[GHC] #9296: Acquire all_tasks_mutex in forkProcess

[GHC]

#9296: Acquire all_tasks_mutex in forkProcess
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       Reporter:  edsko             |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 In `forkProcess` we acquire a bunch of mutexes so make sure that when we
 fork the child doesn't see an inconsistent state of any global data
 structures. However, we do not acquire the `all_tasks_mutex`, which means
 that the child might have an inconsistent view of the `all_tasks` list.

 https://phabricator.haskell.org/D60

 Sadly, I do not have a test case illustrating that this is in fact a
 problem. Found this while working tracking down a deadlock in forkProcess
 (https://ghc.haskell.org/trac/ghc/ticket/9295).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9296>
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