nonmoving: Fix race in shortcutting

We must use an acquire load to read the info table pointer since if we
find an indirection we must be certain that we see the indirectee.

2 files changed, 8 insertions, 1 deletions

diff --git a/rts/include/rts/storage/ClosureMacros.h b/rts/include/rts/storage/ClosureMacros.h
index ba57df1699..ecfca8a81c 100644
--- a/rts/include/rts/storage/ClosureMacros.h
+++ b/rts/include/rts/storage/ClosureMacros.h
@@ -97,6 +97,12 @@ EXTERN_INLINE const StgInfoTable *get_itbl(const StgClosure *c)
     return INFO_PTR_TO_STRUCT(RELAXED_LOAD(&c->header.info));
 }
 
+EXTERN_INLINE const StgInfoTable *get_itbl_acquire(const StgClosure *c);
+EXTERN_INLINE const StgInfoTable *get_itbl_acquire(const StgClosure *c)
+{
+    return INFO_PTR_TO_STRUCT(ACQUIRE_LOAD(&c->header.info));
+}
+
 EXTERN_INLINE const StgRetInfoTable *get_ret_itbl(const StgClosure *c);
 EXTERN_INLINE const StgRetInfoTable *get_ret_itbl(const StgClosure *c)
 {
diff --git a/rts/sm/NonMovingShortcut.c b/rts/sm/NonMovingShortcut.c
index 83c4857677..ee97ba1b70 100644
--- a/rts/sm/NonMovingShortcut.c
+++ b/rts/sm/NonMovingShortcut.c
@@ -153,7 +153,8 @@ selectee_changed:
     // Selectee is a non-moving object, mark it.
     markQueuePushClosure(queue, selectee, NULL);
 
-    const StgInfoTable *selectee_info_tbl = get_itbl(selectee);
+    // This may synchronize with the release in updateWithIndirection.
+    const StgInfoTable *selectee_info_tbl = get_itbl_acquire(selectee);
     switch (selectee_info_tbl->type) {
         case WHITEHOLE: {
             // Probably a loop. Abort.