使用Haskell回調函數調用多線程C FFI時的計劃錯誤

Question

以下是在運行時拋出計劃錯誤（GHC 7.0.3，Mac OS 10.7，x86_64）的Haskell/C FFI代碼。我尋找錯誤的解釋，但沒有找到任何相關的。

C代碼（mt.c）：

#include <pthread.h> 
#include <stdio.h> 

typedef void(*FunctionPtr)(int); 

/* This is our thread function. It is like main(), but for a thread*/ 
void *threadFunc(void *arg) 
{ 
    FunctionPtr fn; 
    fn = (FunctionPtr) arg; 
    fn(1); //call haskell function with a CInt argument to see if it works 
} 

void create_threads(FunctionPtr* fp, int numThreads) 
{ 
    pthread_t pth[numThreads]; // array of pthreads 
    int t; 
    for (t=0; t < numThreads;){ 
    pthread_create(&pth[t],NULL,threadFunc,*(fp + t)); 
    t++; 
    } 

    printf("main waiting for all threads to terminate...\n"); 
    for (t=0; t < numThreads;t++){ 
    pthread_join(pth[t],NULL); 
    } 
} 

Haskell代碼（t.hs） - 它在mt.c上述調用create_threads與FunPtrStorable Vector到Haskell的功能f（施加前三個參數後f）：

{-# LANGUAGE BangPatterns #-} 
import Control.Concurrent (forkIO, threadDelay, MVar, newEmptyMVar, putMVar, takeMVar) 
import qualified Data.Vector.Storable.Mutable as MSV 
import qualified Data.Vector.Storable as SV 
import Control.Monad.Primitive (PrimState) 
import Control.Monad (mapM, forM_) 
import Foreign.Ptr (Ptr, FunPtr) 
import Foreign.C.Types (CInt) 


type Length = CInt 

-- | f is a function that is called back by create_threads in mt.c 
f :: MVar Int -> MSV.MVector (PrimState IO) CInt -> Length -> CInt -> IO() 
f m v l x = do 
       !i <- takeMVar m 
       case (i< fromIntegral l) of 
       True -> MSV.unsafeWrite v i x >> print x >> putMVar m (i+1) 
       False -> return() -- overflow 

-- a "wrapper" import gives us a converter for converting a Haskell function to a foreign function pointer 
foreign import ccall "wrapper" 
    wrap :: (CInt -> IO()) -> IO (FunPtr (CInt -> IO())) 

foreign import ccall safe "create_threads" 
    createThreads :: Ptr (FunPtr (CInt -> IO())) -> CInt -> IO() 

main = do 
    let threads = [1..4] 
    m <- mapM (\x -> newEmptyMVar) $ threads 
    -- intialize mvars with 0 
    forM_ m $ \x -> putMVar x 0 
    let l = 10 
    -- intialize vectors of length 10 that will be filled by function f 
    v <- mapM (\x -> MSV.new l) threads 
    -- create a list of function pointers to partial function - the partial function is obtained by applying first three arguments to   function f 
    lf <- mapM (\(x,y) -> wrap (f x y (fromIntegral l))) $ zip m v 
    -- convert above function list to a storable vector of function pointers 
    let fv = SV.fromList lf 
    -- call createThreads with storable vector of function pointers, and number of threads - createThreads will spawn threads which will use function pointers for callback 
    SV.unsafeWith fv $ \x -> createThreads x (fromIntegral $ length threads) 

請忽略代碼中的不安全部分 - 我的目標是使用Haskell FFI測試多線程回調附帶C代碼。當我編譯它，並運行它，我得到下面的錯誤：

$ ghc -O2 t.hs mt.c -lpthread 
[1 of 1] Compiling Main    (t.hs, t.o) 
Linking t ... 
$ ./t 
main waiting for all threads to terminate... 
t: schedule: re-entered unsafely. 
    Perhaps a 'foreign import unsafe' should be 'safe'? 
$ uname -a 
Darwin desktop.local 11.2.0 Darwin Kernel Version 11.2.0: Tue Aug 9 20:54:00 PDT 2011; root:xnu-1699.24.8~1/RELEASE_X86_64 x86_64 
$ ghc --version 
The Glorious Glasgow Haskell Compilation System, version 7.0.3 

如果我的C線程調用回哈斯克爾函數f時間表錯誤僅發生。我想這很可能是我的代碼中存在一個錯誤，比其中一個庫或GHC中存在一個錯誤。所以，我想首先在這裏查看關於錯誤原因的指針。

Answer 1

在這種情況下，發生了schedule錯誤，因爲haskell代碼編譯時沒有-threaded選項。

haskell代碼正在調用C函數create_threads，該函數爲C函數生成多個線程。 threadFunc回調Haskell函數f。因此，即使Haskell代碼編譯時沒有-threaded選項，它仍然會導致多個C線程執行f。

GHC運行時調度程序很好地捕捉到執行多線程而沒有線程運行時的這種疏忽，並將其標記爲錯誤。這比一個神祕的運行時崩潰要好得多。當我在GHC代碼庫中檢查rts/schedule.c代碼時，我意識到了這個疏忽，並且看到了下面的註釋。它給我帶來了大約threaded運行時未啓用：

// Check whether we have re-entered the RTS from Haskell without 
// going via suspendThread()/resumeThread (i.e. a 'safe' foreign 
// call).