The following code prints "finalized", which means the finalizer runs while the "call" function is still running.

Steps to reproduce:

% ghc finalizer.hs foreign.c -threaded
% ./finalizer

finalizer.hs:

import Control.Concurrent
import Control.Monad
import Foreign.C.Types
import Foreign.ForeignPtr
import Foreign.Ptr

main :: IO ()
main = do
  _ <- forkIO $ do
    fptr <- newForeignPtr finalizer nullPtr
    forever $ withForeignPtr fptr call
  threadDelay 1000000

foreign import ccall "&finalizer" finalizer :: FunPtr (Ptr CInt -> IO ())
foreign import ccall "call" call :: Ptr CInt -> IO ()

foreign.c

#include <stdio.h>

int finalized = 0;

void finalizer(int *a)
{
  finalized = 1;
  puts("finalizer");
}

void call(int *a)
{
  int i;
  unsigned w = 0;
  puts("begin call");
  for(i = 0; i < 100000000; i++)
  {
    if(finalized)
      puts("finalized");
    w += i;
  }
  printf("end call: %u\n", w);
}