Is this multi-threaded Singleton more efficient?

Question

I have a high-performance Singleton in a multi-threaded environment. Normally I would do something like this:

public static Foo GetInstance()
{
    lock (Foo._syncLock)
    {
        if (Foo._instance == null)
            Foo._instance = new Foo();
        return Foo._instance;
    }
}

I am wondering if the following will be more efficient instead, since it will avoid the continuous blocking of the thread, or is there a hidden problem with it?

public static Foo GetInstance()
{
    if (Foo._instance != null)
        return Foo._instance;
    lock (Foo._syncLock)
    {
        if (Foo._instance == null)
            Foo._instance = new Foo();
        return Foo._instance;
    }
}

Answer 1

I am wondering if the following will be more efficient instead, since it will avoid the continuous blocking of the thread, or is there a hidden problem with it?

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static object sync = new object();
static bool b = false;
static int x = 0;
static int GetIt()
{
  if (!b)
  {
    lock(sync)
    {
      if (!b)
      {
        b = true;
        x = ExpensiveComputation();
      }
    }
  }
  return x;
}

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Answer 2

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if(_instance == null)
  lock(_syncRoot)
    if(_instance == null)
      _instance = new Foo();

return _instance;

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Edit

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private static Foo _instance = new Foo();

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