--- old/src/os/windows/vm/os_windows.cpp	Thu Jan 17 10:09:35 2013
+++ new/src/os/windows/vm/os_windows.cpp	Thu Jan 17 10:09:35 2013
@@ -4565,6 +4565,7 @@
     }
     v = _Event ;
     _Event = 0 ;
+    // see comment at end of os::PlatformEvent::park() below:
     OrderAccess::fence() ;
     // If we encounter a nearly simultanous timeout expiry and unpark()
     // we return OS_OK indicating we awoke via unpark().
@@ -4602,25 +4603,25 @@
 
 void os::PlatformEvent::unpark() {
   guarantee (_ParkHandle != NULL, "Invariant") ;
-  int v ;
-  for (;;) {
-      v = _Event ;      // Increment _Event if it's < 1.
-      if (v > 0) {
-         // If it's already signaled just return.
-         // 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.  A degenerate CAS() would also work.
-         // Viz., CAS (v+0, &_Event, v) == v).
-         OrderAccess::fence() ;
-         if (_Event == v) return ;
-         continue ;
-      }
-      if (Atomic::cmpxchg (v+1, &_Event, v) == v) break ;
-  }
-  if (v < 0) {
-     ::SetEvent (_ParkHandle) ;
-  }
+
+  // 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;
+
+  ::SetEvent(_ParkHandle);
 }