[Haskell-cafe] thread killed

[tsuraan]

What sorts of things can cause a thread to get an asynchronous "thread
killed" exception?  I've been seeing rare, inexplicable "thread
killed" messages in my Snap handlers for a long time, but they aren't
from Snap's timeout code.  I recently upgraded to ghc 7.4.1, and that
caused the kills to happen a lot more often, but also gave me some
traceback capabilities.  I tracked the most common kills down to
cryptohash's Crypto.Hash.Tiger.update function, but that's about as
pure a FFI function can be, so I don't know how that would be causing
anything weird to happen.  I also sometimes get the kills in the
Tiger.finalize function, and I get other ones in functions that I
haven't been able to track down yet.  Given that the thread kills
aren't from Snap's timeout code (they happen in under a second, and I
have snap's timeouts turned to an insanely high value), what sort of
other things cause ThreadKilled exceptions?

Thanks for any help; this is really driving me mad :-/