Test case: (compile with ghc 8.2.1 and -threaded option)

module Main where

import Control.Concurrent
import Control.Monad
import Data.Word
import Foreign.Marshal.Alloc
import Foreign.Ptr
import Foreign.Storable

foreign import ccall safe "test"
    c_test :: Ptr Word32 -> IO ()

main :: IO ()
main = do
    replicateM_ 1000 $ threadDelay 1000
    _ <- forkIO $ forever $ threadDelay 100
    allocaBytes 4 $ \p -> forever $ do
        c_test p
        x <- peek p
        unless (x == 0xDEADBEEF) $ putStrLn "value mismatch"

void test(unsigned int *buf) {
    *buf = 0xDEADBEEF;
}

On my machine, it detects a few value mismatches before crashing with sigsegv.

$ time ./.stack-work/install/x86_64-linux-nopie/nightly-2017-10-10/8.2.1/bin/bug
value mismatch
value mismatch
value mismatch
value mismatch
zsh: segmentation fault (core dumped)  ./.stack-work/install/x86_64-linux-nopie/nightly-2017-10-10/8.2.1/bin/bug
./.stack-work/install/x86_64-linux-nopie/nightly-2017-10-10/8.2.1/bin/bug  2.11s user 0.25s system 66% cpu 3.543 total

I believe this is what is causing crashes in xmobar. See discussion: https://github.com/jaor/xmobar/issues/310. Note that the crash in xmobar still happens without -threaded option, while this example only breaks when compiled with -threaded.