thread blocked indefinitely in an MVar operation in unsafePerformIO

Simon Marlow marlowsd at
Mon Jul 30 17:00:17 CEST 2012

On 30/07/2012 15:30, Marco Túlio Gontijo e Silva wrote:
> Hi.
> I'm having a problem calling logM from hsLogger inside
> unsafePerformIO.  I have described the problem in Haskell-cafe, so
> I'll avoid repeating it here:
> I've had this problem both with GHC 7.4.1 and 7.4.2.  Do you have any
> suggestion?

Is it possible that the String you are passing to uLog contains 
unevaluated calls to uLog itself, which would thereby create a deadlock 
as uLog tries to take the MVar that is already being held by the same 

We once had this problem with hPutStr where if the argument string 
contained a call to trace, which is unsafePerformIO $ hPutStr, the 
result would be deadlock.  Now hPutStr has to go to some trouble to 
evaluate the input string while not holding the lock.


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