[GHC] #7970: Thread GC frees roots before thread actually finishes

[GHC]

#7970: Thread GC frees roots before thread actually finishes
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    Reporter:  joeyadams         |       Owner:  simonmar        
        Type:  bug               |      Status:  new             
    Priority:  high              |   Milestone:  7.8.1           
   Component:  Runtime System    |     Version:  7.6.3           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  Runtime crash   
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:  7170              |  
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Comment(by joeyadams):

 Replying to [comment:1 simonmar]:
 > This is technically not a bug - the thread is unreachable when it is
 deadlocked, and so is the weak pointer.

 It only makes sense to call a thread "unreachable" if it will never run
 again.  But when the RTS detects an unreachable thread and prods it with
 `NonTermination`, the thread continues running and accessing its local
 variables, meaning it should be considered reachable again.

 Here are some valid solutions, based on my understanding:

  1. Treat a thread as reachable until it actually finishes or dies.
 Downside: deadlocked threads would no longer be GCed.

  2. Don't throw `BlockedIndefinitelyOnMVar`, `NonTermination`, etc., just
 free the thread.  This keeps the "unreachable" interpretation valid.
 Downside: manually-managed resources (including those managed with
 bracket) would be leaked.  This is like (1), but at least allows Haskell
 objects to be GCed.

  3. When the RTS detects a deadlocked thread, treat it as reachable again
 before making it throw an exception.  This keeps the current behavior, but
 seems difficult to implement.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7970#comment:2>
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