[jvm-l] Re: request for advice: safepoints in the JSR 292 spec

[John Rose]

On Dec 15, 2010, at 7:04 AM, Rémi Forax wrote:

>>> default generic method (reader):
>>> synchonized(masterLock) {
>>>   // check arguments
>>>   // use meta data to create a fast path
>>> }
>> (This next line should also be synchronized.  -- JRR)
>> 
>>> setTarget(guard + fastpath);
> 
> Correct me if I'm wrong.
> Le last line can be in the synchronized block but it don't have to.
> The JMM guarantees that the current thread will see the new target (T2).
> Other threads can see T1 and in that case will update to a T2' which is
> semantically equivalent to T2.
> It's a racy update.

Racy updates have the risk of not getting picked up by Slow Reader threads.

But, as you point out, in this case T1 synchronizes any Slow Reader and forces him to see T2.

So, you're right.

I guess the principle is that racy updates are OK as long as the previous state (T1) is allowed and will eventually force all readers to go to the new state (T2).

-- John