--- a/hotspot/src/share/vm/gc_implementation/g1/vm_operations_g1.cpp	Fri Mar 16 16:14:04 2012 +0100
+++ b/hotspot/src/share/vm/gc_implementation/g1/vm_operations_g1.cpp	Mon Mar 12 14:59:00 2012 -0700
@@ -34,7 +34,8 @@
 VM_G1CollectForAllocation::VM_G1CollectForAllocation(
                                                   unsigned int gc_count_before,
                                                   size_t word_size)
-  : VM_G1OperationWithAllocRequest(gc_count_before, word_size) {
+  : VM_G1OperationWithAllocRequest(gc_count_before, word_size,
+                                   GCCause::_allocation_failure) {
   guarantee(word_size > 0, "an allocation should always be requested");
 }
 
@@ -57,9 +58,10 @@
                                       bool           should_initiate_conc_mark,
                                       double         target_pause_time_ms,
                                       GCCause::Cause gc_cause)
-  : VM_G1OperationWithAllocRequest(gc_count_before, word_size),
+  : VM_G1OperationWithAllocRequest(gc_count_before, word_size, gc_cause),
     _should_initiate_conc_mark(should_initiate_conc_mark),
     _target_pause_time_ms(target_pause_time_ms),
+    _should_retry_gc(false),
     _full_collections_completed_before(0) {
   guarantee(target_pause_time_ms > 0.0,
             err_msg("target_pause_time_ms = %1.6lf should be positive",
@@ -70,6 +72,22 @@
   _gc_cause = gc_cause;
 }
 
+bool VM_G1IncCollectionPause::doit_prologue() {
+  bool res = VM_GC_Operation::doit_prologue();
+  if (!res) {
+    if (_should_initiate_conc_mark) {
+      // The prologue can fail for a couple of reasons. The first is that another GC
+      // got scheduled and prevented the scheduling of the initial mark GC. The
+      // second is that the GC locker may be active and the heap can't be expanded.
+      // In both cases we want to retry the GC so that the initial mark pause is
+      // actually scheduled. In the second case, however, we should stall until
+      // until the GC locker is no longer active and then retry the initial mark GC.
+      _should_retry_gc = true;
+    }
+  }
+  return res;
+}
+
 void VM_G1IncCollectionPause::doit() {
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
   assert(!_should_initiate_conc_mark ||
@@ -106,11 +124,25 @@
     // next GC pause to be an initial mark; it returns false if a
     // marking cycle is already in progress.
     //
-    // If a marking cycle is already in progress just return and skip
-    // the pause - the requesting thread should block in doit_epilogue
-    // until the marking cycle is complete.
+    // If a marking cycle is already in progress just return and skip the
+    // pause below - if the reason for requesting this initial mark pause
+    // was due to a System.gc() then the requesting thread should block in
+    // doit_epilogue() until the marking cycle is complete.
+    //
+    // If this initial mark pause was requested as part of a humongous
+    // allocation then we know that the marking cycle must just have
+    // been started by another thread (possibly also allocating a humongous
+    // object) as there was no active marking cycle when the requesting
+    // thread checked before calling collect() in
+    // attempt_allocation_humongous(). Retrying the GC, in this case,
+    // will cause the requesting thread to spin inside collect() until the
+    // just started marking cycle is complete - which may be a while. So
+    // we do NOT retry the GC.
     if (!res) {
-      assert(_word_size == 0, "ExplicitGCInvokesConcurrent shouldn't be allocating");
+      assert(_word_size == 0, "Concurrent Full GC/Humongous Object IM shouldn't be allocating");
+      if (_gc_cause != GCCause::_g1_humongous_allocation) {
+        _should_retry_gc = true;
+      }
       return;
     }
   }
@@ -123,6 +155,13 @@
                                       true /* expect_null_cur_alloc_region */);
   } else {
     assert(_result == NULL, "invariant");
+    if (!_pause_succeeded) {
+      // Another possible reason reason for the pause to not be successful
+      // is that, again, the GC locker is active (and has become active
+      // since the prologue was executed). In this case we should retry
+      // the pause after waiting for the GC locker to become inactive.
+      _should_retry_gc = true;
+    }
   }
 }
 
@@ -168,6 +207,7 @@
 }
 
 void VM_CGC_Operation::acquire_pending_list_lock() {
+  assert(_needs_pll, "don't call this otherwise");
   // The caller may block while communicating
   // with the SLT thread in order to acquire/release the PLL.
   ConcurrentMarkThread::slt()->
@@ -175,6 +215,7 @@
 }
 
 void VM_CGC_Operation::release_and_notify_pending_list_lock() {
+  assert(_needs_pll, "don't call this otherwise");
   // The caller may block while communicating
   // with the SLT thread in order to acquire/release the PLL.
   ConcurrentMarkThread::slt()->
@@ -198,7 +239,9 @@
 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();
+  if (_needs_pll) {
+    acquire_pending_list_lock();
+  }
 
   Heap_lock->lock();
   SharedHeap::heap()->_thread_holds_heap_lock_for_gc = true;
@@ -210,5 +253,7 @@
   // VM_GC_Operation::doit_epilogue()
   SharedHeap::heap()->_thread_holds_heap_lock_for_gc = false;
   Heap_lock->unlock();
-  release_and_notify_pending_list_lock();
+  if (_needs_pll) {
+    release_and_notify_pending_list_lock();
+  }
 }