If you’re going to use an interlocked operation to generate a unique value, you need to use it before it’s gone

Raymond Chen

Is the Interlocked­Increment function broken? One person seemed to think so.

We’re finding that the Interlocked­Increment is producing duplicate values. Are there are any know bugs in Interlocked­Increment?

Because of course when something doesn’t work, it’s because you are the victim of a vast conspiracy. There is a fundamental flaw in the Interlocked­Increment function that only you can see. You are not a crackpot.

LONG g_lNextAvailableId = 0;

DWORD GetNextId()
{
  // Increment atomically
  InterlockedIncrement(&g_lNextAvailableId);

  // Subtract 1 from the current value to get the value
  // before the increment occurred.
  return (DWORD)g_lNextAvailableId – 1;
}

Recall that Interlocked­Increment function increments a value atomically and returns the incremented value. If you are interested in the result of the increment, you need to use the return value directly and not try to read the variable you incremented, because that variable may have been modified by another thread in the interim.

Consider what happens when two threads call Get­Next­Id simultaneously (or nearly so). Suppose the initial value of g_lNext­Available­Id is 4.

• First thread calls Interlocked­Increment to increment from 4 to 5. The return value is 5.
• Second thread calls Interlocked­Increment to increment from 5 to 6. The return value is 6.
• First thread ignores the return value and instead reads the current value of g_lNext­Available­Id, which is 6. It subtracts 1, leaving 5, and returns it.
• Second thread ignores the return value and instead reads the current value of g_lNext­Available­Id, which is still 6. It subtracts 1, leaving 5, and returns it.

Result: Both calls to Get­Next­Id return 5. Interpretation: “Interlocked­Increment is broken.”

Actually, Interlocked­Increment is working just fine. What happened is that the code threw away the unique information that Interlocked­Increment returned and instead went back to the shared variable, even though the shared variable changed its value in the meantime.

Since this code cares about the result of the increment, it needs to use the value returned by Interlocked­Increment.

DWORD GetNextId()
{
  // Increment atomically and subtract 1 from the
  // incremented value to get the value before the
  // increment occurred.
  return (DWORD)InterlockedIncrement(&g_lNextAvailableId) – 1;
}

Exercise: Criticize this implementation of IUnknown::Release:

STDMETHODIMP_(ULONG) CObject::Release()
{
 InterlockedDecrement(&m_cRef);
 if (m_cRef == 0)
 {
  delete this;
  return 0;
 }
 return m_cRef;
}

Raymond Chen

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