diff --git a/src/share/vm/prims/jvmtiEnvBase.cpp b/src/share/vm/prims/jvmtiEnvBase.cpp
index 56387634179bf431ea84c107424f782fbcb05e45..597d7a164dc514bd74c6ba6b8fbdb81a6479d2ab 100644
--- a/src/share/vm/prims/jvmtiEnvBase.cpp
+++ b/src/share/vm/prims/jvmtiEnvBase.cpp
@@ -997,13 +997,19 @@ JvmtiEnvBase::get_object_monitor_usage(JavaThread* calling_thread, jobject objec
       // move our object at this point. However, our owner value is safe
       // since it is either the Lock word on a stack or a JavaThread *.
       owning_thread = Threads::owning_thread_from_monitor_owner(owner, !at_safepoint);
-      assert(owning_thread != NULL, "sanity check");
-      if (owning_thread != NULL) {  // robustness
+      // Cannot assume (owning_thread != NULL) here because this function
+      // may not have been called at a safepoint and the owning_thread
+      // might not be suspended.
+      if (owning_thread != NULL) {
         // The monitor's owner either has to be the current thread, at safepoint
         // or it has to be suspended. Any of these conditions will prevent both
         // contending and waiting threads from modifying the state of
         // the monitor.
         if (!at_safepoint && !JvmtiEnv::is_thread_fully_suspended(owning_thread, true, &debug_bits)) {
+          // Don't worry! This return of JVMTI_ERROR_THREAD_NOT_SUSPENDED
+          // will not make it back to the JVM/TI agent. The error code will
+          // get intercepted in JvmtiEnv::GetObjectMonitorUsage() which
+          // will retry the call via a VM_GetObjectMonitorUsage VM op.
           return JVMTI_ERROR_THREAD_NOT_SUSPENDED;
         }
         HandleMark hm;
diff --git a/src/share/vm/runtime/synchronizer.cpp b/src/share/vm/runtime/synchronizer.cpp
index bdf416275dccd0040a26a4d5608d3df3ecc87474..bff39625ed4d96d21ed4b685d4720604f6db2779 100644
--- a/src/share/vm/runtime/synchronizer.cpp
+++ b/src/share/vm/runtime/synchronizer.cpp
@@ -813,6 +813,7 @@ JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
   }
 
   if (owner != NULL) {
+    // owning_thread_from_monitor_owner() may also return NULL here
     return Threads::owning_thread_from_monitor_owner(owner, doLock);
   }
 
diff --git a/src/share/vm/runtime/thread.cpp b/src/share/vm/runtime/thread.cpp
index e0cf75b03f20c9e5562291ef29a604e7033c8df7..cb883c5d8215dce3a7ff0f32c7e656bd43746bde 100644
--- a/src/share/vm/runtime/thread.cpp
+++ b/src/share/vm/runtime/thread.cpp
@@ -4285,7 +4285,9 @@ JavaThread *Threads::owning_thread_from_monitor_owner(address owner, bool doLock
       if (owner == (address)p) return p;
     }
   }
-  assert(UseHeavyMonitors == false, "Did not find owning Java thread with UseHeavyMonitors enabled");
+  // Cannot assert on lack of success here since this function may be
+  // used by code that is trying to report useful problem information
+  // like deadlock detection.
   if (UseHeavyMonitors) return NULL;
 
   //
@@ -4303,7 +4305,7 @@ JavaThread *Threads::owning_thread_from_monitor_owner(address owner, bool doLock
       }
     }
   }
-  assert(the_owner != NULL, "Did not find owning Java thread for lock word address");
+  // cannot assert on lack of success here; see above comment
   return the_owner;
 }
 
diff --git a/src/share/vm/services/threadService.cpp b/src/share/vm/services/threadService.cpp
index b1c92c0b1101f855a3a4d6f3b498c6620ad29727..03289c7e9ac2499a4a0e5a5f91477a8dd1c84f6c 100644
--- a/src/share/vm/services/threadService.cpp
+++ b/src/share/vm/services/threadService.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -327,8 +327,28 @@ DeadlockCycle* ThreadService::find_deadlocks_at_safepoint(bool concurrent_locks)
     while (waitingToLockMonitor != NULL || waitingToLockBlocker != NULL) {
       cycle->add_thread(currentThread);
       if (waitingToLockMonitor != NULL) {
-        currentThread = Threads::owning_thread_from_monitor_owner((address)waitingToLockMonitor->owner(),
-                                                                  false /* no locking needed */);
+        currentThread = Threads::owning_thread_from_monitor_owner(
+                          (address)waitingToLockMonitor->owner(),
+                          false /* no locking needed */);
+        if (currentThread == NULL) {
+          // This function is called at a safepoint so the JavaThread
+          // that owns waitingToLockMonitor should be findable, but
+          // if it is not findable, then the previous currentThread is
+          // blocked permanently. We record this as a deadlock.
+          num_deadlocks++;
+
+          cycle->set_deadlock(true);
+
+          // add this cycle to the deadlocks list
+          if (deadlocks == NULL) {
+            deadlocks = cycle;
+          } else {
+            last->set_next(cycle);
+          }
+          last = cycle;
+          cycle = new DeadlockCycle();
+          break;
+        }
       } else {
         if (concurrent_locks) {
           if (waitingToLockBlocker->is_a(SystemDictionary::abstract_ownable_synchronizer_klass())) {
@@ -841,7 +861,17 @@ void DeadlockCycle::print_on(outputStream* st) const {
         owner_desc = " (JVMTI raw monitor),\n  which is held by";
       }
       currentThread = Threads::owning_thread_from_monitor_owner(
-        (address)waitingToLockMonitor->owner(), false /* no locking needed */);
+                        (address)waitingToLockMonitor->owner(),
+                        false /* no locking needed */);
+      if (currentThread == NULL) {
+        // The deadlock was detected at a safepoint so the JavaThread
+        // that owns waitingToLockMonitor should be findable, but
+        // if it is not findable, then the previous currentThread is
+        // blocked permanently.
+        st->print("%s UNKNOWN_owner_addr=" PTR_FORMAT, owner_desc,
+                  (address)waitingToLockMonitor->owner());
+        continue;
+      }
     } else {
       st->print("  waiting for ownable synchronizer " INTPTR_FORMAT ", (a %s)",
                 (address)waitingToLockBlocker,