#1283: thread-safe getStdRandom and newStdGen
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.6.2
Component: libraries/base | Version: 6.6
Severity: normal | Keywords:
Difficulty: Easy (1 hr) | Testcase:
Architecture: Multiple | Os: Multiple
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Implementations of getStdRandom and newStdGen use unsynchronised calls to
getStdGen and setStdGen, allowing a race condition in which duplicate
random numbers can be returned in multiple threads.
Patch attached. Tested against GHC 6.6, on Linux amd64.
The following code used with +RTS -N2 demonstrates the race condition.
{{{
import Control.Concurrent
import Control.Monad
import Data.Sequence hiding (take)
import System.Random
threads = 4
samples = 5000
main = loopTest threads samples
loopTest t s = do
isClean <- testRace t s
putStrLn $ if isClean
then "no race condition found"
else "race condition found"
loopTest t s
testRace t s = do
ref <- liftM (take (t*s) . randoms) getStdGen
iss <- threadRandoms t s
return (isInterleavingOf (ref::[Int]) iss)
threadRandoms t s = do
vs <- sequence $ replicate t $ do
v <- newEmptyMVar
forkIO (sequence (replicate s randomIO) >>= putMVar v)
return v
mapM takeMVar vs
isInterleavingOf xs yss = iio xs (viewl $ fromList yss) EmptyL where
iio (x:xs) ((y:ys) :< yss) zss
| x /= y = iio (x:xs) (viewl yss) (viewl (fromViewL zss |> (y:ys)))
| x == y = iio xs (viewl ((ys <| yss) >< fromViewL zss)) EmptyL
iio xs ([] :< yss) zss = iio xs (viewl yss) zss
iio [] EmptyL EmptyL = True
iio _ _ _ = False
fromViewL (EmptyL) = empty
fromViewL (x :< xs) = x <| xs
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1283>
GHC <http://www.haskell.org/ghc/>
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