Revert "io-sim-por: concurrent threads"

This reverts commit 6b986f16aa96efa61f18c3e2ad59dd96ee0ee8b5.

Author: coot

io-sim/src/Control/Monad/IOSimPOR/Internal.hs

@@ -1463,23 +1463,16 @@ updateRaces newStep@Step{ stepThreadId = tid, stepEffect = newEffect }
           -- then any threads that it wakes up become non-concurrent also.
           let lessConcurrent = foldr Set.delete concurrent (effectWakeup newEffect) in
           if tid `elem` concurrent then
-            let theseStepsRace = isRacyThreadId tid
-                              && step `racingSteps` newStep
+            let theseStepsRace = isRacyThreadId tid && racingSteps step newStep
                 happensBefore  = step `happensBeforeStep` newStep
                 nondep' | happensBefore = nondep
                         | otherwise     = newStep : nondep
-                -- We cannot remove the 'tid' thread from the set of concurrent
-                -- threads if 'theseStepsRace'.  This was done previously in
-                -- order to record only the first race with each thread.  But
-                -- it can happen that the frist race will block a thread that
-                -- is scheduled to run until another later step unblock it
-                -- (e.g. 'modifyTMVar`).  If we remove the 'tid' from the set
-                -- of concurrent threads we can end up in a situation where
-                -- a schedule is not runnable, because the next thread to run
-                -- is still blocked (e.g. 'TMVar' is taken but not yet
-                -- released).
-                concurrent' | happensBefore = Set.delete tid lessConcurrent
-                            | otherwise     = concurrent
+                -- We will only record the first race with each thread---reversing
+                -- the first race makes the next race detectable. Thus we remove a
+                -- thread from the concurrent set after the first race.
+                concurrent' | happensBefore  = Set.delete tid lessConcurrent
+                            | theseStepsRace = Set.delete tid concurrent
+                            | otherwise      = concurrent
                 -- Here we record discovered races.
                 -- We only record a new race if we are following the default schedule,
                 -- to avoid finding the same race in different parts of the search space.