wtf/CrossThreadRefCounted.h

   1 /*
   2  * Copyright (C) 2009 Google Inc. All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are
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   7  *
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   9  * notice, this list of conditions and the following disclaimer.
  10  *     * Redistributions in binary form must reproduce the above
  11  * copyright notice, this list of conditions and the following disclaimer
  12  * in the documentation and/or other materials provided with the
  13  * distribution.
  14  *     * Neither the name of Google Inc. nor the names of its
  15  * contributors may be used to endorse or promote products derived from
  16  * this software without specific prior written permission.
  17  *
  18  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  19  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  20  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  21  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  22  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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  28  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  29  */
  30
  31 #ifndef CrossThreadRefCounted_h
  32 #define CrossThreadRefCounted_h
  33
  34 #include <wtf/Noncopyable.h>
  35 #include <wtf/PassRefPtr.h>
  36 #include <wtf/RefCounted.h>
  37 #include <wtf/Threading.h>
  38
  39 #include <wtf/iphone/WebCoreThread.h>
  40
  41 namespace WTF {
  42
  43     // Used to allowing sharing data across classes and threads (like ThreadedSafeShared).
  44     //
  45     // Why not just use ThreadSafeShared?
  46     // ThreadSafeShared can have a significant perf impact when used in low level classes
  47     // (like UString) that get ref/deref'ed a lot. This class has the benefit of doing fast ref
  48     // counts like RefPtr whenever possible, but it has the downside that you need to copy it
  49     // to use it on another thread.
  50     //
  51     // Is this class threadsafe?
  52     // While each instance of the class is not threadsafe, the copied instance is threadsafe
  53     // with respect to the original and any other copies.  The underlying m_data is jointly
  54     // owned by the original instance and all copies.
  55     template<class T>
  56     class CrossThreadRefCounted : public Noncopyable {
  57     public:
  58         static PassRefPtr<CrossThreadRefCounted<T> > create(T* data)
  59         {
  60             return adoptRef(new CrossThreadRefCounted<T>(data, 0));
  61         }
  62
  63         // Used to make an instance that can be used on another thread.
  64         PassRefPtr<CrossThreadRefCounted<T> > crossThreadCopy();
  65
  66         void ref();
  67         void deref();
  68         T* release();
  69
  70         bool isShared() const
  71         {
  72             return !m_refCounter.hasOneRef() || (m_threadSafeRefCounter && !m_threadSafeRefCounter->hasOneRef());
  73         }
  74
  75     private:
  76         CrossThreadRefCounted(T* data, ThreadSafeSharedBase* threadedCounter)
  77             : m_threadSafeRefCounter(threadedCounter)
  78             , m_data(data)
  79 #ifndef NDEBUG
  80             , m_threadId(0)
  81 #endif
  82         {
  83         }
  84
  85         ~CrossThreadRefCounted()
  86         {
  87             if (!m_threadSafeRefCounter)
  88                 delete m_data;
  89         }
  90
  91         void threadSafeDeref();
  92
  93 #ifndef NDEBUG
  94         bool isOwnedByCurrentThread() const {
  95             return !m_threadId || m_threadId == currentThread() || ((isMainThread() || pthread_main_np()) && WebCoreWebThreadIsLockedOrDisabled());
  96         }
  97 #endif
  98
  99         RefCountedBase m_refCounter;
 100         ThreadSafeSharedBase* m_threadSafeRefCounter;
 101         T* m_data;
 102 #ifndef NDEBUG
 103         ThreadIdentifier m_threadId;
 104 #endif
 105     };
 106
 107     template<class T>
 108     void CrossThreadRefCounted<T>::ref()
 109     {
 110         ASSERT(isOwnedByCurrentThread());
 111
 112         m_refCounter.ref();
 113 #ifndef NDEBUG
 114         // Store the threadId as soon as the ref count gets to 2.
 115         // The class gets created with a ref count of 1 and then passed
 116         // to another thread where to ref count get increased.  This
 117         // is a heuristic but it seems to always work and has helped
 118         // find some bugs.
 119         if (!m_threadId && m_refCounter.refCount() == 2)
 120             m_threadId = currentThread();
 121 #endif
 122     }
 123
 124     template<class T>
 125     void CrossThreadRefCounted<T>::deref()
 126     {
 127         ASSERT(isOwnedByCurrentThread());
 128
 129         if (m_refCounter.derefBase()) {
 130             threadSafeDeref();
 131             delete this;
 132         } else {
 133 #ifndef NDEBUG
 134             // Clear the threadId when the ref goes to 1 because it
 135             // is safe to be passed to another thread at this point.
 136             if (m_threadId && m_refCounter.refCount() == 1)
 137                 m_threadId = 0;
 138 #endif
 139         }
 140     }
 141
 142     template<class T>
 143     T* CrossThreadRefCounted<T>::release()
 144     {
 145         ASSERT(!isShared());
 146
 147         T* data = m_data;
 148         m_data = 0;
 149         return data;
 150     }
 151
 152     template<class T>
 153     PassRefPtr<CrossThreadRefCounted<T> > CrossThreadRefCounted<T>::crossThreadCopy()
 154     {
 155         ASSERT(isOwnedByCurrentThread());
 156
 157         if (m_threadSafeRefCounter)
 158             m_threadSafeRefCounter->ref();
 159         else
 160             m_threadSafeRefCounter = new ThreadSafeSharedBase(2);
 161
 162         return adoptRef(new CrossThreadRefCounted<T>(m_data, m_threadSafeRefCounter));
 163     }
 164
 165
 166     template<class T>
 167     void CrossThreadRefCounted<T>::threadSafeDeref()
 168     {
 169         if (m_threadSafeRefCounter && m_threadSafeRefCounter->derefBase()) {
 170             delete m_threadSafeRefCounter;
 171             m_threadSafeRefCounter = 0;
 172         }
 173     }
 174 } // namespace WTF
 175
 176 using WTF::CrossThreadRefCounted;
 177
 178 #endif // CrossThreadRefCounted_h