[GHC] #4001: Implement an atomic readMVar

[GHC]

#4001: Implement an atomic readMVar
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        Reporter:           |            Owner:  ezyang
  simonmar                  |           Status:  new
            Type:  task     |        Milestone:  7.6.2
        Priority:  low      |          Version:  6.12.2
       Component:  Runtime  |         Keywords:
  System                    |     Architecture:  Unknown/Multiple
      Resolution:           |       Difficulty:  Moderate (less than a day)
Operating System:           |       Blocked By:
  Unknown/Multiple          |  Related Tickets:
 Type of failure:           |
  None/Unknown              |
       Test Case:           |
        Blocking:           |
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Comment (by ezyang):

 Ah, found the bug...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4001#comment:20>
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