199     // This can be a relaxed load as any write of 0 that we observe will leave
 200     // the field in a permanently zero (or `DEAD`) state (so a "stale" read of 0
 201     // is fine), and any other value is confirmed by the CAS below.
 202     //
 203     // This roughly matches rust's Arc::upgrade implementation as of rust 1.49.0
 204     T prev = mValue.load(std::memory_order_relaxed);
 205     while (prev != 0) {
 206       MOZ_ASSERT(prev != detail::DEAD,
 207                  "Cannot IncrementIfNonzero if marked as dead!");
 208       // TODO: It may be possible to use relaxed success ordering here?
 209       if (mValue.compare_exchange_weak(prev, prev + 1,
 210                                        std::memory_order_acquire,
 211                                        std::memory_order_relaxed)) {
 212         return prev + 1;
 213       }
 214     }
 215     return 0;
 216   }
 217
 218  private:
 219   std::atomic<T> mValue;
 220 };
 221
 222 template <typename T, RefCountAtomicity Atomicity>
 223 class RefCounted {
 224  protected:
 225   RefCounted() : mRefCnt(0) {}
 226 #ifdef DEBUG
 227   ~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }
 228 #endif
 229
 230  public:
 231   // Compatibility with RefPtr.
 232   void AddRef() const {
 233     // Note: this method must be thread safe for AtomicRefCounted.
 234     MOZ_ASSERT(int32_t(mRefCnt) >= 0);
 235     MozRefCountType cnt = ++mRefCnt;
 236     detail::RefCountLogger::logAddRef(static_cast<const T*>(this), cnt);
 237   }
 238
 239   void Release() const {
 240     // Note: this method must be thread safe for AtomicRefCounted.
 241     MOZ_ASSERT(int32_t(mRefCnt) > 0);
 242     detail::RefCountLogger::ReleaseLogger logger(static_cast<const T*>(this));
 243     MozRefCountType cnt = --mRefCnt;
 244     // Note: it's not safe to touch |this| after decrementing the refcount,
 245     // except for below.
 246     logger.logRelease(cnt);
 247     if (0 == cnt) {
 248       // Because we have atomically decremented the refcount above, only
 249       // one thread can get a 0 count here, so as long as we can assume that
 250       // everything else in the system is accessing this object through
 251       // RefPtrs, it's safe to access |this| here.
 252 #ifdef DEBUG
 253       mRefCnt = detail::DEAD;
 254 #endif
 255       delete static_cast<const T*>(this);
 256     }
 257   }
 258
 259   // Compatibility with wtf::RefPtr.
 260   void ref() { AddRef(); }
 261   void deref() { Release(); }
 262   MozRefCountType refCount() const { return mRefCnt; }
 263   bool hasOneRef() const {
 264     MOZ_ASSERT(mRefCnt > 0);
 265     return mRefCnt == 1;
 266   }
 267
 268  private:
 269   mutable RC<MozRefCountType, Atomicity> mRefCnt;
 270 };
 271
 272 #ifdef MOZ_REFCOUNTED_LEAK_CHECKING
 273 // Passing override for the optional argument marks the typeName and
 274 // typeSize functions defined by this macro as overrides.
 275 #  define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)           \
 276     virtual const char* typeName() const __VA_ARGS__ { return #T; } \
 277     virtual size_t typeSize() const __VA_ARGS__ { return sizeof(*this); }
 278 #else
 279 #  define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)
 280 #endif
 281
 282 // Note that this macro is expanded unconditionally because it declares only
 283 // two small inline functions which will hopefully get eliminated by the linker
 284 // in non-leak-checking builds.
 285 #define MOZ_DECLARE_REFCOUNTED_TYPENAME(T)    \
 286   const char* typeName() const { return #T; } \
 287   size_t typeSize() const { return sizeof(*this); }
 288
 289 }  // namespace detail
 290
 291 template <typename T>
 292 class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount> {
 293  public:
 294   ~RefCounted() {
 295     static_assert(std::is_base_of<RefCounted, T>::value,
 296                   "T must derive from RefCounted<T>");
 297   }
 298 };
 299
 300 namespace external {
 301
 302 /**
 303  * AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated
 304  * reference counter.
 305  *
 306  * NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING
 307  * instead.
 308  */
 309 template <typename T>
 310 class AtomicRefCounted
 311     : public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount> {
 312  public:
 313   ~AtomicRefCounted() {
 314     static_assert(std::is_base_of<AtomicRefCounted, T>::value,
 315                   "T must derive from AtomicRefCounted<T>");
 316   }
 317 };
 318
 319 }  // namespace external
 320
 321 }  // namespace mozilla
 322
 323 #endif  // mozilla_RefCounted_h