ThreadPaused: Don't zero slop until free vars are pushed

When threadPaused blackholes a thunk it calls `OVERWRITING_CLOSURE` to
zero the slop for the benefit of the sanity checker. Previously this was
done *before* pushing the thunk's free variables to the update
remembered set. Consequently we would pull zero'd pointers to the update
remembered set.

1 files changed, 7 insertions, 6 deletions

diff --git a/rts/ThreadPaused.c b/rts/ThreadPaused.c
index 13fc2b4ca0..9f41fa1ab3 100644
--- a/rts/ThreadPaused.c
+++ b/rts/ThreadPaused.c
@@ -314,10 +314,6 @@ threadPaused(Capability *cap, StgTSO *tso)
                 continue;
             }
 
-            // zero out the slop so that the sanity checker can tell
-            // where the next closure is.
-            OVERWRITING_CLOSURE(bh);
-
             // an EAGER_BLACKHOLE or CAF_BLACKHOLE gets turned into a
             // BLACKHOLE here.
 #if defined(THREADED_RTS)
@@ -345,11 +341,16 @@ threadPaused(Capability *cap, StgTSO *tso)
                     // overwrite to the update remembered set.
                     // N.B. We caught the WHITEHOLE case above.
                     updateRemembSetPushThunkEager(cap,
-                                                 THUNK_INFO_PTR_TO_STRUCT(bh_info),
-                                                 (StgThunk *) bh);
+                                                  THUNK_INFO_PTR_TO_STRUCT(bh_info),
+                                                  (StgThunk *) bh);
                 }
             }
 
+            // zero out the slop so that the sanity checker can tell
+            // where the next closure is. N.B. We mustn't do this until we have
+            // pushed the free variables to the update remembered set above.
+            OVERWRITING_CLOSURE_SIZE(bh, closure_sizeW_(bh, INFO_PTR_TO_STRUCT(bh_info)));
+
             // The payload of the BLACKHOLE points to the TSO
             RELAXED_STORE(&((StgInd *)bh)->indirectee, (StgClosure *)tso);
             SET_INFO_RELEASE(bh,&stg_BLACKHOLE_info);