Much better :-)
I accumulated some more questions while I was looking further.
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(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?
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
}
Paul.
> On Nov 19, 2019, at 9:49 AM, Vladimir Ivanov <vladimir.x.iva...@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.iva...@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
