[GHC] #12019: Profiling option -hb is not thread safe

[GHC]

#12019: Profiling option -hb is not thread safe
-------------------------------------+-------------------------------------
        Reporter:  erikd             |                Owner:  erikd
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Runtime System    |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11978, #12009    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by erikd):

 Fixing this is definitely not trivial.

 I've placed locks around some of the shared mutable data structures, but I
 still get the same assertion (in function `processHeapClosureForDead`)
 failing:

 {{{
 ASSERT(((LDVW(c) & LDV_CREATE_MASK) >> LDV_SHIFT) > 0);
 }}}

 because the `overwritingClosure` has already been called on it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12019#comment:3>
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