[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:  patch
        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):  Phab:D2516
       Wiki Page:                    |
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Changes (by bgamari):

 * status:  new => patch
 * differential:   => Phab:D2516


@@ -7,1 +7,1 @@
- {{{
+ {{{#!hs

New description:

 This ticket is a continuation of #11978 and #12009. After fixing a couple
 of issues in those two tickets I found that the profiling run time is not
 thread safe.

 Have a trivial test program (written as one of the tests for #11978):

 {{{#!hs
 import Control.Concurrent
 import Control.Concurrent.MVar
 import Control.Exception
 import Control.Monad

 main :: IO ()
 main = do
     putStrLn "Start ..."
     mvar <- newMVar (0 :: Int)

     let count = 50

     forM_ [ 1 .. count ] $ const $ forkIO $ do
             threadDelay 100
             i <- takeMVar mvar
             putMVar mvar $! i + 1

     threadDelay 1000000
     end <- takeMVar mvar
     putStrLn $ "Final result " ++ show end
     assert (end == count) $ return ()
 }}}

 Compiling that with a compiler that has bug fixes arising from #11978 and
 #12009 as:

 {{{
 inplace/bin/ghc-stage2 testsuite/tests/profiling/should_run/T11978b.hs \
     -fforce-recomp -rtsopts -fno-warn-tabs -O -prof -static -auto-all \
     -threaded -debug -o T11978b
 }}}

 and run as:

 {{{
 ./T11978b +RTS -p -hb -N10
 }}}

 crashes in a number of different ways. I've seen at least 3 different
 assertion failures and numerous segfaults (in different `stg_ap_*`
 functions).

 Replace `-hb` with other profiling options like `-hr` etc do not seem to
 crash.

 Looking at code, one example of lack of thread safetly is the function
 `LDV_recordDead` which mutates global variable `censuses` which does not
 have any locking around it. Only figured this out because the following
 assert (in `LDV_recordDead`) was being triggered occasionally.

 {{{
     ASSERT(censuses[t].void_total < censuses[t].not_used);
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12019#comment:5>
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