mfbt/RefCounted.h
author James Graham <james@hoppipolla.co.uk>
Mon, 21 May 2018 11:37:35 +0100
changeset 1555220 4c1f3fe87bc0049ab9f4138994d3c10d1e4a8514
parent 1334111 cde4f36e7c2372a61bab8ac0709febe754897bb1
child 1336103 2f3379c2229ed7aece95604692004e0c7db2f3d3
child 1336492 46ef8bf6c68664b27953e4680747da7a985c30b2
child 1337085 cde6a955130a186c679dce1a6be1838c59d24f54
child 1339214 b656faee9e138198ecf787fba317de5a6fe5a788
child 1343097 98e2b4ee9775a2a2b5cc1bb8a390bfe901814f38
child 1398386 80ed235cff6a319ed3171c949f352a5d6e41726e
child 1429249 90f5cc4128a22e072d42666fa2861cf6a461f7f8
child 1439387 bdeb90147ea2810041bc066dc608122e52ba3992
permissions -rw-r--r--
Bug 1354232 - Add support for LSAN to mozlog, r=ahal, mccr8 This adds two new actions to mozlog, one for reporting an LSAN failure, and one for reporting the summary. MozReview-Commit-ID: D7ep27SrI1n

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* CRTP refcounting templates.  Do not use unless you are an Expert. */

#ifndef mozilla_RefCounted_h
#define mozilla_RefCounted_h

#include "mozilla/AlreadyAddRefed.h"
#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"
#include "mozilla/Attributes.h"
#include "mozilla/Move.h"
#include "mozilla/RefCountType.h"
#include "mozilla/TypeTraits.h"

#include <atomic>

#if defined(MOZILLA_INTERNAL_API)
#include "nsXPCOM.h"
#endif

#if defined(MOZILLA_INTERNAL_API) && \
    (defined(DEBUG) || defined(FORCE_BUILD_REFCNT_LOGGING))
#define MOZ_REFCOUNTED_LEAK_CHECKING
#endif

namespace mozilla {

/**
 * RefCounted<T> is a sort of a "mixin" for a class T.  RefCounted
 * manages, well, refcounting for T, and because RefCounted is
 * parameterized on T, RefCounted<T> can call T's destructor directly.
 * This means T doesn't need to have a virtual dtor and so doesn't
 * need a vtable.
 *
 * RefCounted<T> is created with refcount == 0.  Newly-allocated
 * RefCounted<T> must immediately be assigned to a RefPtr to make the
 * refcount > 0.  It's an error to allocate and free a bare
 * RefCounted<T>, i.e. outside of the RefPtr machinery.  Attempts to
 * do so will abort DEBUG builds.
 *
 * Live RefCounted<T> have refcount > 0.  The lifetime (refcounts) of
 * live RefCounted<T> are controlled by RefPtr<T> and
 * RefPtr<super/subclass of T>.  Upon a transition from refcounted==1
 * to 0, the RefCounted<T> "dies" and is destroyed.  The "destroyed"
 * state is represented in DEBUG builds by refcount==0xffffdead.  This
 * state distinguishes use-before-ref (refcount==0) from
 * use-after-destroy (refcount==0xffffdead).
 *
 * Note that when deriving from RefCounted or AtomicRefCounted, you
 * should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public
 * section of your class, where ClassName is the name of your class.
 *
 * Note: SpiderMonkey should use js::RefCounted instead since that type
 * will use appropriate js_delete and also not break ref-count logging.
 */
namespace detail {
const MozRefCountType DEAD = 0xffffdead;

// When building code that gets compiled into Gecko, try to use the
// trace-refcount leak logging facilities.
#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
class RefCountLogger
{
public:
  static void logAddRef(const void* aPointer, MozRefCountType aRefCount,
                        const char* aTypeName, uint32_t aInstanceSize)
  {
    MOZ_ASSERT(aRefCount != DEAD);
    NS_LogAddRef(const_cast<void*>(aPointer), aRefCount, aTypeName,
                 aInstanceSize);
  }

  static void logRelease(const void* aPointer, MozRefCountType aRefCount,
                         const char* aTypeName)
  {
    MOZ_ASSERT(aRefCount != DEAD);
    NS_LogRelease(const_cast<void*>(aPointer), aRefCount, aTypeName);
  }
};
#endif

// This is used WeakPtr.h as well as this file.
enum RefCountAtomicity
{
  AtomicRefCount,
  NonAtomicRefCount
};

template<typename T, RefCountAtomicity Atomicity>
class RC
{
public:
  explicit RC(T aCount) : mValue(aCount) {}

  T operator++() { return ++mValue; }
  T operator--() { return --mValue; }

  void operator=(const T& aValue) { mValue = aValue; }

  operator T() const { return mValue; }

private:
  T mValue;
};

template<typename T>
class RC<T, AtomicRefCount>
{
public:
  explicit RC(T aCount) : mValue(aCount) {}

