On 03/05/2012 17:14, Bas van Dijk wrote:
On 3 May 2012 17:31, Edward Z. Yangezy...@mit.edu wrote:
Excerpts from Bas van Dijk's message of Thu May 03 11:10:38 -0400 2012:
As can be seen, the putMVar is executed successfully. So why do I get
the message: thread blocked indefinitely in an MVar operation?
GHC will send BlockedIndefinitelyOnMVar to all threads involved
in the deadlock, so it's not unusual that this can interact with
error handlers to cause the system to become undeadlocked.
But why is the BlockedIndefinitelyOnMVar thrown in the first place?
According to the its documentation and your very enlightening article
it is thrown when:
The thread is blocked on an MVar, but there are no other references
to the MVar so it can't ever continue.
The first condition holds for the main thread since it's executing
takeMVar. But the second condition doesn't hold since the forked
thread still has a reference to the MVar.
The forked thread is deadlocked, so the MVar is considered unreachable
and the main thread is also unreachable. Hence both threads get sent
the exception.
The RTS does this analysis using the GC, tracing the reachable objects
starting from the roots. It then send an exception to any threads which
were not reachable, which in this case is both the main thread and the
child, since neither is reachable.
We (the user) knows that waking up the child thread will unblock the
main thread, but the RTS doesn't know this, and it's not clear how it
could find out easily (i.e. without multiple scans of the heap).
Cheers,
Simon
I just tried delaying the thread before the putMVar:
-
main :: IO ()
main = do
mv- newEmptyMVar
_- forkIO $ do
catch action
(\e - putStrLn $ I solved the Halting Problem: ++
show (e :: SomeException))
putStrLn Delaying for 2 seconds...
threadDelay 200
putStrLn putting MVar...
putMVar mv ()
putStrLn putted MVar
takeMVar mv
-
Now I get the following output:
loop: thread blocked indefinitely in an MVar operation
I solved the Halting Problem:loop
Delaying for 2 seconds...
Now it seems the thread is killed while delaying. But why is it
killed? It could be a BlockedIndefinitelyOnMVar that is thrown.
However I get the same output when I catch and print all exceptions in
the forked thread:
main :: IO ()
main = do
mv- newEmptyMVar
_- forkIO $
handle (\e - putStrLn $ Oh nooo: ++
show (e :: SomeException)) $ do
catch action
(\e - putStrLn $ I solved the Halting Problem: ++
show (e :: SomeException))
putStrLn Delaying for 2 seconds...
threadDelay 200
putStrLn putting MVar...
putMVar mv ()
putStrLn putted MVar
takeMVar mv
Bas
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