Hi. I'm using the hslogger library for logging pure code, using the following function: uLog :: Priority -> String -> a -> a uLog pri str x = unsafePerformIO $ logM rootLoggerName pri str >> return x Everything was working fine, till I used uLog on a very accessed function. Then the program started to fail with: thread blocked indefinitely in an MVar operation I've changed uLog to: uLog :: Priority -> String -> a -> a uLog pri str x = unsafePerformIO $ hPutStrLn stderr (show str) >> return x And I noticed on the log that two messages were sent "at the same time" on the point where the version with hslogger fails: "canonicate 4 "canonicate 1 \r\n\t\t\t\t\t\t\t\tRio Branco = rio branco" "canonicate 2 AC\r\n\t\t\t\t\t\t\t\t = ac" ac = ac" They were sent as (in any order): "canonicate 4 ac = ac" "canonicate 1 \r\n\t\t\t\t\t\t\t\tRio Branco = rio branco" "canonicate 2 AC\r\n\t\t\t\t\t\t\t\t = ac" I tried changing hslogger to avoid calling modifyMVar when getting logger for rootLoggerName, and using withMVar instead, but the problem persisted. I tried to create a simple test case to reproduce the problem, but I couldn't, all test cases I created worked as expected. As I was with no other ideas, I tried some things that make little sense. This happens with optimization or without it. Following http://hackage.haskell.org/trac/ghc/ticket/5859 I tried using -fno-state-hack, but the problem persisted. I tried adding NOINLINE to uLog, without success. I also tried changing unsafePerformIO to unsafeDupablePerformIO. I thought that the only thing I needed to take care while using unsafePerformIO was knowing that the time of execution is undetermined and that it could even run more than once. This is not a problem for my logging. Is there something else I should be aware while using unsafePerformIO? Greetings. -- marcot http://marcot.eti.br/