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

1 files changed, 6 insertions, 2 deletions

diff --git a/rts/STM.c b/rts/STM.c
index eee0f46bbc..8f4bdfbecb 100644
--- 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;