[Haskell-cafe] strange hangs with -threaded runtime

I have a rather large program that works fine with ghc's default runtime, but now I'd like to build it with -threaded, and this causes it to hang in a way I have not been able to pin down to anything specific. It hangs at different points each time it's run; running it in strace will consistently avoid the problem. If I look at it once it's hung, I always see a pair of processes, both blocked in futex: joey@wren:~/src/git-annex>strace -p 21749 Process 21749 attached - interrupt to quit futex(0x8b03930, FUTEX_WAIT_PRIVATE, 69, NULL^C Process 21749 detached joey@wren:~/src/git-annex>strace -p 21811 Process 21811 attached - interrupt to quit futex(0x8b03df8, FUTEX_WAIT_PRIVATE, 2, NULL Are there any common gotchas with converting to the threaded runtime that might provide a hint to what's wrong? My code does not currently use Control.Concurrent or threads, although it does use some libraries like MissingH that use forkIO or forkOS. GHC 7.4.1, 7.4.2 on Debian Linux -- see shy jo