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.
Trac metadata
Trac field | Value |
---|---|
Version | 6.10.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | libraries/base |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |