[commit: ghc] ghc-parmake-gsoc: TcRnMonad: make forkM thread-safe (b0a20f2)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-parmake-gsoc
Link       : http://ghc.haskell.org/trac/ghc/changeset/b0a20f26a777566a6f87f2e597682004513ef186/ghc

>---------------------------------------------------------------

commit b0a20f26a777566a6f87f2e597682004513ef186
Author: Patrick Palka <patrick at parcs.ath.cx>
Date:   Thu Aug 22 21:04:34 2013 -0400

    TcRnMonad: make forkM thread-safe
    
    A forkM'd action cannot safely share a UniqSupply with its parent.
    
    [ Originally I resolved this issue in another way: by atomically
      updating env_us in newUnique/newUniqueSupply. But I think this
      (equivalent) change is more sensible. ]


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b0a20f26a777566a6f87f2e597682004513ef186
 compiler/typecheck/TcRnMonad.lhs |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/compiler/typecheck/TcRnMonad.lhs b/compiler/typecheck/TcRnMonad.lhs
index 84dcecf..1146302 100644
--- a/compiler/typecheck/TcRnMonad.lhs
+++ b/compiler/typecheck/TcRnMonad.lhs
@@ -1266,7 +1266,11 @@ forkM_maybe :: SDoc -> IfL a -> IfL (Maybe a)
 -- signatures, which is pretty benign
 
 forkM_maybe doc thing_inside
- = do { unsafeInterleaveM $
+ -- NB: Don't share the mutable env_us with the interleaved thread since env_us
+ --     does not get updated atomically (e.g. in newUnique and newUniqueSupply).
+ = do { child_us <- newUniqueSupply
+      ; child_env_us <- newMutVar child_us
+      ; unsafeInterleaveM $ updEnv (\env -> env { env_us = child_env_us }) $
         do { traceIf (text "Starting fork {" <+> doc)
            ; mb_res <- tryM $
                        updLclEnv (\env -> env { if_loc = if_loc env $$ doc }) $