[GHC] #8402: throwTo segfaults under -prof -threaded -with-rtsopts=N2

[GHC]

#8402: throwTo segfaults under -prof -threaded -with-rtsopts=N2
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        Reporter:  akio           |            Owner:
            Type:  bug            |           Status:  patch
        Priority:  highest        |        Milestone:  7.8.1
       Component:  Profiling      |          Version:  7.7
      Resolution:                 |         Keywords:
Operating System:  Linux          |     Architecture:  x86_64 (amd64)
 Type of failure:  Runtime crash  |       Difficulty:  Unknown
       Test Case:                 |       Blocked By:
        Blocking:                 |  Related Tickets:
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Changes (by simonmar):

 * cc: ezyang (added)
 * priority:  normal => highest
 * milestone:   => 7.8.1


Comment:

 Just to be clear for those watching this ticket: this just affects
 profiling with `+RTS -N`.

 Here are the rules, documented next to `overwritingClosure`:

  * We cannot zero slop when THREADED_RTS && +RTS -N
  * LDV profiling requires slop-zeroing, so we cannot do LDV profiling with
 +RTS -N

 If THREADED_RTS is on, then we only get into `overwritingClosure` if
 `PROFILING` is on.  Originally we returned immediately if (PROFILING &&
 !LDV), but Edward removed this, basically making it so that slop is always
 overwritten when PROFILING is on, which breaks the first rule above.

 I'd like to understand more about what Edward was fixing with his patch.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8402#comment:4>
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