modify    : mutex
noreaders : event
readers : int = 0
countmodify : mutex

WriteLock:
  request(modify)
  wait(noreaders)

WriteUnlock:
  release(modify)

ReadLock:
  request(modify)
    request(countmodify)
      if(readers==0)
        reset(noreaders)
      readers++
    release(countmodify)
  release(modify)

ReadUnlock:
  request(countmodify)
    readers--
    if(readers==0)
      signal(noreaders)
  release(countmodify)