[jmm-dev] jdk9 APIs

[Aleksey Shipilev]

On 08/12/2015 11:31 AM, Aleksey Shipilev wrote:
> On 08/10/2015 09:22 PM, Doug Lea wrote:
>> /**
>>  * Stand-in for spec purposes of jdk9 java.lang.invoke.VarHandle
>>  */
>> abstract class NotReallyVarHandle<T> {
>>     boolean weakCompareAndSet(Object owner, T cmp, T val);
>>     boolean weakCompareAndSetAcquire(Object owner, T cmp, T val);
>>     boolean weakCompareAndSetRelease(Object owner, T cmp, T val);
> 
> I have a question about these. I understand the acq/rel relaxations for
> strong CASes, but what is the relaxation for already weak CAS? In other
> words, weakCompareAndSet is spec'ed in j.u.c.atomic as:
> 
> "weakCompareAndSet atomically reads and conditionally writes a variable
> but does not create any happens-before orderings, so provides no
> guarantees with respect to previous or subsequent reads and writes of
> any variables other than the target of the weakCompareAndSet."
> 
> ...and
> 
> "Additionally weakCompareAndSet does not provide ordering guarantees
> that are usually needed for synchronization control. However, the method
> may be useful for updating counters and statistics when such updates are
> unrelated to the other happens-before orderings of a program. When a
> thread sees an update to an atomic variable caused by a
> weakCompareAndSet, it does not necessarily see updates to any other
> variables that occurred before the weakCompareAndSet."

Or is it the other way around: weakCAS is relaxed by default, and
acq/rel weakCAS version are actually *stronger* than default weakCAS?

-Aleksey