Re: Solving Lazy Initialization and double checked locking problem

From: Vinay Aggarwal (vinay_at_t-e-c-h-l-o-b-b-y.com)
Date: 11/29/03 

Date: Fri, 28 Nov 2003 23:33:00 GMT

Hey Roger,

That was a nice catch. I missed that one.

Now lets follow this logic. The compiler can only reorder the statements
if it does not change anything for the currently executing thread.
So can we create a side effect?

What if I change the inner class to something as follows.

    static class HeavyObjectInitSync extends
HeavyObjectInit // line 9
    {
        public volatile int i=0;
        public HeavyObject getHeavyObject(LazyInit
lazyInit) // line 11
        {
            HeavyObject heavyObject =
getHeavyObjectInternal(lazyInit); // line 13
            if(i>0)
                lazyInit.heavyObjectInit =
heavyObjectInitStatic; // line 14
            return
heavyObject;
// line 15
        }

        private synchronized HeavyObject getHeavyObjectInternal(LazyInit
lazyInit) // line 18
        {
            if(lazyInit.heavyObject ==
null) // line 20
            {
                lazyInit.heavyObject = new
HeavyObject(); // line 22
            }
            i++;
            return
lazyInit.heavyObject;
// line 24
        }
    }

Now the compiler is bound to keep them in straight order. What do you
think?

Vinay Aggarwal
CTO Techlobby
vinay at t e c h l o b b y dot com
h t t p : / / w w w. t e c h l o b b y . c o m /

Roger Lindsjö wrote:
>
> On Fri, 28 Nov 2003 19:32:23 +0100, Vinay Aggarwal wrote:
>
> > I have been thinking about the lazy initialization and double checked
> > locking problem. This problem is explain in detail here
> > http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html I
> > am not fully convinced that this problem cannot be solved. I am going to
> > propose a solution here. For the sake of discussion I will post my
> > solution here. It is possible that the proposed solution does not work,
> > feedback and comments are welcome.
>
> Sorry, but I don't think it works.
>
> > Since the thread is exiting a
> > syncronized method and the variable is volatile, the HeavyObject will be
> > fully initialized before the thread releases the lock. Now the lucky
> > thread will switch the implementation of heavyObjectInit to a non
> > syncronized initializer (line 14). Any threads reaching line 39 after
> > this will call the non syncronized version of getHeavyObject() at line
> > 31. At the same time, all the threads blocked at line 18 will enter the
> > getHeavyObjectInternal() method one by one and return with the singleton
> > heavy object instance.
>
> Here I think it goes wrong. The JIT should be free to reassign the
> instructions as long as it does not change the execution of the current
> thread according to the specs. This would allow it to:
>
> lazyInit.heavyObjectInit = heavyObjectInitStatic; // line 14
>
> before line 13 since this does not change anything for the executing
> thread. However, now another thread can enter LazyInit.getHeavyObject()
> and get the non synchronized implementation with an uninitialized object
> before the first thread has had a chance to fully create the heavy
> object.
>
> So the second thread could return a null object or a not fully
> initialized object.
>
> BTW, comp.lang.java does not exist.
>
> //Roger Lindsjö

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