Bug 1404742 - add ThreadSafeWeakPointer to MFBT. r=nfroyd

mfbt/ThreadSafeWeakPtr.h

@@ -0,0 +1,382 @@
+/* -*- 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/. */
+
+/* A thread-safe weak pointer */
+
+/**
+ * Derive from SupportsThreadSafeWeakPtr to allow thread-safe weak pointers to an
+ * atomically refcounted derived class. These thread-safe weak pointers may be safely
+ * accessed and converted to strong pointers on multiple threads.
+ *
+ * Note that SupportsThreadSafeWeakPtr necessarily already inherits from AtomicRefCounted,
+ * so you should not separately inherit from AtomicRefCounted.
+ *
+ * ThreadSafeWeakPtr and its implementation is distinct from the normal WeakPtr which is
+ * not thread-safe. The interface discipline and implementation details are different enough
+ * that these two implementations are separated for now for efficiency reasons. If you don't
+ * actually need to use weak pointers on multiple threads, you can just use WeakPtr instead.
+ *
+ * When deriving from SupportsThreadSafeWeakPtr, you should add
+ * MOZ_DECLARE_THREADSAFEWEAKREFERENCE_TYPENAME(ClassName) and
+ * MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public section of your class,
+ * where ClassName is the name of your class.
+ *
+ * Example usage:
+ *
+ *   class C : public SupportsThreadSafeWeakPtr<C>
+ *   {
+ *   public:
+ *     MOZ_DECLARE_THREADSAFEWEAKREFERENCE_TYPENAME(C)
+ *     MOZ_DECLARE_REFCOUNTED_TYPENAME(C)
+ *     void doStuff();
+ *   };
+ *
+ *   ThreadSafeWeakPtr<C> weak;
+ *   {
+ *     RefPtr<C> strong = new C;
+ *     if (strong) {
+ *       strong->doStuff();
+ *     }
+ *     // Make a new weak reference to the object from the strong reference.
+ *     weak = strong;
+ *   }
+ *   MOZ_ASSERT(!bool(weak), "Weak pointers are cleared after all strong references are released.");
+ *
+ *   // Convert the weak reference to a strong reference for usage.
+ *   RefPtr<C> other(weak);
+ *   if (other) {
+ *     other->doStuff();
+ *   }
+ */
+
+#ifndef mozilla_ThreadSafeWeakPtr_h
+#define mozilla_ThreadSafeWeakPtr_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/RefCounted.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TypeTraits.h"
+#include "mozilla/Unused.h"
+
+#include <limits>
+
+namespace mozilla {
+
+template<typename T> class ThreadSafeWeakPtr;
+template<typename T> class SupportsThreadSafeWeakPtr;
+
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+  #define MOZ_DECLARE_THREADSAFEWEAKREFERENCE_TYPENAME(T) \
+    static const char* threadSafeWeakReferenceTypeName() { return "ThreadSafeWeakReference<" #T ">"; }
+#else
+  #define MOZ_DECLARE_THREADSAFEWEAKREFERENCE_TYPENAME(T)
+#endif
+
+namespace detail {
+
+// A multiple reader, single writer spin-lock.
+// This lock maintains an atomic counter which is incremented every time the lock is acquired
+// reading. So long as the counter remains positive, it may be incremented for reading multiple
+// times. When acquiring the lock for writing, we must ensure the counter is 0 (no readers),
+// and if so, set it to a negative value to indicate that no new readers may take the lock.
+class ReadWriteSpinLock
+{
+  // Only need a type large enough to represent the number of simultaneously accessing threads.
+  typedef int32_t CounterType;
+
+public:
+  // Try to increment the counter for reading, so long as it is positive.
+  void readLock()
+  {
+    for (;;)
+    {
+      CounterType oldCounter = mCounter & std::numeric_limits<CounterType>::max();
+      CounterType newCounter = oldCounter + 1;
+      if (mCounter.compareExchange(oldCounter, newCounter)) {
+        break;
+      }
+    }
+  }
+
+  // Decrement the counter to remove a read lock.
+  void readUnlock()
+  {
+    mCounter--;
+  }
+
+  // Try to acquire the write lock, but only if there are no readers.
+  // If successful, sets the counter to a negative value.
+  bool tryWriteLock()
+  {
+    return mCounter.compareExchange(0, std::numeric_limits<CounterType>::min());
+  }
+
+  // Reset the counter to 0.
