[GHC] #12019: Profiling option -hb is not thread safe

6 May 2016

      #12019: Profiling option -hb is not thread safe
-------------------------------------+-------------------------------------
           Reporter:  erikd          |             Owner:  erikd
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.2
          Component:  Runtime        |           Version:  8.1
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime crash
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11978, #12009
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 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):

 {{{
 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);
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12019
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