[14] RFR (S): 8234401: ConstantCallSite may stuck in non-frozen state

[Vladimir Ivanov]

Thanks, Paul.

> CallSite:
> 
>   public class CallSite {
>   
> - // The actual payload of this call site:
> + // The actual payload of this call site.
> + // Can be modified using {@link 
> MethodHandleNatives#setCallSiteTargetNormal} or {@link 
> MethodHandleNatives#setCallSiteTargetVolatile}.
>       /*package-private*/
> - MethodHandle target; // Note: This field is known to the JVM. Do not 
> change.
> + final MethodHandle target; // Note: This field is known to the JVM.
> 
> 
> Is there any benefit to making target final, even though it's not really 
> for mutable call sites? (With the recent discussion of "final means 
> final" it would be nice to not introduce more special case stomping on 
> final fields if we can avoid it).

CallSite.target is already treated specially: all updates go through 
MethodHandleNatives and JIT-compiler treat it as "final" irrespective of 
the flags.

My main interest in marking it final is to enforce proper initialization 
on JDK side to make it easier to reason about:

 
https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-November/036021.html 


>       CallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
> - this(targetType);
> + this(targetType); // need to initialize target to make CallSite.type() 
> work in createTargetHook
>           ConstantCallSite selfCCS = (ConstantCallSite) this;
>           MethodHandle boundTarget = (MethodHandle) createTargetHook.invokeWithArguments(selfCCS);
> - checkTargetChange(this.target, boundTarget);
> - this.target = boundTarget;
> + setTargetNormal(boundTarget); // ConstantCallSite doesn't publish 
> CallSite.target
> + UNSAFE.storeStoreFence(); // barrier between target and isFrozen updates
>       }
> 
> 
> I wonder if instead of introducing the store store fence here we could 
> move it into ConstantCallSite?

Sure, if you prefer to see it on ConstantCallSite side, we can move it 
there.

By putting it in CallSite near the call site update, I wanted to stress 
there's a CallSite.target update happening on partially published 
instance.

Best regards,
Vladimir Ivanov

> 
>       protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
> - super(targetType, createTargetHook);
> + super(targetType, createTargetHook); // "this" instance leaks into 
> createTargetHook
> 
> + UNSAFE.storeStoreFence(); // barrier between target and isFrozen 
> updates<— Add here
> 
>           isFrozen = true;
> + UNSAFE.storeStoreFence(); // properly publish isFrozen
>       }
>> On Nov 19, 2019, at 9:49 AM, Vladimir Ivanov 
>> <vladimir.x.ivanov at oracle.com <mailto:vladimir.x.ivanov at oracle.com>> 
>> wrote:
>>
>>
>>> Subtle, so I could be misunderstanding something, did you intend to 
>>> remove the assignment of isFrozen in the ConstantCallSite constructor?
>>
>> Oh, good catch. It is my fault: the update should be there. (The 
>> barriers are added just to preserve final field semantics for isFrozen.)
>>
>> Published the wrong version (with some leftovers from last-minute 
>> failed experiment). Updated in place.
>>
>> Best regards,
>> Vladimir Ivanov
>>
>>>> On Nov 19, 2019, at 8:53 AM, Vladimir Ivanov 
>>>> <vladimir.x.ivanov at oracle.com <mailto:vladimir.x.ivanov at oracle.com>> 
>>>> wrote:
>>>>
>>>> http://cr.openjdk.java.net/~vlivanov/8234401/webrev.00/
>>>> https://bugs.openjdk.java.net/browse/JDK-8234401
>>>>
>>>> ConstantCallSite has a ctor which deliberately leaks partially 
>>>> initialized instance into user code. isFrozen is declared final and 
>>>> if user code is obstinate enough, it can end up with non-frozen 
>>>> state embedded into the generated code. It manifests as a 
>>>> ConstantCallSite instance which is stuck in non-frozen state.
>>>>
>>>> I switched isFrozen from final to @Stable, so non-frozen state is 
>>>> never constant folded. Put some store-store barriers along the way 
>>>> to restore final field handling.
>>>>
>>>> I deliberately stopped there (just restoring isFrozen final field 
>>>> behavior). Without proper synchronization, there's still a 
>>>> theoretical possibility of transiently observing a call site in 
>>>> non-frozen state right after ctor is over. But at least there's no 
>>>> way anymore to accidentally break an instance in such a way it 
>>>> becomes permanently unusable.
>>>>
>>>> PS: converted CallSite.target to final along the way and made some 
>>>> other minor refactorings.
>>>>
>>>> Testing: regression test, tier1-2
>>>>
>>>> Best regards,
>>>> Vladimir Ivanov
>