[GHC] #9384: setNumCapabilities call breaks eventlog events

[GHC]

#9384: setNumCapabilities call breaks eventlog events
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       Reporter:  slyfox             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Profiling          |                 Version:  7.8.3
       Keywords:                     |        Operating System:  Linux
   Architecture:  x86_64 (amd64)     |         Type of failure:  Incorrect
     Difficulty:  Unknown            |  result at runtime
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 The problem was found when i tried to eventlog '''ghc --make''' itself.
 I've missinterpreted is as a threadscope bug:
 https://github.com/haskell/ThreadScope/issues/37

 Here is small program to reliably reproduce the bug:

 {{{#!hs
 module Main where

 import qualified Data.List as L
 import qualified System.Environment as E
 import Control.Monad
 import qualified Control.Concurrent as CC
 import qualified Control.Concurrent.MVar as CC

 slow_and_silly :: Int -> IO Int
 slow_and_silly i = return $ length $ L.foldl' (\a v -> a ++ [v]) [] [1..i]

 -- build as:
 -- $ ghc --make a -O2 -threaded -eventlog

 -- valid eventlog:
 -- $ ./a 2 7000 +RTS -ls -N2
 -- $ ghc-events validate threads a.eventlog
 -- Valid eventlog:
 -- ...

 -- invalid eventlog
 -- $ ./a 2 7000 +RTS -ls
 -- $ ghc-events validate threads a.eventlog
 -- Invalid eventlog:
 -- ...

 main = do
     [caps, count] <- E.getArgs
     let n_caps :: Int
         n_caps = read caps
         max_n :: Int
         max_n = read count

     CC.setNumCapabilities n_caps

     waits <- replicateM n_caps $ CC.newEmptyMVar

     forM_ waits $ \w -> CC.forkIO $ do
         slow_and_silly max_n >>= print
         CC.putMVar w ()

     forM_ waits $ \w -> CC.takeMVar w
 }}}

 How to reproduce (comments have '''ghc-events''' version):
 {{{
 $ ghc --make a -O2 -threaded -eventlog
 $ ./a 2 7000 +RTS -ls -N2
 $ threadscope a.eventlog # works
 $ ./a 2 7000 +RTS -ls
 $ threadscope a.eventlog # crashes
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9384>
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