[reactive] Re: black hole detection and concurrency

[Bertram Felgenhauer]

Conal Elliott wrote:
> Thanks very much for these ideas.  Peter Verswyvelen suggested running the
> example repeatedly to see if it always runs correctly.  He found, and I
> verified, that the example runs fine with Bertram's last version of unamb
> below, *unless* it's compiled with -threaded and run with +RTS -N2.  In the
> latter case, it locks up after a while.

It seems that we've found an RTS bug. If a thread is started with
exceptions blocked, then throwTo might never deliver its exception and
block forever, as can be seen with the following test program, which
locks up after a while (immediately with the non-threaded RTS)

  import Control.Exception
  import Control.Concurrent
  import Control.Monad
  
  test n = do
      t <- block $ forkIO yield
      yield
      putStr $ show n ++ ": kill\n"
      killThread t
  
  main = forM_ [1..] test

Or, even more convincing:

  import Control.Exception
  import GHC.Conc
  
  main = do
      t1 <- block $ forkIO yield
      t2 <- forkIO $ killThread t1
      yield
      yield
      threadStatus t1 >>= print
      threadStatus t2 >>= print

prints (fairly reliably, it seems):

  ThreadFinished
  ThreadBlocked BlockedOnException

(Trac is giving me errors right now. I'm planning to report this later.)

> I also tried a version with brackAsync and found that it eventually locks up
> even under ghci.  When compiled & run multi-threaded, it locks up almost
> immediately.

> -- This one locks up after a while even in ghci.  When compiled -threaded
> -- and run +RTS -N2, it locks up almost immediately.
> a `race` b = do
>    v <- newEmptyMVar
>    let t x = x >>= putMVar v
>    withThread (t a) $ withThread (t b) $ takeMVar v
>  where
>   withThread u v = brackAsync (forkIO u) killThread (const v)

At the point the 'forkIO' is run, exceptions are blocked, making the
thread basically immortal. Using

>   withThread u v = brackAsync (forkIO $ unblock u) killThread (const v)

we get the same behaviour as with my 'race' - it works for a while, but
locks up eventually.

I believe that the actual lockup is similar to the test programs above
in all cases - what's different is just the probability of triggering
it.

regards,

Bertram