[GHC] #8209: Race condition in setNumCapabilities

1 Sep 2013

      #8209: Race condition in setNumCapabilities
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       Reporter:  akio            |             Owner:
           Type:  bug             |            Status:  new
       Priority:  normal          |         Milestone:
      Component:  Runtime System  |           Version:  7.7
       Keywords:                  |  Operating System:  Unknown/Multiple
   Architecture:  x86_64 (amd64)  |   Type of failure:  Runtime crash
     Difficulty:  Unknown         |         Test Case:
     Blocked By:                  |          Blocking:
Related Tickets:                  |
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 In HEAD, the following program sometimes deadlocks (about 1/10 of the
 time).

 {{{
 import Control.Concurrent
 import Control.Monad
 import GHC.Conc

 main = do
   mainTid <- myThreadId
   labelThread mainTid "main"
   forM_ [0..0] $ \i -> forkIO $ do
     subTid <- myThreadId
     labelThread subTid $ "sub " ++ show i
     forM_ [0..100000000] $ \j -> putStrLn $ "sub " ++ show i ++ ": " ++
 show j
   yield
   setNumCapabilities 2
 }}}

 The problem seems to be that there is a race condition between
 setNumCapabilites and a Task returning from a foreign call. Specifically,
 a sequence of events like the following can happen:

 1. Task 0 makes a foreign call.
 2. Task 1 calls setNumCapabilities()
 3. The call on task 0 returns; it reads task->cap from the memory.
 4. Task 1 moves the Capabilities, invalidating all pointers to them.
 5. Task 0 takes over the invalidated Capability.

 The attached patch adds an ASSERT to the RTS to demonstrate the problem
 (it does not fix the problem).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8209
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