Comments on Nirav's Contemplations: Why Concurrency is hard

Hi Nirav, I know the AtomicXXX implementations ve...

2011-03-09T01:20:34.909-05:00

Hi Nirav,

I know the AtomicXXX implementations very well.

The operation you want to do, setting a value, doesn't need more 'atomicity' than an atomic set. The new value is a local variable and not subject to any currency issues.

For the getAndSet it is a different matter since you want to have the value that existed prior to doing the write, so the get and set needs to be atomic, hence you need to do it in a loop and use a compareAndSwap.

Peter Veentjer
Multiverse:Software Transactional Memory for the JVM
http://multiverse.codehaus.org

Not sure why URLs was truncated. Here's the li...

2011-03-08T19:33:15.999-05:00

Not sure why URLs was truncated. Here's the link
AtomicReference

I agree with your first suggestions but not with s...

2011-03-08T19:31:00.112-05:00

I agree with your first suggestions but not with second one, the atomicity is only guaranteed if you use atomicRef.get() see correct implementation for getAndSet() http://www.docjar.com/html/api/java/util/concurrent/atomic/AtomicReference.java.html

Thanks for pointing out the typo!

Ps: the following code can be simplified: while (...

2011-03-08T10:35:14.065-05:00

Ps:
the following code can be simplified:

while (true) {
V x = atomicRef.get();
if (atomicRef.cas(x,newValue))
return atomicRef.get();
}

To

atomicRef.set(newValue)
return newValue

No need to do that in a loop.

I think you have a JMM problem here depending on i...

2011-03-08T10:28:20.455-05:00

I think you have a JMM problem here depending on if the aRef is or is not volatile

synchronized(this){ aRef = newVal;} return aRef;

It could be that the aRef is updated by another thread after the {...} logic is executed. This could lead to various jmm problems.

It is better to use this:

synchronized(this){
aRef = newVal;
return aRef;
}

But I agree that concurrency can be hard and having it all over the place in a normal system, doesn't make me a happy man.