The following test code (F#) is not returning the result I'd expect:

let safeCount() =
  let n = 1000000
  let counter = ref 0
  let spinlock = ref <| SpinLock(false)
  let run i0 i1 () =
    for i=i0 to i1-1 do
      let locked = ref false
      try
        (!spinlock).Enter locked
        if !locked then
          counter := !counter + 1
      finally
        if !locked then
          (!spinlock).Exit()
  let thread = System.Threading.Thread(run 0 (n/2))
  thread.Start()
  run (n/2) n ()
  thread.Join()
  !counter

I'd expect the SpinLock to mutually exclude the counter and, therefore, for it to return counts of 1,000,000 but, instead, it returns smaller values as if no mutual exclusion is occurring.

Any ideas what's wrong?