RFR(S): 8173013: JVMTI tagged object access needs G1 pre-barrier

[David Holmes]

Hi Sangheon,

On 3/02/2017 5:11 AM, sangheon wrote:
> Hi all,
>
> Could I have some reviews for this change that adds G1 pre-barrier?
>
> JvmtiTagHashmapEntry has a bare oop member and this is a weak reference.
> So any place that allows this oop to 'escape' needs the G1 pre-barrier.
> JvmtiEnv::GetObjectsWithTags() provides such an escape path.
>
> For G1, in order to maintain the SATB invariants, reading the referent
> of a weak reference must ensure the referent is marked alive.
>
> So this proposal includes adding the pre-barrier at
> TagObjectCollector::do_entry(JvmtiTagHashmapEntry* entry) which I see
> the only place interacts(except 'peek' operations) with the bare oop member.

Pardon my GC ignorance but it seems odd to me that this barrier is 
inserted immediately before we create a local JNIHandle. Won't the 
JNIHandle ensure the object is seen as live?

Thanks,
David

> As writing stable test could take some more time, Stefan Karlsson and I
> did some tests to provoke this problem.
> ( Stefan Karlsson kindly provided the test,
> http://cr.openjdk.java.net/~stefank/8173013/reproducer/
> <http://cr.openjdk.java.net/%7Estefank/8173013/reproducer/> )
> With this proposed patch, the problem goes away.
>
> CR: https://bugs.openjdk.java.net/browse/JDK-8173013
> Webrev: http://cr.openjdk.java.net/~sangheki/8173013/webrev.0
> Testing: JPRT, some closed tests which use JVMTI and JDI.
>
> Thanks,
> Sangheon
>