[GHC] #11978: running a profiled build of shake test suite with rts args +RTS -hb -N10 triggers SIGSEGV

[GHC]

#11978: running a profiled build of shake test suite with rts args  +RTS -hb -N10
triggers SIGSEGV
---------------------------------+----------------------------------------
        Reporter:  carter        |                Owner:
            Type:  bug           |               Status:  patch
        Priority:  highest       |            Milestone:
       Component:  Compiler      |              Version:  8.0.1
      Resolution:                |             Keywords:
Operating System:  MacOS X       |         Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:  #4820         |  Differential Rev(s):  phab:D2159
       Wiki Page:                |
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Comment (by simonmar):

 Yeah, this actually looks like a bug.  `overwritingClosure()` wants to see
 the `MSG_THROWTO` closure, but we have locked it with `lockClosure()`,
 which replaces the header with a `WHITEHOLE`.

 We can't replace the header before calling `overwritingClosure()`, because
 that would cause race conditions - another thread could execute the
 `MSG_THROWTO`.  We can't replace the header with `MSG_NULL` before calling
 `overwritingClosure()`, because a `MSG_NULL` is smaller than a
 `MSG_THROWTO`, and we won't overwrite the payload correctliy.

 So I think the only alternative is to have another version of
 `overwritingClosure()` that takes the size of the original closure as an
 argument.  The size is `sizeofW(MessageThrowTo)`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11978#comment:16>
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