[GHC] #10793: Incorrect blocked on MVar detection

[GHC]

#10793: Incorrect blocked on MVar detection
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        Reporter:  NeilMitchell      |                   Owner:  simonmar
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Runtime System    |                 Version:  7.10.1
      Resolution:  wontfix           |                Keywords:
Operating System:  Windows           |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonmar):

 Well, thinking about it a bit more, in this case it would be possible to
 do something differently.  We have

 * a main thread, blocked on an MVar
 * a child thread, in an infinite loop
 * the MVar, reachable from both threads.

 All of these things are unreachable from the roots, so we definitely can't
 make progress without throwing an exception.  Normally we throw exceptions
 to all the deadlocked threads in this case, because it's the only
 deterministic thing we can do. (picking a thread randomly to receive the
 exception doesn't seem good).

 We could refine it slightly so that if there are both NonTermination
 exceptions and BlockedIndefinitelyOnMVar exceptions, we only throw the
 NonTermination exceptions.  We know that waking up the
 BlockedIndefinitelyOnMVar threads can't unblock the NonTermination
 threads, but the reverse might be true.

 That wouldn't change the more common case of this, which is where both
 threads are blocked on MVars:

 {{{
 main = do
     v <- newEmptyMVar
     v2 <- newEmptyMVar
     forkFinally (takeMVar v2) (putMVar v)
     print =<< takeMVar v
 }}}

 here we'd still get both exceptions at the same time.  So I'm not sure how
 I feel about doing something different for `NonTermination`, possibly
 making the situation more complicated.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10793#comment:3>
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