为了解决GHC finalizers的并发问题，可以使用下列代码示例中的技术。当多个线程同时访问finalizer时，互斥量可以防止并发访问。此外，使用标志变量可以帮助在代码执行过程中跟踪finalizer的状态。

import Control.Concurrent
import Control.Monad
import Foreign.ForeignPtr

-- | An example data type containing a finalized resource.
data Example = Example
    { exFinalizer :: !(ForeignPtr ())
    -- ^ A pointer to the finalizer for the resource in question.
    }

-- | A dummy finalizer that is used to demonstrate the use of finalizers.
-- In practice, this finalizer would perform some kind of useful cleanup.
exampleFinalizer :: IO ()
exampleFinalizer = forever (threadDelay 1000000)

-- | Create a new example resource.
newExample :: IO Example
newExample = do
    fPtr <- newForeignPtr_ nullPtr
    addForeignPtrFinalizer fPtr exampleFinalizer
    return Example { exFinalizer = fPtr }

-- | Free the finalizer for an example resource.
freeExampleFinalizer :: Example -> IO ()
freeExampleFinalizer ex = do
    -- Set the finalizer flag to indicate that the finalizer should
    -- be skipped if called by a finalizer from another thread.
    writeIORef (finalizing ex) True
    -- Wait for any currently executing finalizers to terminate.
    -- This is an important step to ensure that the finalizer is not
    -- modified or executed after it has been freed.
    waitForFinalizer (exFinalizer ex)
    -- Remove the finalizer from the finalizer table.
    touchForeignPtr (exFinalizer ex)
    -- Set the finalizer flag to indicate that the finalizer has been removed.
    writeIORef (finalized ex) True

-- | A signal indicating that the finalizer is executing.
finalizing :: Example -> IORef Bool
finalizing ex = unsafePerformIO (newIORef False)

-- | A signal indicating that the finalizer has been removed.
finalized :: Example -> IORef Bool
finalized ex = unsafePerformIO (newIORef False)

-- | Wait for the finalizer associated with a given 'ForeignPtr' to terminate.
waitForFinalizer :: ForeignPtr a -> IO ()
waitForFinalizer fptr = withForeignPtr fptr $ \ptr ->
    waitForFinalizer' (castPtr ptr) False
    where
        waitForFinalizer' :: Ptr a -> Bool -> IO ()
        waitForFinalizer' ptr prevFinalizerStatus = do
            threadDelay 10000
            final