  T operator++()
  {
    // Memory synchronization is not required when incrementing a
    // reference count.  The first increment of a reference count on a
    // thread is not important, since the first use of the object on a
    // thread can happen before it.  What is important is the transfer
    // of the pointer to that thread, which may happen prior to the
    // first increment on that thread.  The necessary memory
    // synchronization is done by the mechanism that transfers the
    // pointer between threads.
    return mValue.fetch_add(1, std::memory_order_relaxed) + 1;
  }

  T operator--()
  {
    // Since this may be the last release on this thread, we need
    // release semantics so that prior writes on this thread are visible
    // to the thread that destroys the object when it reads mValue with
    // acquire semantics.
    T result = mValue.fetch_sub(1, std::memory_order_release) - 1;
    if (result == 0) {
      // We're going to destroy the object on this thread, so we need
      // acquire semantics to synchronize with the memory released by
      // the last release on other threads, that is, to ensure that
      // writes prior to that release are now visible on this thread.
      std::atomic_thread_fence(std::memory_order_acquire);
    }
    return result;
  }

  // This method is only called in debug builds, so we're not too concerned
  // about its performance.
  void operator=(const T& aValue) { mValue.store(aValue, std::memory_order_seq_cst); }

  operator T() const
  {
    // Use acquire semantics since we're not sure what the caller is
    // doing.
    return mValue.load(std::memory_order_acquire);
  }

private:
  std::atomic<T> mValue;
};

template<typename T, RefCountAtomicity Atomicity>
class RefCounted
{
protected:
  RefCounted() : mRefCnt(0) {}
  ~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }

public:
  // Compatibility with nsRefPtr.
  void AddRef() const
  {
    // Note: this method must be thread safe for AtomicRefCounted.
    MOZ_ASSERT(int32_t(mRefCnt) >= 0);
#ifndef MOZ_REFCOUNTED_LEAK_CHECKING
    ++mRefCnt;
#else
    const char* type = static_cast<const T*>(this)->typeName();
    uint32_t size = static_cast<const T*>(this)->typeSize();
    const void* ptr = static_cast<const T*>(this);
    MozRefCountType cnt = ++mRefCnt;
    detail::RefCountLogger::logAddRef(ptr, cnt, type, size);
#endif
  }

  void Release() const
  {
    // Note: this method must be thread safe for AtomicRefCounted.
    MOZ_ASSERT(int32_t(mRefCnt) > 0);
#ifndef MOZ_REFCOUNTED_LEAK_CHECKING
    MozRefCountType cnt = --mRefCnt;
#else
    const char* type = static_cast<const T*>(this)->typeName();
    const void* ptr = static_cast<const T*>(this);
    MozRefCountType cnt = --mRefCnt;
    // Note: it's not safe to touch |this| after decrementing the refcount,
    // except for below.
    detail::RefCountLogger::logRelease(ptr, cnt, type);
#endif
    if (0 == cnt) {
      // Because we have atomically decremented the refcount above, only
      // one thread can get a 0 count here, so as long as we can assume that
      // everything else in the system is accessing this object through
      // RefPtrs, it's safe to access |this| here.
#ifdef DEBUG
      mRefCnt = detail::DEAD;
#endif
      delete static_cast<const T*>(this);
    }
  }

  // Compatibility with wtf::RefPtr.
  void ref() { AddRef(); }
  void deref() { Release(); }
  MozRefCountType refCount() const { return mRefCnt; }
  bool hasOneRef() const
  {
    MOZ_ASSERT(mRefCnt > 0);
    return mRefCnt == 1;
  }

private:
  mutable RC<MozRefCountType, Atomicity> mRefCnt;
};

#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
// Passing override for the optional argument marks the typeName and
// typeSize functions defined by this macro as overrides.
#define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...) \
  virtual const char* typeName() const __VA_ARGS__ { return #T; } \
  virtual size_t typeSize() const __VA_ARGS__ { return sizeof(*this); }
#else
#define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)
#endif

// Note that this macro is expanded unconditionally because it declares only
// two small inline functions which will hopefully get eliminated by the linker
// in non-leak-checking builds.
#define MOZ_DECLARE_REFCOUNTED_TYPENAME(T) \
  const char* typeName() const { return #T; } \
  size_t typeSize() const { return sizeof(*this); }

} // namespace detail

template<typename T>
class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount>
{
public:
  ~RefCounted()
  {
    static_assert(IsBaseOf<RefCounted, T>::value,
                  "T must derive from RefCounted<T>");
  }
};

namespace external {

/**
 * AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated
 * reference counter.
 *
 * NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING
 * instead.
 */
template<typename T>
class AtomicRefCounted :
  public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount>
{
public:
  ~AtomicRefCounted()
  {
    static_assert(IsBaseOf<AtomicRefCounted, T>::value,
                  "T must derive from AtomicRefCounted<T>");
  }
};

} // namespace external

} // namespace mozilla

#endif // mozilla_RefCounted_h