Fix a bug introduced with allocation counters

rts/Schedule.c

@@ -2233,6 +2233,9 @@ suspendThread (StgRegTable *reg, rtsBool interruptible)
   task->incall->suspended_tso = tso;
   task->incall->suspended_cap = cap;
 
+  // Otherwise allocate() will write to invalid memory.
+  cap->r.rCurrentTSO = NULL
+
   ACQUIRE_LOCK(&cap->lock);
 
   suspendTask(cap,task);

testsuite/tests/ffi/should_run/all.T

@@ -209,3 +209,7 @@ test('T8083',
      compile_and_run,
      ['T8083_c.c'])
 
+test('ffi023', [ omit_ways(['ghci']),
+                extra_clean(['ffi023_c.o']),
+                extra_run_opts('1000 4') ],
+              compile_and_run, ['ffi023_c.c'])

testsuite/tests/ffi/should_run/ffi023.hs

+-- Tests for a bug fixed in
+
+module Main where
+
+import System.Environment
+import Control.Concurrent
+import Control.Monad
+
+foreign import ccall safe "out"
+  out :: Int -> IO Int
+
+foreign export ccall "incall"  incall :: Int -> IO Int
+
+incall :: Int -> IO Int
+incall x = return $ x + 1
+
+main = do
+  [n, m] <- fmap (fmap read) getArgs
+  ms <- replicateM m $ do
+    v <- newEmptyMVar
+    forkIO $ do mapM out [0..n]; putMVar v ()
+    return v
+  mapM_ takeMVar ms

testsuite/tests/ffi/should_run/ffi023_c.c

+#include "ffi023_stub.h"
+#include "HsFFI.h"
+#include "Rts.h"
+
+HsInt out (HsInt x)
+{
+    performMajorGC();
+    return incall(x);
+}