Race condition in setNumCapabilities

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).

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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