[commit: ghc] master: Check to see if TVar's are locked in check_read_only (fixes #7815) (87baa31)

[Ian Lynagh]

Repository : http://darcs.haskell.org/ghc.git/

On branch  : master

https://github.com/ghc/ghc/commit/87baa31cb117181ab4f4b26387653996d1792167

>---------------------------------------------------------------

commit 87baa31cb117181ab4f4b26387653996d1792167
Author: Ryan Yates <ryates at cs.rochester.edu>
Date:   Mon Apr 8 19:18:51 2013 -0400

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

>---------------------------------------------------------------

 rts/STM.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/rts/STM.c b/rts/STM.c
index eee0f46..8f4bdfb 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;