+  void writeUnlock()
+  {
+    mCounter = 0;
+  }
+
+private:
+  Atomic<CounterType> mCounter;
+};
+
+// A shared weak reference that is used to track a SupportsThreadSafeWeakPtr object.
+// It guards access to that object via a read-write spinlock.
+template<typename T>
+class ThreadSafeWeakReference : public external::AtomicRefCounted<ThreadSafeWeakReference<T>>
+{
+public:
+  typedef T ElementType;
+
+  explicit ThreadSafeWeakReference(T* aPtr)
+  {
+    mPtr = aPtr;
+  }
+
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+  const char* typeName() const
+  {
+    // The first time this is called mPtr is null, so don't
+    // invoke any methods on mPtr.
+    return T::threadSafeWeakReferenceTypeName();
+  }
+  size_t typeSize() const { return sizeof(*this); }
+#endif
+
+private:
+  friend class mozilla::SupportsThreadSafeWeakPtr<T>;
+  template<typename U> friend class mozilla::ThreadSafeWeakPtr;
+
+  // Does an unsafe read of the raw weak pointer.
+  T* get() const
+  {
+    return mPtr;
+  }
+
+  // Creates a new RefPtr to the tracked object.
+  // We need to acquire the read lock while we do this, as we need to atomically
+  // both read the pointer and then increment the refcount on it within the scope
+  // of the lock. This guards against the object being destroyed while in the middle
+  // of creating the new RefPtr.
+  already_AddRefed<T> getRefPtr()
+  {
+    mLock.readLock();
+    RefPtr<T> result(get());
+    mLock.readUnlock();
+    return result.forget();
+  }
+
+  // Try to detach the weak reference from the tracked object.
+  // We need to acquire the write lock while we do this, to ensure that no
+  // RefPtr is created to this while detaching. Once acquired, it is safe
+  // to check the refcount and verify that this is the last reference to
+  // the tracked object, so the weak reference can be safely detached.
+  void tryDetach(const SupportsThreadSafeWeakPtr<T>* aOwner)
+  {
+    if (mLock.tryWriteLock()) {
+      if (aOwner->hasOneRef()) {
+        mPtr = nullptr;
+      }
+      mLock.writeUnlock();
+    }
+  }
+
+  ReadWriteSpinLock mLock;
+  Atomic<T*> mPtr;
+};
+
+} // namespace detail
+
+template<typename T>
+class SupportsThreadSafeWeakPtr : public external::AtomicRefCounted<T>
+{
+protected:
+  typedef external::AtomicRefCounted<T> AtomicRefCounted;
+  typedef detail::ThreadSafeWeakReference<T> ThreadSafeWeakReference;
+
+public:
+  ~SupportsThreadSafeWeakPtr()
+  {
+    // Clean up the shared weak reference if one exists.
+    if (ThreadSafeWeakReference* ptr = mRef) {
+      ptr->Release();
+    }
+  }
+
+  void Release() const
+  {
+    // If there is only one remaining reference to the object when trying to release,
+    // then attempt to detach it from its weak reference. New references could possibly
+    // be created to the object while this happens, so take care to do this atomically
+    // inside tryDetach.
+    if (AtomicRefCounted::hasOneRef()) {
+      if (ThreadSafeWeakReference* ptr = mRef) {
+        ptr->tryDetach(this);
+      }
+    }
+
+    // Once possibly detached, it is now safe to continue to decrement the refcount.
+    AtomicRefCounted::Release();
+  }
+
+private:
+  template<typename U> friend class ThreadSafeWeakPtr;
+
+  // Creates a shared weak reference for the object if one does not exist. Note that the
+  // object may be of an actual derived type U, but the weak reference is created for the
+  // supplied type T of SupportsThreadSafeWeakPtr<T>.
+  already_AddRefed<ThreadSafeWeakReference> getThreadSafeWeakReference()
+  {
+    static_assert(IsBaseOf<SupportsThreadSafeWeakPtr<T>, T>::value,
+                  "T must derive from SupportsThreadSafeWeakPtr<T>");
+
+    if (!mRef) {
+      RefPtr<ThreadSafeWeakReference> ptr(new ThreadSafeWeakReference(static_cast<T*>(this)));
+      // Only set the new weak reference if one does not exist (== nullptr).
+      // If there is already a weak reference, just let this superflous weak reference get
+      // destroyed when it goes out of scope.
+      if (mRef.compareExchange(nullptr, ptr)) {
+        // If successful, forget the refcount so that the weak reference stays alive.
+        Unused << ptr.forget();
+      }
+    }
+
+    // Create a new RefPtr to weak reference.
