QSem fails with negative quantities
The following program should always give 100 (I think). It doesn't:
import Data.IORef import Control.Concurrent main = do sem <- newQSem (-99) r <- newIORef 0 let incRef = atomicModifyIORef r (\a -> (a+1,a)) sequence_ $ replicate 100 $ forkIO $ incRef >> signalQSem sem waitQSem sem v <- readIORef r print v
With a 2 processor machine on Windows, using GHC 6.8.3 and 6.10.2 and +RTS -N3 I usually get 100, but occasionally get answers such as 49, 82, 95. With +RTS -N2 it almost always works.
From reading the implementation of QSem, it doesn't seem that negative availability was considered. A quick look suggests a better implementation might be:
-- Invariant: avail >= 1 ==> null blocked waitQSem :: QSem -> IO () waitQSem (QSem sem) = do (avail,blocked) <- takeMVar sem -- gain ex. access if avail > 0 then putMVar sem (avail-1,) else do block <- newEmptyMVar putMVar sem (avail, blocked++[block]) -- changed line takeMVar block signalQSem :: QSem -> IO () signalQSem (QSem sem) = do (avail,blocked) <- takeMVar sem -- changed below if null blocked || avail < 0 then putMVar sem (avail+1,blocked) else putMVar sem (avail, tail blocked) putMVar (head blocked) ()
Writing parallel code is hard, so I could have easily got this wrong. I haven't looked at QSemN, which may need similar fixes (or may already deal with this)
Marking as severity major because it can cause incorrect parallel behaviour.