Check to see if TVar's are locked in check_read_only (fixes #7815)

ghc · Apr 17, 2013 · 87baa31 · 87baa31
1 parent 78f9189
commit 87baa31
Showing 1 changed file with 6 additions and 2 deletions.
diff --git a/rts/STM.c b/rts/STM.c
@@ -905,8 +905,12 @@ static StgBool check_read_only(StgTRecHeader *trec STG_UNUSED) {
       s = e -> tvar;
       if (entry_is_read_only(e)) {
         TRACE("%p : check_read_only for TVar %p, saw %ld", trec, s, e -> num_updates);
-        if (s -> num_updates != e -> num_updates) {
-          // ||s -> current_value != e -> expected_value) {
+
+        // Note we need both checks and in this order as the TVar could be
+        // locked by another transaction that is committing but has not yet
+        // incremented `num_updates` (See #7815).
+        if (s -> current_value != e -> expected_value ||
+            s -> num_updates != e -> num_updates) {
           TRACE("%p : mismatch", trec);
           result = FALSE;
           BREAK_FOR_EACH;