+    RefPtr<ThreadSafeWeakReference> ptr(mRef);
+    return ptr.forget();
+  }
+
+  Atomic<ThreadSafeWeakReference*> mRef;
+};
+
+// A thread-safe variant of a weak pointer
+template<typename T>
+class ThreadSafeWeakPtr
+{
+  // Be careful to use the weak reference type T in the SupportsThreadSafeWeakPtr<T> definition.
+  typedef typename T::ThreadSafeWeakReference ThreadSafeWeakReference;
+
+public:
+  ThreadSafeWeakPtr()
+  {}
+
+  ThreadSafeWeakPtr& operator=(const ThreadSafeWeakPtr& aOther)
+  {
+    mRef = aOther.mRef;
+    return *this;
+  }
+
+  ThreadSafeWeakPtr(const ThreadSafeWeakPtr& aOther)
+    : mRef(aOther.mRef)
+  {
+  }
+
+  ThreadSafeWeakPtr& operator=(ThreadSafeWeakPtr&& aOther)
+  {
+    mRef = aOther.mRef.forget();
+    return *this;
+  }
+
+  ThreadSafeWeakPtr(ThreadSafeWeakPtr&& aOther)
+    : mRef(aOther.mRef.forget())
+  {
+  }
+
+  ThreadSafeWeakPtr& operator=(const RefPtr<T>& aOther)
+  {
+    if (aOther) {
+      // Get the underlying shared weak reference to the object, creating one if necessary.
+      mRef = aOther->getThreadSafeWeakReference();
+    } else {
+      mRef = nullptr;
+    }
+    return *this;
+  }
+
+  explicit ThreadSafeWeakPtr(const RefPtr<T>& aOther)
+  {
+    *this = aOther;
+  }
+
+  ThreadSafeWeakPtr& operator=(decltype(nullptr))
+  {
+    mRef = nullptr;
+    return *this;
+  }
+
+  explicit ThreadSafeWeakPtr(decltype(nullptr))
+  {}
+
+  explicit operator bool() const
+  {
+    return !!get();
+  }
+
+  bool operator==(const ThreadSafeWeakPtr& aOther) const
+  {
+    return get() == aOther.get();
+  }
+
+  bool operator==(const RefPtr<T>& aOther) const
+  {
+    return get() == aOther.get();
+  }
+
+  bool operator==(const T* aOther) const
+  {
+    return get() == aOther;
+  }
+
+  template<typename U>
+  bool operator!=(const U& aOther) const
+  {
+    return !(*this == aOther);
+  }
+
+  // Convert the weak pointer to a strong RefPtr.
+  explicit operator RefPtr<T>() const
+  {
+    return getRefPtr();
+  }
+
+private:
+  // Gets a new strong reference of the proper type T to the tracked object.
+  already_AddRefed<T> getRefPtr() const
+  {
+    static_assert(IsBaseOf<typename ThreadSafeWeakReference::ElementType, T>::value,
+                  "T must derive from ThreadSafeWeakReference::ElementType");
+    return mRef ? mRef->getRefPtr().template downcast<T>() : nullptr;
+  }
+
+  // Get a pointer to the tracked object, downcasting to the proper type T.
+  // Note that this operation is unsafe as it may cause races if downwind
+  // code depends on the value not to change after reading.
+  T* get() const
+  {
+    static_assert(IsBaseOf<typename ThreadSafeWeakReference::ElementType, T>::value,
+                  "T must derive from ThreadSafeWeakReference::ElementType");
+    return mRef ? static_cast<T*>(mRef->get()) : nullptr;
+  }
+
+  // A shared weak reference to an object. Note that this may be null so as to save memory
+  // (at the slight cost of an extra null check) if no object is being tracked.
+  RefPtr<ThreadSafeWeakReference> mRef;
+};
+
+} // namespace mozilla
+
+template<typename T>
+inline already_AddRefed<T>
+do_AddRef(const mozilla::ThreadSafeWeakPtr<T>& aObj)
+{
+  RefPtr<T> ref(aObj);
+  return ref.forget();
+}
+
+#endif /* mozilla_ThreadSafeWeakPtr_h */
+

mfbt/moz.build

@@ -91,6 +91,7 @@ EXPORTS.mozilla = [
     'TaggedAnonymousMemory.h',
     'TemplateLib.h',
     'ThreadLocal.h',
+    'ThreadSafeWeakPtr.h',
     'ToString.h',
     'Tuple.h',
     'TypedEnumBits.h',