[GHC] #10545: Deadlock in the threaded RTS

19 Jun 2015

      #10545: Deadlock in the threaded RTS
-------------------------------------+-------------------------------------
              Reporter:  simonmar    |             Owner:  simonmar
                  Type:  bug         |            Status:  new
              Priority:  highest     |         Milestone:  7.10.2
             Component:  Runtime     |           Version:  7.10.1
  System                             |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 The following program deadlocks with high probability:

 {{{
 -- ghc -rtsopts -threaded -debug performGC.hs
 -- ./performGC 1000 +RTS -qg -N2

 -- -qg turns off parallel GC, needed to trigger the bug
 -- -N2 or greater is needed

 module Main (main) where

 import System.Environment
 import Control.Concurrent
 import Control.Exception
 import Control.Monad
 import System.Random
 import System.Mem
 import qualified Data.Set as Set

 main = do
   [n] <- getArgs
   forkIO $ doSomeWork
   forM [1..read n] $ \n -> do print n; threadDelay 1000; performMinorGC

 doSomeWork :: IO ()
 doSomeWork = forever $ do
   ns <- replicateM 10000 randomIO :: IO [Int]
   ms <- replicateM 1000 randomIO
   let set = Set.fromList ns
       elems = filter (`Set.member` set) ms
  evaluate $ sum elems
 }}}

 There are a few ways that this bug can be triggered:

 * At shutdown, when there are other threads still running.  This is how we
 first encountered it.
 * Using `performGC`, as above.  I think it's necessary to call it from a
 bound thread (e.g. the main thread) to get bad things to happen.
 * I think `forkProcess` might also trigger it, but I haven't observed it.

 I'm working on a fix.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10545
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler