improve conc039 a little bit, and omit it for threaded1

testsuite/tests/ghc-regress/concurrent/should_run/all.T

@@ -90,9 +90,9 @@ test('conc037', only_ways(['threaded1','threaded2']), compile_and_run, [''])
 test('conc038', only_ways(['threaded1','threaded2']), compile_and_run, [''])
 
 # Omit for GHCi, uses foreign export
-# Omit for SMP (threaded2), because in this case the main thread is allowed to 
+# Omit for the threaded ways, because in this case the main thread is allowed to 
 # complete, which causes the child thread to be interrupted.
-test('conc039', omit_ways(['ghci','threaded2']), compile_and_run, [''])
+test('conc039', omit_ways(['ghci','threaded1','threaded2']), compile_and_run, [''])
 
 # Omit for GHCi, uses foreign export
 test('conc040', compose(only_compiler_types(['ghc']),

testsuite/tests/ghc-regress/concurrent/should_run/conc039.hs

@@ -5,13 +5,16 @@ import System.Mem
 import Control.Concurrent
 
 foreign export ccall "performGC_" performGC' :: IO ()
-performGC' = do yield; performGC
+performGC' = do putMVar m (); yield; performGC
 
 foreign import ccall "performGC_" f :: IO ()
 
+{-# NOINLINE m #-}
+m = unsafePerformIO newEmptyMVar
+
 main = do
   forkIO f
-  yield
+  takeMVar m
 
 -- This tests for a bug in the garbage collector, whereby a main
 -- thread that has completed may be GC'd before its return value is