[Haskell-cafe] Barrier implementation

[Peter Eriksen]

Greeting,

Something is not working for me, and I could use some more eyes on this.
What I'm trying to accomplish is to implement a simpel barrier for ten
worker threads (id = 0..9) using STM.  With or without the barrier, the
program produces an unordered interleaving of the output from the
workers.  Here's what I get with the program below:

$ ghc --make Main.lhs
$ a.out
0134568027913457896012579026813423904671238455702468159367839684012570279134685049137825901642375689134057892610462578903156012389473268457910267801345923924567801304689235714013679458256702465913878...

And here's what I get without the line "atomically $ barrier tv id":

$ a.out
1249056782934567210845619720538461975203698469175203469850123485076912348579406123894625738942106381592740631859274163841092315768491302578416930728254169302785693024917853029640217390856490...

The first run should've been something like:
012345678901234567890123456789012345...
since each worker thread 0..9 should write its id out once per
iteration,
and the workers should iterate in sync.

Here's the code:

> module Main where
> 
> import Control.Concurrent
> import Control.Concurrent.STM
> import System.Random
> 
> worker :: Int -> TVar Int -> IO ()
> worker id tv = do
> 	sleepingTime <- randomRIO (0, 50000)
> 	threadDelay sleepingTime 
> 	putStr $ show id
> 	atomically $ barrier tv id
> 	worker id tv

Each worker sleeps for some time, then outputs its id and waits at 
the barrier for all the other workers to finish their sleep+output.
 
> barrier :: TVar Int -> Int -> STM ()
> barrier tv id = do
> 	passed <- readTVar tv
> 	if (passed `mod` 10 == id) 
> 		then writeTVar tv (passed+1)
> 		else retry

The barrier is simply a global variable, tv, which holds the number of
times any worker passed the barrier.  Now, a worker may only pass the 
barrier iff the worker with an id one less just passed, or else it 
should block. 

> main :: IO ()
> main = do
> 	tv <- atomically $ newTVar 0
> 	for [0..9] $ \i -> forkIO $ worker i tv 
> 	threadDelay (10*10^6)
> 
> for = flip mapM_

The main thread just initializes the pass counter, starts 10 worker
threads, and waits for ten seconds.

I'd like to hear some comments on the approach, and perhaps even some
insight into why it doesn't work.

Regards,

Peter Eriksen