RFR: ShenandoahControlThread should not get GCLABs

[Aleksey Shipilev]

This seems to be the bug that we are chasing now in sh/jdk. ShenandoahControlThread accidentally got
the GCLAB, but never retired it. When traversal-precleaning started, it walked some unmarked
objects, evacuating them, which happened in the ShControlThread context. Which means we have
allocated something in GCLAB we don't otherwise know about. And the whole thing crashes if we get to
recycle the region holding that GCLAB before it retires.

There are two possible fixes: let GCLAB retirement code know about ShControlThread, or make
ShControlThread go via shared allocs all the time, like other non-Java threads do. I think the
second option is better, because doing the GCLAB retirement from the ShControlThread might be prone
to other errors (e.g. deadlocks). We intended that only Java threads and GC worker threads have
GCLABs to simplify GCLAB tracking, but this is not true it current code, see below.

(Separately, we shall fix traversal-precleaning to get to worker threads so it can use GCLABs there).

Fix:

diff -r ea2f39b37e25 src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp        Fri Aug 31 13:29:24 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp        Fri Aug 31 15:49:05 2018 +0200
@@ -564,12 +564,11 @@
 }

 class ShenandoahInitGCLABClosure : public ThreadClosure {
 public:
   void do_thread(Thread* thread) {
-    if (thread != NULL && (thread->is_Java_thread() || thread->is_Worker_thread() ||
-                           thread->is_ConcurrentGC_thread())) {
+    if (thread != NULL && (thread->is_Java_thread() || thread->is_Worker_thread())) {
       ShenandoahThreadLocalData::initialize_gclab(thread);
     }
   }
 };

diff -r ea2f39b37e25 src/hotspot/share/gc/shenandoah/shenandoahThreadLocalData.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahThreadLocalData.hpp     Fri Aug 31 13:29:24 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahThreadLocalData.hpp     Fri Aug 31 15:49:05 2018 +0200
@@ -120,10 +120,11 @@
   static bool is_force_satb_flush(Thread* thread) {
     return data(thread)->_force_satb_flush;
   }

   static void initialize_gclab(Thread* thread) {
+    assert (thread->is_Java_thread() || thread->is_Worker_thread(), "Only Java and GC worker
threads are allowed to get GCLABs");
     data(thread)->_gclab = new PLAB(PLAB::min_size());
     data(thread)->_gclab_size = 0;
   }

   static PLAB* gclab(Thread* thread) {

Testing: failing tests on my desktop and gotland, tier3_gc_shenandoah (running)

Thanks,
-Aleksey