ReadWriteLock fail

ReadWriteLockStress.hs

module Main where

import Control.Concurrent ( forkIO, threadDelay )
import Control.Monad ( forever, replicateM_ )
import qualified Control.Concurrent.ReadWriteLock as RWL
import System.Random ( randomRIO )

main :: IO ()
main = do
  lock <- RWL.new
  
  let
    reader = forever $ do
      randomRIO (0, 100) >>= threadDelay
      RWL.withRead lock $ putStr "r"
      
    writer = forever $ do
      randomRIO (0, 100) >>= threadDelay
      RWL.withWrite lock $ putStr "w"
    
  replicateM_ 10 $ forkIO reader
  replicateM_ 10 $ forkIO writer
  writer

An example run (GHC 7.6.3, Linux 64):

./Main 
rrrrrrrrrrrrrwrrrrrrrrwwwwwwwwwwrrwrrrrrrrrwwwwwwwwwwrrwrrrrrrrrwwwwwwwwwwrrrwrrrrrrrwwwwwwwwwwrrrrwrrrrrrwwrwwwwwwwwrrrrwrrrrrwwrwwwwwwwrwrrrwrrrrwrwrwwwwwwwrrwrrwrrrrwrrwrwwwwwwwrrwrwrrrrwrrwrwwwwwwrrwwrrrwrrwrrrwwwwwwwrrwwrrrwrwrrrrwwwwwwwrrwrwrrrwrrwrrrwwwwwwwrrwrwrrwrrrwrrwwwwwwrwwrrwrrwrrwrrrwwwwwwrwwrrwrrwrrwrrrwwwwwwrwwrrwrrwrrrwrwrwwwwwwrwrrwrrwrrwrrwrwwwwwwwrrrwrrwrrwrrrwwwwwwwwwrrrrwrrrwwrwrrwwwwwwwrrrrwrrrwwwrrrwwwwwwwrrrwrrrrwwwrwrrwwwwwwrrrwrrwwrrwrwrwwrwwrwwwrrrwrwrwwrrwrrwwwwrwwrrrwwrrrwwwrwwrrwwrrwwwwrwrrrwwrrwwrwrrrwwwwwrwrrwrwrrwwwrrrwrwwrwwwwrwrrrwrrwrwwrwrwwwrwwrwrrrwrwrwwrrwrwwrwwrwwrrwwrrrrwrwwwrwwrrwwrwrwwrrrrwrrwwwwrwrwwwrrwwrrrwrrwrrwwwwwrwwwrwrrrrwrrwrwrwwwwrwrwwwrrrrwrrrwrwwwwwrwwrwrwrwrrrwrrrwwwwwrwwrwrwrwrrrrrrwwwrwwwwwrrrwwwrrrrrrrwwwwwwwwrrrwrwrwrrrrwrwwwwwwrrwwrwrrwrrwrrwrwwwwwrwrrwrwwrrrwrwrrwwwrwwwrrrwwrwrrrwrwwwrwrwwrwrrwrrrrwwwrwwwrrwwwwrrrrwrrwwwwrrwwwwwrrwrrwrwrrwwrwwwwrwrwrrwwrwrrwrwrrwwwrwrrwrwwwrwrrwwrwwrrwrrrwwwrwrwwwrwrrwwrrwwwwrrrrwwwwwrrrrwwrwrrwrwrwwrwwwwrrwrwrrrwwrrrwwwwwrwrwrwwrrrrrwwrrwwwwrwwwrrrrwrrwwwrrrwwwwwwrwrrrrrrwrwwwrwwrwwwrrwwrrrrwrrwwwwwrrwwwrwrrrwwrrwrrwwrrwwwrrrwwwrwrwrwrrrwwwwrwrwrrrwwrwwrrrwwrwrwwwrrrrwwrrwrwwwrwrwrwwwrrrrrwwrwrwwwrwrwwwrwrrrrrwrrwwwwwrrwwrwwwrrrrwwrrrwwwrwwrwwrrwrwwrrrwrwrwwwwwwrrrwwrrrwrwrrwwwrwrwwrrwwwrwwrrrrrwwwrwrwwrwrrwwwrrwrrwrwrwrwrwrwwrwwwrrwrwwrrwwwrrrrwrrwwwwrrwwwwrrrwwrwrrrwwrwwrwrrwwwrwrrwrwwrwrrwrwrrwwwrwwwrrwrwwrrwrrrrwwwrwwrwrwrwwrwrwrrrrwwwrrrwwwrrwwwwrwrrrrwwwrrrwwwrrwwwwwrrrrrwwrwwrrrwrwwwrwwwrrrrwwrwwwrrrwrrwwwwrwrwrrwwwrrwwrrrrwwrwrwwwrrwwwrwrrwwwrrrrwwrwwrrwwrrrwwwwwrrrwrrwrrwwwwrrrwwwwwrrwwrrwrwrrrrwwwwwrwwrrwwrrwwrrrwwwrrwwrrrwwrwrwwrrwwwwrrwrrrrwwrwwrrwwrrwwwwrwrwrrwrwrwwwrrrrwwwrwwrwrrwwwrrwrwrwrrwwwrwrrwrwrwwwrrwrrwrwwwwrrwwwwrrrwrrwrrrwwwwrwwrwwrrwrwrwrrwwrrwwwrwrwwwrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrwrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrwwrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrwwwwrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrwwrrrrrrrrwrrrwMain: Control.Concurrent.ReadWriteLock.releaseRead: already released
Main: Control.Concurrent.ReadWriteLock.releaseRead: already released
Main: Control.Concurrent.ReadWriteLock.releaseRead: already released
wMain: thread blocked indefinitely in an MVar operation