--- a/hotspot/src/os/linux/vm/os_linux.cpp	Tue Jan 22 05:55:04 2013 -0800
+++ b/hotspot/src/os/linux/vm/os_linux.cpp	Tue Jan 22 05:56:42 2013 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 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
@@ -5001,11 +5001,12 @@
      }
      -- _nParked ;
 
-    // In theory we could move the ST of 0 into _Event past the unlock(),
-    // but then we'd need a MEMBAR after the ST.
     _Event = 0 ;
      status = pthread_mutex_unlock(_mutex);
      assert_status(status == 0, status, "mutex_unlock");
+    // Paranoia to ensure our locked and lock-free paths interact
+    // correctly with each other.
+    OrderAccess::fence();
   }
   guarantee (_Event >= 0, "invariant") ;
 }
@@ -5068,40 +5069,44 @@
   status = pthread_mutex_unlock(_mutex);
   assert_status(status == 0, status, "mutex_unlock");
   assert (_nParked == 0, "invariant") ;
+  // Paranoia to ensure our locked and lock-free paths interact
+  // correctly with each other.
+  OrderAccess::fence();
   return ret;
 }
 
 void os::PlatformEvent::unpark() {
-  int v, AnyWaiters ;
-  for (;;) {
-      v = _Event ;
-      if (v > 0) {
-         // The LD of _Event could have reordered or be satisfied
-         // by a read-aside from this processor's write buffer.
-         // To avoid problems execute a barrier and then
-         // ratify the value.
-         OrderAccess::fence() ;
-         if (_Event == v) return ;
-         continue ;
-      }
-      if (Atomic::cmpxchg (v+1, &_Event, v) == v) break ;
-  }
-  if (v < 0) {
-     // Wait for the thread associated with the event to vacate
-     int status = pthread_mutex_lock(_mutex);
-     assert_status(status == 0, status, "mutex_lock");
-     AnyWaiters = _nParked ;
-     assert (AnyWaiters == 0 || AnyWaiters == 1, "invariant") ;
-     if (AnyWaiters != 0 && WorkAroundNPTLTimedWaitHang) {
-        AnyWaiters = 0 ;
-        pthread_cond_signal (_cond);
-     }
-     status = pthread_mutex_unlock(_mutex);
-     assert_status(status == 0, status, "mutex_unlock");
-     if (AnyWaiters != 0) {
-        status = pthread_cond_signal(_cond);
-        assert_status(status == 0, status, "cond_signal");
-     }
+  // Transitions for _Event:
+  //    0 :=> 1
+  //    1 :=> 1
+  //   -1 :=> either 0 or 1; must signal target thread
+  //          That is, we can safely transition _Event from -1 to either
+  //          0 or 1. Forcing 1 is slightly more efficient for back-to-back
+  //          unpark() calls.
+  // See also: "Semaphores in Plan 9" by Mullender & Cox
+  //
+  // Note: Forcing a transition from "-1" to "1" on an unpark() means
+  // that it will take two back-to-back park() calls for the owning
+  // thread to block. This has the benefit of forcing a spurious return
+  // from the first park() call after an unpark() call which will help
+  // shake out uses of park() and unpark() without condition variables.
+
+  if (Atomic::xchg(1, &_Event) >= 0) return;
+
+  // Wait for the thread associated with the event to vacate
+  int status = pthread_mutex_lock(_mutex);
+  assert_status(status == 0, status, "mutex_lock");
+  int AnyWaiters = _nParked;
+  assert(AnyWaiters == 0 || AnyWaiters == 1, "invariant");
+  if (AnyWaiters != 0 && WorkAroundNPTLTimedWaitHang) {
+    AnyWaiters = 0;
+    pthread_cond_signal(_cond);
+  }
+  status = pthread_mutex_unlock(_mutex);
+  assert_status(status == 0, status, "mutex_unlock");
+  if (AnyWaiters != 0) {
+    status = pthread_cond_signal(_cond);
+    assert_status(status == 0, status, "cond_signal");
   }
 
   // Note that we signal() _after dropping the lock for "immortal" Events.
@@ -5187,13 +5192,14 @@
 }
 
 void Parker::park(bool isAbsolute, jlong time) {
+  // Ideally we'd do something useful while spinning, such
+  // as calling unpackTime().
+
   // Optional fast-path check:
   // Return immediately if a permit is available.
-  if (_counter > 0) {
-      _counter = 0 ;
-      OrderAccess::fence();
-      return ;
-  }
+  // We depend on Atomic::xchg() having full barrier semantics
+  // since we are doing a lock-free update to _counter.
+  if (Atomic::xchg(0, &_counter) > 0) return;
 
   Thread* thread = Thread::current();
   assert(thread->is_Java_thread(), "Must be JavaThread");
@@ -5234,6 +5240,8 @@
     _counter = 0;
     status = pthread_mutex_unlock(_mutex);
     assert (status == 0, "invariant") ;
+    // Paranoia to ensure our locked and lock-free paths interact
+    // correctly with each other and Java-level accesses.
     OrderAccess::fence();
     return;
   }
@@ -5270,12 +5278,14 @@
   _counter = 0 ;
   status = pthread_mutex_unlock(_mutex) ;
   assert_status(status == 0, status, "invariant") ;
+  // Paranoia to ensure our locked and lock-free paths interact
+  // correctly with each other and Java-level accesses.
+  OrderAccess::fence();
+
   // If externally suspended while waiting, re-suspend
   if (jt->handle_special_suspend_equivalent_condition()) {
     jt->java_suspend_self();
   }
-
-  OrderAccess::fence();
 }
 
 void Parker::unpark() {