On Wed, Oct 17, 2012 at 3:13 AM, Jason Greene
<jgreene@redhat.com> wrote:
Sent from my iPhone
On Oct 16, 2012, at 10:39 AM, Jason Greene <
jason.greene@redhat.com> wrote:
>> from _get_ to lock on an eventually created new instance.
>
> Yes if you get the ordering right, it can certainly be done. You might have to introduce a state-based CAS or secondary lock though for some scenarios (I haven't thought through them all) I think Manik's point though was just that getting it right is harder than just making the whole thing atomic.
Right, you still need a retry loop on acquire, so why not use reference counting and actually reuse the lock?
I think all you need is a tryAddRef method, something like this:
public boolean tryAddRef() {
int newCount;
int oldCount;
do {
oldCount = refCount.get();
if (oldCount == 0)
return false;
newCount = oldCount + 1;
} while (!refCount.compareAndSet(oldCount, newCount));
return true;
}
Then in your acquireLock you call tryAddRef in a loop and when it returns true you can go on and really acquire the lock - knowing that another thread can't remove it from the map.