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

[GHC]

#8402: throwTo segfaults under -prof -threaded -with-rtsopts=N2
----------------------------------+----------------------------------
        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:
----------------------------------+----------------------------------

Comment (by kazu-yamamoto):

 I'm trying to profile my web server with GHC head (+RTS -Nx -p). Without
 ezyang's patch, it causes a segfault when I type C-c to stop the server.
 With GHC 7.6.3, C-c stops the server and generates a profile file. With
 ezyang's patch, this segfault disappeared.

 However, all counters are displayed 0:

 {{{
 individual     inherited
 COST CENTRE MODULE                                    no.     entries
 %time %alloc   %time %alloc

 MAIN        MAIN                                      347           0
 100.0  100.0   100.0  100.0
  CAF        Main                                      693           0
 0.0    0.0     0.0    0.0
  CAF        Server                                    692           0
 0.0    0.0     0.0    0.0
  CAF        WaiApp                                    690           0
 0.0    0.0     0.0    0.0
  CAF        Program.Mighty.Config                     688           0
 0.0    0.0     0.0    0.0
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8402#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler