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Are AddRef and Release really thread safe ???

From: Benny (benny.nguyenUSA.XEROX.COM)
Date: Mon Mar 17 2003 - 21:13:37 CST

Typical COM implementation

ULONG ClassXXX::AddRef()
{
   return InterlockedIncrement(&m_dwRef);
}

ULONG ClassXXX::Release()
{
   ULONG l = InterlockedDecrement(&m_dwRef);
   if (l == 0)
     delete this;
   return l;
}

The InterlockedXXXX functions guarantee that the variable will not be
updated simutaneously by more than one thread and the return value is the
resulting updated value (NT 4.0 or later) but consider this scenario

1. m_dwRef is at 0
2. Thread A enters AddRef and calls InterlockedIncrement(&m_dwRef)
3. Thread A returns from InterlockedIncrement and now m_dwRef is 1
4. Thread B enters Release and calls InterlockedDecrement(&m_dwRef)
5. Thread B returns from InterlockedDecrement and now m_dwRef is 0
6. Thread A returns from AddRef() and continues executing using the COM ptr
7. Thread B sees that ref count now is 0 and call delete this
8. Thread A crashes

Is it possible ????

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