[SOLVED] Forked IORef reader function seems to stall main thread

Forked IORef reader function seems to stall main thread

I was doing some experiments with concurrency and memory visibility and ran into this strange behavior (see comments inline):

module Main
    where

import Data.IORef
import Control.Concurrent
import System.CPUTime

import System.IO

main = do
    hSetBuffering stdout NoBuffering

    r <- newIORef False
    putStrLn "forking..."  -- PRINTED
    forkIO $ f r
    threadDelay 1000000

    putStrLn "writeIORef"  -- NEVER PRINTED
    writeIORef r True

    threadDelay maxBound

f :: IORef Bool -> IO ()
f r = readIORef r >>= \b-> if b then print "NEVER PRINTED" else f r

I was expecting perhaps the writeIORef not to be visible to the child thread, but not for the main thread to simply (apparently) stall.

Compiled on ghc 7.8.3

 cabal exec ghc -- --make -fforce-recomp -O2 -threaded visibility.hs

and run with

./visibility +RTS -N

What's happening here?

EDIT: So my machine has two real cores and two hyperthreading cores, so with +RTS -N GHC sees 4 capabilities. Per Gabriel Gonzalez's answer I tried out the following to see if maybe the scheduler was putting both threads on the same physical processor:

module Main
    where

import Data.IORef
import Control.Concurrent    
import GHC.Conc(threadCapability,myThreadId,forkOn)

main = do    
    r <- newIORef False
    putStrLn "going..."

    (cap,_) <- threadCapability =<< myThreadId
    forkOn (cap+1) $ f r                    -- TRIED cap+1, +2, +3....
    threadDelay 1000000

    putStrLn "writeIORef"                   -- BUT THIS STILL NEVER RUNS
    writeIORef r True

    threadDelay maxBound

f :: IORef Bool -> IO ()
f r = readIORef r >>= \b-> if b then print "A" else f r

Solution

It looks like this is probably an ancient ghc bug #367.