4605   int v ;

  4606 

  4606   for (;;) {

  4607   // Transitions for _Event:

  4607       v = _Event ;      // Increment _Event if it's < 1.

  4608   //    0 :=> 1

  4608       if (v > 0) {

  4609   //    1 :=> 1

  4609          // If it's already signaled just return.

  4610   //   -1 :=> either 0 or 1; must signal target thread

  4610          // The LD of _Event could have reordered or be satisfied

  4611   //          That is, we can safely transition _Event from -1 to either

  4611          // by a read-aside from this processor's write buffer.

  4612   //          0 or 1. Forcing 1 is slightly more efficient for back-to-back

  4612          // To avoid problems execute a barrier and then

  4613   //          unpark() calls.

  4613          // ratify the value.  A degenerate CAS() would also work.

  4614   // See also: "Semaphores in Plan 9" by Mullender & Cox

  4614          // Viz., CAS (v+0, &_Event, v) == v).

  4615   //

  4615          OrderAccess::fence() ;

  4616   // Note: Forcing a transition from "-1" to "1" on an unpark() means

  4616          if (_Event == v) return ;

  4617   // that it will take two back-to-back park() calls for the owning

  4617          continue ;

  4618   // thread to block. This has the benefit of forcing a spurious return

  4618       }

  4619   // from the first park() call after an unpark() call which will help

  4619       if (Atomic::cmpxchg (v+1, &_Event, v) == v) break ;

  4620   // shake out uses of park() and unpark() without condition variables.

  4620   }

  4621 

  4621   if (v < 0) {

  4622   if (Atomic::xchg(1, &_Event) >= 0) return;

  4622      ::SetEvent (_ParkHandle) ;

  4623 

  4623   }

  4624   ::SetEvent(_ParkHandle);