--- old/src/share/vm/gc_implementation/g1/concurrentMarkThread.cpp	2011-10-17 14:23:36.370149118 -0700
+++ new/src/share/vm/gc_implementation/g1/concurrentMarkThread.cpp	2011-10-17 14:23:36.163979935 -0700
@@ -147,12 +147,8 @@
           }
         }
       } while (cm()->restart_for_overflow());
-      double counting_start_time = os::elapsedVTime();
-
-      // YSR: These look dubious (i.e. redundant) !!! FIX ME
-      slt()->manipulatePLL(SurrogateLockerThread::acquirePLL);
-      slt()->manipulatePLL(SurrogateLockerThread::releaseAndNotifyPLL);
 
+      double counting_start_time = os::elapsedVTime();
       if (!cm()->has_aborted()) {
         double count_start_sec = os::elapsedTime();
         if (PrintGC) {
@@ -175,6 +171,7 @@
           }
         }
       }
+
       double end_time = os::elapsedVTime();
       _vtime_count_accum += (end_time - counting_start_time);
       // Update the total virtual time before doing this, since it will try
@@ -335,13 +332,15 @@
   clear_started();
 }
 
-// Note: this method, although exported by the ConcurrentMarkSweepThread,
+// Note: this method, although exported by the ConcurrentMarkThread,
 // which is a non-JavaThread, can only be called by a JavaThread.
 // Currently this is done at vm creation time (post-vm-init) by the
 // main/Primordial (Java)Thread.
-// XXX Consider changing this in the future to allow the CMS thread
+// XXX Consider changing this in the future to allow the CM thread
 // itself to create this thread?
 void ConcurrentMarkThread::makeSurrogateLockerThread(TRAPS) {
+  assert(UseG1GC, "SLT thread needed only for concurrent GC");
+  assert(THREAD->is_Java_thread(), "must be a Java thread");
   assert(_slt == NULL, "SLT already created");
   _slt = SurrogateLockerThread::make(THREAD);
 }
--- old/src/share/vm/gc_implementation/g1/vm_operations_g1.cpp	2011-10-17 14:23:37.449265344 -0700
+++ new/src/share/vm/gc_implementation/g1/vm_operations_g1.cpp	2011-10-17 14:23:37.242502289 -0700
@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "gc_implementation/g1/concurrentMarkThread.inline.hpp"
 #include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
 #include "gc_implementation/g1/g1CollectorPolicy.hpp"
 #include "gc_implementation/g1/vm_operations_g1.hpp"
@@ -165,6 +166,20 @@
   }
 }
 
+void VM_CGC_Operation::acquire_pending_list_lock() {
+  // The caller may block while communicating
+  // with the SLT thread in order to acquire/release the PLL.
+  ConcurrentMarkThread::slt()->
+    manipulatePLL(SurrogateLockerThread::acquirePLL);
+}
+
+void VM_CGC_Operation::release_and_notify_pending_list_lock() {
+  // The caller may block while communicating
+  // with the SLT thread in order to acquire/release the PLL.
+  ConcurrentMarkThread::slt()->
+    manipulatePLL(SurrogateLockerThread::releaseAndNotifyPLL);
+}
+
 void VM_CGC_Operation::doit() {
   gclog_or_tty->date_stamp(PrintGC && PrintGCDateStamps);
   TraceCPUTime tcpu(PrintGCDetails, true, gclog_or_tty);
@@ -180,12 +195,19 @@
 }
 
 bool VM_CGC_Operation::doit_prologue() {
+  // Note the relative order of the locks must match that in
+  // VM_GC_Operation::doit_prologue() or deadlocks can occur
+  acquire_pending_list_lock();
+
   Heap_lock->lock();
   SharedHeap::heap()->_thread_holds_heap_lock_for_gc = true;
   return true;
 }
 
 void VM_CGC_Operation::doit_epilogue() {
+  // Note the relative order of the unlocks must match that in
+  // VM_GC_Operation::doit_epilogue()
   SharedHeap::heap()->_thread_holds_heap_lock_for_gc = false;
   Heap_lock->unlock();
+  release_and_notify_pending_list_lock();
 }
--- old/src/share/vm/gc_implementation/g1/vm_operations_g1.hpp	2011-10-17 14:23:38.511326176 -0700
+++ new/src/share/vm/gc_implementation/g1/vm_operations_g1.hpp	2011-10-17 14:23:38.306085444 -0700
@@ -93,11 +93,17 @@
   }
 };
 
-// Concurrent GC stop-the-world operations such as initial and final mark;
+// Concurrent GC stop-the-world operations such as final mark and cleanup;
 // consider sharing these with CMS's counterparts.
 class VM_CGC_Operation: public VM_Operation {
   VoidClosure* _cl;
   const char* _printGCMessage;
+
+protected:
+  // java.lang.ref.Reference support
+  void acquire_pending_list_lock();
+  void release_and_notify_pending_list_lock();
+  
 public:
   VM_CGC_Operation(VoidClosure* cl, const char *printGCMsg)
     : _cl(cl), _printGCMessage(printGCMsg) { }
--- old/src/share/vm/gc_implementation/shared/concurrentGCThread.cpp	2011-10-17 14:23:39.572186112 -0700
+++ new/src/share/vm/gc_implementation/shared/concurrentGCThread.cpp	2011-10-17 14:23:39.367175698 -0700
@@ -224,6 +224,8 @@
   MutexLockerEx x(&_monitor, Mutex::_no_safepoint_check_flag);
   assert(_buffer == empty, "Should be empty");
   assert(msg != empty, "empty message");
+  assert(!Heap_lock->owned_by_self(), "Heap_lock owned by requesting thread");
+
   _buffer = msg;
   while (_buffer != empty) {
     _monitor.notify();