-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 So, the Hint library was recently updated and I was editing Mueval to run with (i386) 6.10, when I discovered that for some reason, DoS'ing expressions were succeeding in rendering my machine unusable. Eventually, I discovered that my watchdog thread didn't seem to be running. But with +RTS -N2 -RTS all my tests did pass! Here's a simple example of what I mean; the following is basically a very lightly adapted version of the actual Mueval code: - ---------------- import Control.Concurrent (forkIO, killThread, myThreadId, threadDelay, throwTo, yield, ThreadId) import System.Posix.Signals (sigXCPU, installHandler, Handler(CatchOnce)) import Control.OldException (Exception(ErrorCall)) main :: IO () main = do tid ThreadId -> IO () watchDog tout tid = do installHandler sigXCPU (CatchOnce $ throwTo tid $ ErrorCall "Time limit exceeded.") Nothing forkIO $ do threadDelay (tout * 100000) -- Time's up. It's a good day to die. throwTo tid (ErrorCall "Time limit exceeded") yield -- give the other thread a chance killThread tid -- Die now, srsly. error "Time expired" return () -- Never reached. Either we error out in -- watchDog, or the evaluation thread finishes. - -------------- Now, from the looks of it, this should always error out with "Time limit exceeded." And the threading is done via forkIO, so it shouldn't matter how it's compiled (be it threaded or no) or run; nor do we need to worry about optimizations, since x has no sensible value for any input - it's always bottom. But the results are very different: gwern@craft:31542~>=ghc -fforce-recomp -O0 example.hs && ./a.out ^C^C gwern@craft:31532~>=ghc -fforce-recomp -O0 -threaded example.hs && ./a.out ^C^C gwern@craft:31536~>=ghc -threaded -fforce-recomp -O0 -threaded example.hs && ./a.out +RTS -N1 -RTS [a minute later] ^C^C^C^C^C^C gwern@craft:31540~>=ghc -threaded -fforce-recomp -O0 -threaded example.hs && ./a.out +RTS -N2 -RTS a.out: Time limit exceeded gwern@craft:31543~>=ghc -fforce-recomp -O2 example.hs && ./a.out a.out: Time limit exceeded gwern@craft:31544~>=ghc -threaded -fforce-recomp -O2 example.hs && ./a.out a.out: Time limit exceeded gwern@craft:31545~>=ghc -threaded -fforce-recomp -O2 example.hs && ./a.out +RTS -N1 -RTS a.out: Time limit exceeded gwern@craft:31546~>=ghc -threaded -fforce-recomp -O2 example.hs && ./a.out +RTS -N2 -RTS a.out: Time limit exceeded So it seems that without optimizations, or without explicit/multiple OS threads, the watchdog thread never gets called! I'm not any sort of expert on parallelism or GHC, but this seems like a bad thing to me. One final note: I suspect there are further ways this can manifest. When compiled with -O2, example.hs terminates (as it should). But Mueval does in fact set -O2 as a ghc-option:, and yet I still found it looping. So, does anyone know whether this is an already known bug or whether it's something else? - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkk63+EACgkQvpDo5Pfl1oITZgCdG2p4VcB6m5IhfqzOT0fi5qrI VagAnRILZSxBzadSj2wlzoOWfBuwdbJo =X61b -----END PGP SIGNATURE-----