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

[Kim Barrett]

> On Feb 5, 2017, at 10:19 PM, David Holmes <david.holmes at oracle.com> wrote:
> Thanks for explaining. I must say that I find this treatment rather ad-hoc as we seem to be discovering by trial-and-error where these places occur. I would have hoped this was all encapsulated within the weakreference code itself (in this case).

The GC team is taking a hard look at weak reference handling, as part
of preparing for some improvements we are considering.  That's turned
up some holes that probably should have been dealt with as part of
productizing G1, but seem to have gone unnoticed until recently.

For now we're patching over these holes. I expect some cleanup tasks
for JDK 10; we're not happy with this littering of otherwise generic
runtime code with G1-specific stuff.  OTOH, I have to say that some of
the abstraction violations I'm finding around jobject, Handle, and
oop* in non-GC code are not helpful.

> Does the SATB occur at a global safepoint?

SATB is the approach G1 uses for a part of concurrent collection.  It
defines the invariants that must be maintained by the mutator so that
the collector can find all the live objects.  The barriers in question
are executed by the mutator concurrently with the collector, in order
to inform the collector of reachability graph modifications.